human
***task***
the task is to summarize an input biomedical literature in six sentences

***input***
the input is a biomedical literature

***output***
the output is the summary of an input biomedical literature in six sentences

***documentation***

***examples***

input: eukaryotic cells can enclose their own cytoplasmic components in a double - membrane structure , the autophagosome , and deliver it to a lytic compartment , the vacuole / lysosome , where the contents are then degraded . this conserved system is involved not only in the recycling of proteins under starvation conditions but also in the clearance of organelles and aberrant aggregate - prone proteins , digestion of invading pathogens , and so on . genes involved in autophagy were first identified by yeast genetic screenings . at present , more than 30 autophagy - related ( atg ) genes have been identified in yeast , and of them at least 18 genes are essential for autophagosome formation , a crucial process in autophagy . most of these 18 genes are conserved in mammals , suggesting that the mechanism of autophagosome formation is basically conserved from yeast to mammals . the 18 atg proteins can be divided into five groups according to their functions . one group consists of subunits of a class iii phosphatidylinositol ( ptdins ) 3 - kinase complex ( hereafter , ptdins 3 - kinase indicates the class iii ptdins 3 - kinase ) . atg14 and vps30 / atg6 are two such proteins and are included in this group together with vps34 and vps15 , catalytic and regulatory subunits , respectively , ( the functions of vps34 and vps15 in the vacuolar protein sorting pathway have been studied in detail , and ; thus , they are not designated as atg proteins although they are essential for autophagy ) . atg14 is a key subunit in determining the function of the ptdins 3 - kinase complex and is the focus of this paper ( see following sections ) . ptdins 3 - kinase phosphorylates ptdins at the d - 3 position of the inositol ring , generating ptdins ( 3 ) p . in yeast , vps34 is the sole ptdins 3 - kinase . vps34 is essential to both the vacuolar protein - sorting pathway and to autophagy . autophagic activity is completely abolished in vps34 cells expressing a lipid kinase - dead form of vps34 , indicating that production of ptdins ( 3 ) p is essential for autophagy . in yeast , it was shown that ptdins ( 3 ) p is enriched in the inner surface of the isolation membrane and autophagosome ( figure 1 ) . produced ptdins ( 3 ) p recruits downstream molecules , such as atg18 , that are considered to be directly involved in autophagosome formation . for a general introduction to the function of ptdins 3 - kinase and ptdins ( 3 ) p in autophagy , please refer to other reviews . . inhibitors of ptdins 3 - kinase , such as wortmannin and 3 - methyladenine , suppress autophagy in mammalian cells . conversely , supplementation with ptdins ( 3 ) p , but not other phosphoinositides , enhances autophagic degradation in ht - 29 cells . like in yeast , ptdins 3 - kinase is also required for vesicular trafficking toward the lytic compartment , the lysosome . as mentioned above , ptdins 3 - kinase is required for both autophagy and vacuolar protein sorting . the means by which the sole ptdins 3 - kinase , vps34 , is involved in these two distinct processes is explained by the existence of multiple ptdins 3 - kinase complexes ( figure 1 ( b ) ) . in yeast , vps34 forms two distinct ptdins 3 - kinase complexes ( complexes i and ii ) that are involved in different processes . complex i specifically functions in autophagy while complex ii is required in the vacuolar protein sorting pathway . both complexes have ptdins 3 - kinase activity and share three common subunits , vps34 , vps15 , and vps30 / atg6 . the ptdins 3 - kinase activity of vps34 is not required to form the ptdins 3 - kinase complexes but is essential for autophagy and for the vacuolar protein sorting pathway . vps15 is a serine / threonine protein kinase that phosphorylates vps34 and recruits it to the membrane fraction . vps15 is myristoylated , but the membrane association of the ptdins 3 - kinase complexes does not solely depend on myristoylation . however , beclin 1 , a mammalian homolog of vps30 / atg6 , interacts with various proteins involved in other processes and is proposed to serve as a platform upon which multiple cellular signals converge , thereby regulating the balance between autophagy and other biological processes . atg14 is specifically integrated into complex i , while complex ii contains vps38 as a specific subunit . these specific factors play an essential role in sorting the ptdins 3 - kinase