ed by Sec63, while the translocation channel remains blocked by the plug. A protein substrate destined for post-translational translocation is kept in an unfolded state through association with cytosolic chaperones, that include Hsp70s, and target the polypeptide to the Sec61 complex. Here, substrate proteins are captured by Sec72 that interacts with Hsp70, thereby positioning the substrate for transfer into the translocation channel. Subsequently, the polypeptide inserts as a hairpin into the channel (C) with the signal sequence (SS) exiting through the lateral gate and binds to a groove on the outside of the gate. The following polypeptide is retained in the translocation channel and at this stage chaperones can no longer remain bound because they don’t fit between Sec63 and Sec61. Therefore, all substrates are released from the polypeptide following insertion into the translocation channel. After exit of the signal sequence, the plug domain is probably displaced, which opens the channel towards the lumen. This enables translocation of the polypeptide.
It is well established that the highly conserved Sec complex plays a crucial role in protein export in all organisms (1,2). For example, in bacteria the SecY complex facilitates transport and insertion of proteins into the cytoplasmic membrane, while in eukaryotes the Sec61 channel performs the same functions in the endoplasmic membrane (2). Protein transport can occur during or after translation via the so-called post-translational or co-translational pathway. Both pathways are utilized by all organisms but their importance seems species-specific. In mammals the vast majority of proteins is directed towards the endoplasmic membrane co-translationally, while in yeast and bacteria both pathways are equally important. In eukaryotes, co-translational translocation is initiated by binding of the SRP to a hydrophobic targeting signal of the substrate protein as it emerges from the ribosome. This temporarily halts translation and the SRP-ribosome-nascent chain complex is subsequently delivered at the Sec61 channel embedded into the endoplasmic membrane. After transfer of the nascent chain to the Sec complex, translation continues and the polypeptide is pushed through the translocation channel (1). Secretory proteins with a moderately hydrophobic signal sequence escape recognition by the SRP system and are therefore funneled into the post-translational pathway. These proteins are kept in an unfolded state after release from the ribosome through association with cytosolic chaperones, which also target the polypeptide to the Sec channel. Following capture of the substrate protein by the Sec61 channel, the polypeptide is bound by the luminal Hsp70 chaperone BiP which prevents it’s backsliding into and in fact “pulls” the substrate into the lumen (2). Although both pathways converge at the Sec61 channel, both translocation modes require a structurally different Sec61 complex. In yeast, co-translational translocation is performed by the Sec61 complex made up of Sec61, Sss1 and Sbh1, while, during post-translational translocation the Sec61 channel associates with another membrane protein complex, known as the Sec62/Sec63 complex (2). This represents a tetrameric complex comprising Sec62, Sec63, Sec71 and Sec72. Of these, Sec62 and 63 are essential for cell viability unlike Sec71 and 72. These proteins are not found in mammals, while in humans, Sec62 and 63 are frequently mutated or over expressed in different tumors. The molecular details of the co-translational pathway are reasonably well understood, in contrast to post-translational translocation. This is primarily caused by a lack of structural information and with regards to this the recently determined atomic structures of the post-translational Sec complex from yeast (5,6) are of great importance because they provide mechanistic insight into post-translational protein export.
1. Wickner W, Schekman R. 2005. Protein translocation across biological membranes. Science. 310: 1452-1456.
2. Rapoport TA. 2007. Protein translocation ac