Many cytoplasmic plant proteins involved in plant resistance to pathogens, generally referred to as “R” proteins, possess both a nucleotide binding site (NBS) and a leucine rich repeat (LRR). R proteins are involved in both a rapid defense response (hypersensitive response) and more long-term nonspecific resistance (systemic acquired resistance). The hypersensitive response involves a form of programmed death localized to the site of infection and changes in gene expression that are thought to prevent further infection. The LRR of the R proteins is believed to recognize and bind to pathogen-derived proteins, triggering the defensive responses and resulting in a rapid and localized host cell death. Many R proteins have an amino terminal effector domain (e.g., a TIR domain or a leucine zipper domain) that is thought to play a role in downstream signaling of events triggered by infection and, possibly, other stresses.
The R proteins are structurally similar to APAF-1, which mediates the activation of caspases, the proteases directly responsible for the degradation of cellular proteins that leads to the morphological changes seen in cells undergoing apoptosis. A domain, designated the NB-ARC domain (“nucleotide-binding adaptor shared by APAF-1, certain R gene products and CED-4”), contains a series of motifs and residues that are conserved among plant resistance proteins (e.g., R proteins) and regulators of cell death (e.g., APAF-1 and CED-4) (van der Bizen and Jones (1999) Current Biology 8:226–228). In addition to the NBS, APAF-1 has a CARD domain, functionally analogous to the effector domain of R proteins, and a WD-40 domain, functionally analogous to the LRR domain of R proteins.
The mechanisms that mediate apoptosis have been intensively studied. These mechanisms involve the activation of endogenous proteases, loss of mitochondrial function, and structural changes such as disruption of the cytoskeleton, cell shrinkage, membrane blebbing, and nuclear condensation due to degradation of DNA.
The various signals that trigger apoptosis are thought to bring about these events by converging on a common cell death pathway, the core components of which are highly conserved from worms, such as C. elegans, to humans. In fact, invertebrate model systems have been invaluable tools in identifying and characterizing the genes that control apoptosis. Despite this conservation of certain core components, apoptotic signaling in mammals is much more complex than in invertebrates. For example, in mammals there are multiple homologues of the core components in the cell death signaling pathway.
Caspases, a class of proteins central to the apoptotic program, are responsible for the degradation of cellular proteins that leads to the morphological changes seen in cells undergoing apoptosis. Caspases (cysteinyl aspartate-specific proteinases) are cysteine proteases having specificity for aspartate at the substrate cleavage site. Generally, caspases are classified as either initiator caspases or effector caspases, both of which are zymogens that are activated by proteolysis that generates an active species. An effector caspase is activated by an initiator caspase which cleaves the effector caspase. Initiator caspases are activated by an autoproteolytic mechanism that is often dependent upon oligomerization directed by association of the caspase with an adapter molecule.
Nuclear factor-κB (NF-κB) is a transcription factor expressed in many cell types and which activates homologous or heterologous genes that have κB sites in their promoters. Molecules that regulate NF-κB activation play a critical role in both apoptosis and inflammation. Quiescent NF-κB resides in the cytoplasm as a heterodimer of proteins referred to as p50 and p65 and is complexed with the regulatory protein IκB. NF-κB binding to IκB causes NF-κB to remain in the cytoplasm. At least two dozen stimuli that activate NF-κB are known (New England Journal of Medicine 336:1066, 1997) and they include cytokines, protein kinase C activators, oxidants, viruses, and immune system stimuli. NF-κB activating stimuli activate specific IκB kinases that phosphorylate IκB leading to its degradation. Once liberated from IκB, NF-κB translocates to the nucleus and activates genes with κB sites in their promoters. The proinflammatory cytokines TNF-α and IL-1 induce NF-κB activation by binding their cell-surface receptors and activating the NF-κB-inducing kinase, NIK, and NF-κB. NIK phosphorylates the IκB kinases α and β which phosphorylate IκB, leading to its degradation.
NF-κB and the NF-κB pathway has been implicated in mediating chronic inflammation in inflammatory diseases such as asthma, ulcerative colitis, rheumatoid arthritis (Epstein, New England Journal of Medicine 336:1066, 1997) and inhibiting NF-κB or NF-κB pathways may be an effective way of treating these diseases. NF-κB and the NF-κB pathway has also been implicated in atherosclerosis (Navab et al., American Journal of Cardiology 76:18C, 1995), especially in mediating fatty streak formation, and inhibiting NF-κB or NF-κB pathways may be an effective therapy for atherosclerosis. Among the genes activated by NF-κB are cIAP-1, cIAP-2, TRAF1, and TRAF2, all of which have been shown to protect cells from TNF-α induced cell death (Wang et al., Science 281:1680–83, 1998). CLAP, a protein which includes a CARD, activates the Apaf-1-caspase-9 pathway and activates NF-κB by acting upstream of NIK and IκB kinase (Srinivasula et al., supra).
CARD-4 is a member of the CED-4/Apaf-1 family that interacts with RICK, a serine threonine kinase, and induces NF-κB via the signaling protein TRAF-6 and NIK (Bertin et al. (1999) J. Biol. Chem. 274:12955). CARD-4 includes domains that are similar to the nucleotide binding site domain (NBS) and leucine rich repeat (LRR) domains found in plant R proteins that mediate resistance to pathogens.