complexes to the distinct processes . atg14 bridges vps30 / atg6 and vps34 to allow formation of complex i. similarly , vps38 bridges vps30 / atg6 and vps34 to form complex ii . thus , these unique subunits serve as connectors to form ptdins 3 - kinase complexes . in addition , deletion of atg14 does not affect vacuolar protein sorting , and , conversely , disruption of vps38 does not suppress autophagy . thus , the ptdins 3 - kinase complexes are strictly sorted to distinct functions depending on the specific subunits , atg14 and vps38 . complex i is present on the vacuolar membrane and at a perivacuolar structure called the preautophagosomal structure ( pas ) . most of atg proteins localize to the pas , and ; thus , the pas is considered to be closely related to autophagosome formation . pas localization of vps34 ( the catalytic subunit ) and vps30 / atg6 is abolished in atg14 cells , indicating that complex i is targeted to the pas in an atg14 - dependent manner ( figure 1 ( b ) ) . on the other hand , deletion of vps38 does not affect pas localization of complex i. complex ii localizes to the vacuolar membrane and the endosome . whether or not the endosomal localization of complex ii depends on vps38 is not a simple question . in vps38 cells , the endosomal localization of vps30 / atg6thus , the endosomal localization of the complete complex ii , including vps30 / atg6 , is dependent on vps38 , whereas that of the catalytic and regulatory subunits , vps34 and vps15 , is not . , atg14 has at least two functions in autophagy : ( i ) atg14 acts as a connector to form the ptdins 3 - kinase complex and ( ii ) atg14 directs complex i to function in autophagy by targeting it to the pas . a simple blast database search failed to identify a mammalian homolog of atg14 . recently ; however , several groups succeeded in identifying mammalian atg14 . itakura et al . identified a candidate for mammalian atg14 by psi - blast , a method for detecting weak but biologically relevant sequence similarities that is more sensitive than conventional blast , and experimentally confirmed that it was the bona fide mammalian atg14 . other groups identified mammalian atg14 using a biochemical approach , that is , identifying proteins that are copurified with beclin 1 ( mammalian atg14 is hereafter called barkor / atg14 ( l ) ) . similar to yeast atg14 , barkor / atg14 ( l ) forms a ptdins 3 - kinase complex with beclin 1 and mammalian homologs of vps34 and vps15 . on the other hand , uvrag , originally identified as a protein related to uv resistance , is considered to be a counterpart for vps38 . thus , ptdins 3 - kinase complexes that correspond to yeast complexes i and ii exist in mammals . in addition , mammalian cells have another ptdins 3 - kinase complex composed of vps34 , vps15 , beclin 1 , uvrag , and rubicon , a rab7 effecter containing a run domain . the rubicon - containing complex negatively regulates maturation of the autophagosome . as is the case in yeast , the mammalian ptdins 3 - kinasediscovery of barkor / atg14 ( l ) and subsequent research uncovered a similar mechanism that sorts the ptdins 3 - kinase complexes to distinct functions . barkor / atg14 ( l ) targets the ptdins 3 - kinase complex to an er subdomain . the pas is not identified in mammalian cells . instead , at least in some cases , autophagosomes are formed inside omegasomes , which are formed at specialized er subdomains prior to autophagosome formation . thus , in both yeast and mammals , atg14 directs one of the ptdins 3 - kinase complexes to function specifically in autophagy by targeting it to the proposed site of autophagosome formation . the difference in this sorting system between yeast and mammals is the targeting site of the ptdins 3 - kinase complex ; yeast complex i is targeted to the pas , whereas mammalian autophagy - specific ptdins 3 - kinase complex is targeted to the er subdomain . the amino acid sequence of atg14 is not significantly conserved between yeast and mammals , which is consistent with the fact that a sensitive psi - blast method was needed to identify barkor / atg14 ( l ) . nevertheless , we can still find similarities between yeast atg14 and barkor / atg14 ( l ) at the levels of local amino acid sequences and of secondary structures . both yeast atg14 and barkor / atg14 ( l ) contain a cysteine - rich domain at the n - terminal region ( figure 2 ) . conserved cysteine residues in this region are essential for er targeting of the ptdins 3 - kinase complex in mammals . at the secondary structural level , both yeast atg14 and barkor / atg14 ( l ) deletion analysis of yeast atg14 revealed that the n - terminal half region containing the coiled - coil domains is sufficient to support autophagic activity , although at a significantly reduced level . the n - terminal half of atg14 still has the ability to form complex i and to localize to the pas . these results suggest that the functions of atg14bridging vps34 and atg6 and targeting complex i to the pas are exerted by the n - terminal half region . atg14 is unstable without the interaction with vps30 / atg6 and vps34 through these coiled - coil domains . although the c - terminal half of atg14 is not essential for the minimal level of autophagy , it is required to support a normal level of autophagic activity ( this issue is discussed later ) . similarly , the coiled - coil domains of barkor / atg14 ( l ) play an essential role in autophagy . coiled - coil domains are required for formation of the autophagy - specific ptdins 3 - kinase complex . of these , the second coiled - coil domain is involved in interaction with beclin 1 , as is the case in yeast atg14 . endogenous barkor / atg14 ( l ) is stabilized by binding to beclin 1 and vps34 through the coiled - coil domains . interestingly , an exogenously expressed barkor / atg14 ( l ) variant that lacks the coiled - coils still localizes to the isolation membrane or its precursor , suggesting that other regions are also important for proper localization of barkor / atg14 ( l ) . recently , fan et al . identified a novel domain , called the barkor / atg14 ( l ) autophagosome targeting sequence ( bats ) domain , at the c - terminal region of barkor / atg14 ( l ) . the bats domain is essential and sufficient for localizing barkor / atg14 ( l ) to the autophagosome . an amphiphilic alpha helix resides at the c - terminus of the bats domain , and its hydrophobic side plays a crucial role in localizing to the isolation membrane and the autophagosome . it was proposed that the bats domain senses membrane curvature and binds to the membrane through the hydrophobic side of the amphiphilic alpha helix , thereby , targeting barkor / atg14 ( l ) to the isolation membrane to efficiently produce ptdins ( 3 ) p there . however , several prediction programs anticipate that a clear amphiphilic helix also resides within the c - terminal half of the yeast atg14 ( figure 2 ) . overexpression of barkor / atg14 ( l ) enhances autophagic activity even under nutrient - rich conditions . thus , atg14 seems to be one of the limiting factors regulating autophagic activity . autophagic activity is reduced in yeast cells expressing an atg14 variant lacking the c - terminal half ( hereafter , atg14 - c ) compared to cells expressing the full - length atg14 ( atg14 - fl ) . cells expressing the atg14 - c variant accumulate smaller autophagic bodies , indicating that atg14 has a close relationship with the size of the autophagosome . we performed electron microscopy and measured the diameter of the smaller autophagic bodies accumulated in atg14 - c cells ( figure 3 ) . the average diameter of autophagic bodies accumulated in atg14 - c cells is approximately 66 % of that in cells expressing atg14 - fl . thus , the volume of each autophagic body in atg14 - c cells is estimated to be 29 % ( the cube of 66 % ) of that in atg14 - fl cells . consistent with the estimation based on this electron microscopy , the actual autophagic activity in atg14 - c cells , measured by an established biochemical assay , is approximately 33 % of that in atg14 - fl cells . thus , the c - terminal half of atg14 is likely to be required to form a normal - sized autophagosome rather than to regulate the number of autophagosomes . it is possible that the c - terminal half of atg14 is directly involved in the modulation of autophagosome size . in this sense , it would be interesting to examine whether the amphiphilic helix within the c - terminal half is involved in modulating the curvature of the isolation membrane . alternatively , the c - terminal half of atg14 may regulate autophagosome size indirectly through one or more downstream molecules . deletion of atg14 affects the localization of atg8 , the atg12 - atg5 - atg16 complex , and the atg2 - atg18 complex . similarly , the size of the autophagosome correlates with the protein levels of atg8 . thus , it is possible that atg14 regulates autophagosome size indirectly through modulating atg8 recruitment to the pas . as mentioned above , overexpression of barkor / atg14 ( l ) activates autophagy in mammalian cells even under nutrient - rich conditions , indicating that barkor / atg14 ( l ) is one of the key players regulating autophagic activity in mammals . other subunits of ptdins 3 - kinase complexes are also involved in the regulation of autophagic activity . conversely , rubicon negatively regulates maturation of the endosome and the autophagosome by sequestering uvrag from the class c - vps / hops complex . beclin 1 interacts with multiple proteins in addition to the core subunits of the ptdins 3 - kinase complexes . one of these , ambra1 , positively regulates autophagy and plays a crucial role in neural development . beclin 1 also interacts with bcl - 2 , an antiapoptotic protein , that is believed to regulate the balance between autophagy and apoptosis . thus , beclin 1 may serve as a platform upon which cellular signals converge and function to regulate the crosstalk of multiple processes , including autophagy . this function of regulating the balance of multiple cellular events has not been reported for yeast vps30 / atg6 , which implies that beclin 1 has obtained this regulatory role during evolution . in addition to generating ptdins ( 3 ) p by the autophagy - specific ptdins 3 - kinase , dephosphorylation of ptdins ( 3 ) p also plays an important role in regulating autophagy in mammals . overexpression of ptdins ( 3 ) p phosphatases decreases autophagic activity , while the knockdown or the expression of a dominant - negative form of the phosphatases enhances autophagy . taken together , atg14 regulates autophagic activity , at least partially , both in yeast and mammals . however , the barkor / atg14 ( l ) - containing ptdins 3 - kinase complex seems to play a more crucial role in determining autophagic activity than yeast complex i , and the regulation of the barkor / atg14 ( l ) complex may have evolved to function in a more sophisticated manner . a conserved function of atg14 in autophagy is to target the ptdins 3 - kinase complex to the probable site of autophagosome formation . an important problem to be solved is the mechanism whereby atg14 targets the ptdins 3 - kinase complex to the pas in yeast and to the er subdomain in mammals . there are some reports concerning the regulation of ptdins 3 - kinase complex localization . according to a comprehensive analysis ofthe hierarchy of atg protein localization , proper targeting of atg14 is dependent on atg17 and fip200 in yeast and mammals , respectively , both of which are scaffold proteins for atg assembly . the conserved cysteine residues at the n - terminal region of barkor / atg14 ( l ) are required for er localization of barkor / atg14 ( l ) . yeast vps15 , a regulatory subunit of ptdins 3 - kinase complexes , can localize to the pas even in atg14 cells while vps34 and vps30 / atg6 can not , indicating that vps15 also contains information related to targeting to the pas . the function of the n - terminal half of atg14 has been largely , if not completely , determined . on the other hand , the function of the c - terminal half of atg14 is still unclear . the c - terminal half of atg14 is likely involved in forming a normal - sized autophagosome , directly or indirectly . in this sense , it is interesting that the c - terminal bats domain of barkor / atg14 ( l ) binds to the membrane through the hydrophobic surface of the amphiphilic alpha helix . the bats domain favors highly curved membranes that contain ptdins ( 3 ) p , which are considered to be the properties of the isolation membrane . although the amino acid sequence of the bats domain is not conserved in yeast atg14 , a clear amphiphilic alpha helix is predicted within the c - terminal half of yeast atg14 . whether these amphiphilic alpha helixes are involved in the regulation of autophagosome size or not is an interesting issue for future research .
output:

agent
phosphorylation of phosphatidylinositol ( ptdins ) by a ptdins 3 - kinase is an essential process in autophagy . atg14 , a specific subunit of one of the ptdins 3 - kinase complexes , targets the complex to the probable site of autophagosome formation , thereby , sorting the complex to function specifically in autophagy . the n - terminal half of atg14 , containing coiled - coil domains , is required to form the ptdins 3 - kinase complex and target it to the proper site . the c - terminal half of yeast atg14 is suggested to be involved in the formation of a normal - sized autophagosome . the c - terminal half of mammalian atg14 contains the barkor / atg14 ( l ) autophagosome - targeting sequence ( bats ) domain that preferentially binds to the highly curved membranes containing ptdins ( 3 ) p and is proposed to target the ptdins 3 - kinase complex efficiently to the isolation membrane . thus , the n - and c - terminal halves of atg14 are likely to have an essential core function and a regulatory role , respectively .
***task***
the task is to summarize an input biomedical literature in six sentences

***input***
the input is a biomedical literature

***output***
the output is the summary of an input biomedical literature in six sentences

***documentation***

***examples***

input: eukaryotic cells can enclose their own cytoplasmic components in a double - membrane structure , the autophagosome , and deliver it to a lytic compartment , the vacuole / lysosome , where the contents are then degraded . this conserved system is involved not only in the recycling of proteins under starvation conditions but also in the clearance of organelles and aberrant aggregate - prone proteins , digestion of invading pathogens , and so on . genes involved in autophagy were first identified by yeast genetic screenings . at present , more than 30 autophagy - related ( atg ) genes have been identified in yeast , and of them at least 18 genes are essential for autophagosome formation , a crucial process in autophagy . most of these 18 genes are conserved in mammals , suggesting that the mechanism of autophagosome formation is basically conserved from yeast to mammals . the 18 atg proteins can be divided into five groups according to their functions . one group consists of subunits of a class iii phosphatidylinositol ( ptdins ) 3 - kinase complex ( hereafter , ptdins 3 - kinase indicates the class iii ptdins 3 - kinase ) . atg14 and vps30 / atg6 are two such proteins and are included in this group together with vps34 and vps15 , catalytic and regulatory subunits , respectively , ( the functions of vps34 and vps15 in the vacuolar protein sorting pathway have been studied in detail , and ; thus , they are not designated as atg proteins although they are essential for autophagy ) . atg14 is a key subunit in determining the function of the ptdins 3 - kinase complex and is the focus of this paper ( see following sections ) . ptdins 3 - kinase phosphorylates ptdins at the d - 3 position of the inositol ring , generating ptdins ( 3 ) p . in yeast , vps34 is the sole ptdins 3 - kinase . vps34 is essential to both the vacuolar protein - sorting pathway and to autophagy . autophagic activity is completely abolished in vps34 cells expressing a lipid kinase - dead form of vps34 , indicating that production of ptdins ( 3 ) p is essential for autophagy . in yeast , it was shown that ptdins ( 3 ) p is enriched in the inner surface of the isolation membrane and autophagosome ( figure 1 ) . produced ptdins ( 3 ) p recruits downstream molecules , such as atg18 , that are considered to be directly involved in autophagosome formation . for a general introduction to the function of ptdins 3 - kinase and ptdins ( 3 ) p in autophagy , please refer to other reviews . . inhibitors of ptdins 3 - kinase , such as wortmannin and 3 - methyladenine , suppress autophagy in mammalian cells . conversely , supplementation with ptdins ( 3 ) p , but not other phosphoinositides , enhances autophagic degradation in ht - 29 cells . like in yeast , ptdins 3 - kinase is also required for vesicular trafficking toward the lytic compartment , the lysosome . as mentioned above , ptdins 3 - kinase is required for both autophagy and vacuolar protein sorting . the means by which the sole ptdins 3 - kinase , vps34 , is involved in these two distinct processes is explained by the existence of multiple ptdins 3 - kinase complexes ( figure 1 ( b ) ) . in yeast , vps34 forms two distinct ptdins 3 - kinase complexes ( complexes i and ii ) that are involved in different processes . complex i specifically functions in autophagy while complex ii is required in the vacuolar protein sorting pathway . both complexes have ptdins 3 - kinase activity and share three common subunits , vps34 , vps15 , and vps30 / atg6 . the ptdins 3 - kinase activity of vps34 is not required to form the ptdins 3 - kinase complexes but is essential for autophagy and for the vacuolar protein sorting pathway . vps15 is a serine / threonine protein kinase that phosphorylates vps34 and recruits it to the membrane fraction . vps15 is myristoylated , but the membrane association of the ptdins 3 - kinase complexes does not solely depend on myristoylation . however , beclin 1 , a mammalian homolog of vps30 / atg6 , interacts with various proteins involved in other processes and is proposed to serve as a platform upon which multiple cellular signals converge , thereby regulating the balance between autophagy and other biological processes . atg14 is specifically integrated into complex i , while complex ii contains vps38 as a specific subunit . these specific factors play an essential role in sorting the ptdins 3 - kinase complexes to the distinct processes . atg14 bridges vps30 / atg6 and vps34 to allow formation of complex i. similarly , vps38 bridges vps30 / atg6 and vps34 to form complex ii . thus , these unique subunits serve as connectors to form ptdins 3 - kinase complexes . in addition , deletion of atg14 does not affect vacuolar protein sorting , and , conversely , disruption of vps38 does not suppress autophagy . thus , the ptdins 3 - kinase complexes are strictly sorted to distinct functions depending on the specific subunits , atg14 and vps38 . complex i is present on the vacuolar membrane and at a perivacuolar structure called the preautophagosomal structure ( pas ) . most of atg proteins localize to the pas , and ; thus , the pas is considered to be closely related to autophagosome formation . pas localization of vps34 ( the catalytic subunit ) and vps30 / atg6 is abolished in atg14 cells , indicating that complex i is targeted to the pas in an atg14 - dependent manner ( figure 1 ( b ) ) . on the other hand , deletion of vps38 does not affect pas localization of complex i. complex ii localizes to the vacuolar membrane and the endosome . whether or not the endosomal localization of complex ii depends on vps38 is not a simple question . in vps38 cells , the endosomal localization of vps30 / atg6thus , the endosomal localization of the complete complex ii , including vps30 / atg6 , is dependent on vps38 , whereas that of the catalytic and regulatory subunits , vps34 and vps15 , is not . , atg14 has at least two functions in autophagy : ( i ) atg14 acts as a connector to form the ptdins 3 - kinase complex and ( ii ) atg14 directs complex i to function in autophagy by targeting it to the pas . a simple blast database search failed to identify a mammalian homolog of atg14 . recently ; however , several groups succeeded in identifying mammalian atg14 . itakura et al . identified a candidate for mammalian atg14 by psi - blast , a method for detecting weak but biologically relevant sequence similarities that is more sensitive than conventional blast , and experimentally confirmed that it was the bona fide mammalian atg14 . other groups identified mammalian atg14 using a biochemical approach , that is , identifying proteins that are copurified with beclin 1 ( mammalian atg14 is hereafter called barkor / atg14 ( l ) ) . similar to yeast atg14 , barkor / atg14 ( l ) forms a ptdins 3 - kinase complex with beclin 1 and mammalian homologs of vps34 and vps15 . on the other hand , uvrag , originally identified as a protein related to uv resistance , is considered to be a counterpart for vps38 . thus , ptdins 3 - kinase complexes that correspond to yeast complexes i and ii exist in mammals . in addition , mammalian cells have another ptdins 3 - kinase complex composed of vps34 , vps15 , beclin 1 , uvrag , and rubicon , a rab7 effecter containing a run domain . the rubicon - containing complex negatively regulates maturation of the autophagosome . as is the case in yeast , the mammalian ptdins 3 - kinasediscovery of barkor / atg14 ( l ) and subsequent research uncovered a similar mechanism that sorts the ptdins 3 - kinase complexes to distinct functions . barkor / atg14 ( l ) targets the ptdins 3 - kinase complex to an er subdomain . the pas is not identified in mammalian cells . instead , at least in some cases , autophagosomes are formed inside omegasomes , which are formed at specialized er subdomains prior to autophagosome formation . thus , in both yeast and mammals , atg14 directs one of the ptdins 3 - kinase complexes to function specifically in autophagy by targeting it to the proposed site of autophagosome formation . the difference in this sorting system between yeast and mammals is the targeting site of the ptdins 3 - kinase complex ; yeast complex i is targeted to the pas , whereas mammalian autophagy - specific ptdins 3 - kinase complex is targeted to the er subdomain . the amino acid sequence of atg14 is not significantly conserved between yeast and mammals , which is consistent with the fact that a sensitive psi - blast method was needed to identify barkor / atg14 ( l ) . nevertheless , we can still find similarities between yeast atg14 and barkor / atg14 ( l ) at the levels of local amino acid sequences and of secondary structures . both yeast atg14 and barkor / atg14 ( l ) contain a cysteine - rich domain at the n - terminal region ( figure 2 ) . conserved cysteine residues in this region are essential for er targeting of the ptdins 3 - kinase complex in mammals . at the secondary structural level , both yeast atg14 and barkor / atg14 ( l ) deletion analysis of yeast atg14 revealed that the n - terminal half region containing the coiled - coil domains is sufficient to support autophagic activity , although at a significantly reduced level . the n - terminal half of atg14 still has the ability to form complex i and to localize to the pas . these results suggest that the functions of atg14bridging vps34 and atg6 and targeting complex i to the pas are exerted by the n - terminal half region . atg14 is unstable without the interaction with vps30 / atg6 and vps34 through these coiled - coil domains . although the c - terminal half of atg14 is not essential for the minimal level of autophagy , it is required to support a normal level of autophagic activity ( this issue is discussed later ) . similarly , the coiled - coil domains of barkor / atg14 ( l ) play an essential role in autophagy . coiled - coil domains are required for formation of the autophagy - specific ptdins 3 - kinase complex . of these , the second coiled - coil domain is involved in interaction with beclin 1 , as is the case in yeast atg14 . endogenous barkor / atg14 ( l ) is stabilized by binding to beclin 1 and vps34 through the coiled - coil domains . interestingly , an exogenously expressed barkor / atg14 ( l ) variant that lacks the coiled - coils still localizes to the isolation membrane or its precursor , suggesting that other regions are also important for proper localization of barkor / atg14 ( l ) . recently , fan et al . identified a novel domain , called the barkor / atg14 ( l ) autophagosome targeting sequence ( bats ) domain , at the c - terminal region of barkor / atg14 ( l ) . the bats domain is essential and sufficient for localizing barkor / atg14 ( l ) to the autophagosome . an amphiphilic alpha helix resides at the c - terminus of the bats domain , and its hydrophobic side plays a crucial role in localizing to the isolation membrane and the autophagosome . it was proposed that the bats domain senses membrane curvature and binds to the membrane through the hydrophobic side of the amphiphilic alpha helix , thereby , targeting barkor / atg14 ( l ) to the isolation membrane to efficiently produce ptdins ( 3 ) p there . however , several prediction programs anticipate that a clear amphiphilic helix also resides within the c - terminal half of the yeast atg14 ( figure 2 ) . overexpression of barkor / atg14 ( l ) enhances autophagic activity even under nutrient - rich conditions . thus , atg14 seems to be one of the limiting factors regulating autophagic activity . autophagic activity is reduced in yeast cells expressing an atg14 variant lacking the c - terminal half ( hereafter , atg14 - c ) compared to cells expressing the full - length atg14 ( atg14 - fl ) . cells expressing the atg14 - c variant accumulate smaller autophagic bodies , indicating that atg14 has a close relationship with the size of the autophagosome . we performed electron microscopy and measured the diameter of the smaller autophagic bodies accumulated in atg14 - c cells ( figure 3 ) . the average diameter of autophagic bodies accumulated in atg14 - c cells is approximately 66 % of that in cells expressing atg14 - fl . thus , the volume of each autophagic body in atg14 - c cells is estimated to be 29 % ( the cube of 66 % ) of that in atg14 - fl cells . consistent with the estimation based on this electron microscopy , the actual autophagic activity in atg14 - c cells , measured by an established biochemical assay , is approximately 33 % of that in atg14 - fl cells . thus , the c - terminal half of atg14 is likely to be required to form a normal - sized autophagosome rather than to regulate the number of autophagosomes . it is possible that the c - terminal half of atg14 is directly involved in the modulation of autophagosome size . in this sense , it would be interesting to examine whether the amphiphilic helix within the c - terminal half is involved in modulating the curvature of the isolation membrane . alternatively , the c - terminal half of atg14 may regulate autophagosome size indirectly through one or more downstream molecules . deletion of atg14 affects the localization of atg8 , the atg12 - atg5 - atg16 complex , and the atg2 - atg18 complex . similarly , the size of the autophagosome correlates with the protein levels of atg8 . thus , it is possible that atg14 regulates autophagosome size indirectly through modulating atg8 recruitment to the pas . as mentioned above , overexpression of barkor / atg14 ( l ) activates autophagy in mammalian cells even under nutrient - rich conditions , indicating that barkor / atg14 ( l ) is one of the key players regulating autophagic activity in mammals . other subunits of ptdins 3 - kinase complexes are also involved in the regulation of autophagic activity . conversely , rubicon negatively regulates maturation of the endosome and the autophagosome by sequestering uvrag from the class c - vps / hops complex . beclin 1 interacts with multiple proteins in addition to the core subunits of the ptdins 3 - kinase complexes . one of these , ambra1 , positively regulates autophagy and plays a crucial role in neural development . beclin 1 also interacts with bcl - 2 , an antiapoptotic protein , that is believed to regulate the balance between autophagy and apoptosis . thus , beclin 1 may serve as a platform upon which cellular signals converge and function to regulate the crosstalk of multiple processes , including autophagy . this function of regulating the balance of multiple cellular events has not been reported for yeast vps30 / atg6 , which implies that beclin 1 has obtained this regulatory role during evolution . in addition to generating ptdins ( 3 ) p by the autophagy - specific ptdins 3 - kinase , dephosphorylation of ptdins ( 3 ) p also plays an important role in regulating autophagy in mammals . overexpression of ptdins ( 3 ) p phosphatases decreases autophagic activity , while the knockdown or the expression of a dominant - negative form of the phosphatases enhances autophagy . taken together , atg14 regulates autophagic activity , at least partially , both in yeast and mammals . however , the barkor / atg14 ( l ) - containing ptdins 3 - kinase complex seems to play a more crucial role in determining autophagic activity than yeast complex i , and the regulation of the barkor / atg14 ( l ) complex may have evolved to function in a more sophisticated manner . a conserved function of atg14 in autophagy is to target the ptdins 3 - kinase complex to the probable site of autophagosome formation . an important problem to be solved is the mechanism whereby atg14 targets the ptdins 3 - kinase complex to the pas in yeast and to the er subdomain in mammals . there are some reports concerning the regulation of ptdins 3 - kinase complex localization . according to a comprehensive analysis ofthe hierarchy of atg protein localization , proper targeting of atg14 is dependent on atg17 and fip200 in yeast and mammals , respectively , both of which are scaffold proteins for atg assembly . the conserved cysteine residues at the n - terminal region of barkor / atg14 ( l ) are required for er localization of barkor / atg14 ( l ) . yeast vps15 , a regulatory subunit of ptdins 3 - kinase complexes , can localize to the pas even in atg14 cells while vps34 and vps30 / atg6 can not , indicating that vps15 also contains information related to targeting to the pas . the function of the n - terminal half of atg14 has been largely , if not completely , determined . on the other hand , the function of the c - terminal half of atg14 is still unclear . the c - terminal half of atg14 is likely involved in forming a normal - sized autophagosome , directly or indirectly . in this sense , it is interesting that the c - terminal bats domain of barkor / atg14 ( l ) binds to the membrane through the hydrophobic surface of the amphiphilic alpha helix . the bats domain favors highly curved membranes that contain ptdins ( 3 ) p , which are considered to be the properties of the isolation membrane . although the amino acid sequence of the bats domain is not conserved in yeast atg14 , a clear amphiphilic alpha helix is predicted within the c - terminal half of yeast atg14 . whether these amphiphilic alpha helixes are involved in the regulation of autophagosome size or not is an interesting issue for future research .
output:
