Atypical PKCs A World Apart

Much of what has been discussed to this point applies only to the DAG-responsive PKC isoforms, which include the conventional (a, pi, piI, y) and novel (5, e, 8, n) subgroups. Two atypical PKC isoforms (X and Z) live in a world of their own, including a unique set of scaffolds and a distinct relationship with lipids (Suzuki et al. 2003; Moscat and Diaz-Meco 2000; Hirai and Chida 2003). The atypical PKCs are classified within the PKC family on the basis of modest sequence homol-ogy in the kinase catalytic domain with conventional and novel PKC isoforms (Suzuki et al. 2003). Atypical PKCs show very low sequence homology with other PKCs in their regulatory domains except for a single cysteine-rich C1 motif that appears to bind PIP3 rather than DAG. A PB1 domain unique to atypical PKCs represents a focal point in dictating isotype-specific protein-protein interactions (Hirai and Chida 2003).

Atypical PKC (aPKC) has been shown to play a role in signaling downstream of insulin receptors (Liu et al. 2007), which nicely highlights the involvement of lipids in regulating this subclass of PKCs. A common downstream effector recruited to active tyrosine phosphorylated insulin receptors is PI3-kinase. This enzyme converts the membrane-resident phospholipid PIP2 to PIP3. In turn, PIP3 can then bind and activate a number of kinases, including aPKCZ, PDK1 and the abundant and widely studied growth regulating kinase Akt/PKB (Moscat and Diaz-Meco 2000; Hirai and Chida 2003). Of these, PDK1 has the highest affinity for PIP3. However, aPKCZ becomes activated as a result of coincident PIP3 binding to its C1 domain and phosphorylation of its activation loop residue T410 by PDK1 (Liu et al. 2007). This activation of aPKCZ is critical for insulin-dependent recruitment of the GLUT4 glucose transporter to the cell surface, which promotes glucose uptake in insulin target tissues (Liu et al. 2007).

The multitude of proteins involved in insulin receptor signaling is vast, and the nature of protein assemblies involving aPKCs may be easier to illustrate for other growth-signaling pathways. For example, the TNF-a (tumor necrosis factor-a) receptor signals through a complex made up of the proteins TRAF2, RIP, ZIP/p62 and aPKCZ (Moscat and Diaz-Meco 2000). Through this complex, TNF-a stimulates aPKCZ-mediated phosphorylation of the NF-kB regulatory complex, thereby initiating gene expression patterns involved in inflammatory responses. Analogous signaling complexes featuring aPKCZ may explain intracellular signaling resulting from binding of IL-1 (interleukin-1) and NGF (nerve growth factor) to their respective cell surface receptors (Moscat and Diaz-Meco 2000). These functionally relevant protein-protein interactions involving aPKCZ are mediated through its PB1 subdomain at the N-terminus of the regulatory domain.

A major role for atypical PKCs in embryonic development was revealed by genetic and RNAi experiments in C. elegans, and subsequently recapitulated in mouse knockout models. In C. elegans, at least three mutants were found to be defective in anterior-posterior polarization of embryos during early development: PKC-3, PAR-3 and PAR-6. PKC-3 is the C. elegans homolog of aPKC. PAR-3 is a scaffolding protein with three PDZ domains that binds PKC-3 in its catalytic domain. The mammalian homolog of PAR-3, known as ASIP, binds (through a PDZ domain) proteins such as JAM that are critical in the formation of tight j unctions in polarized epithelial cells (Suzuki et al. 2003). Interestingly, JAM is also a direct molecular partner of PICK1, suggesting that conventional and atypical PKC isoforms may converge during the formation of cell-cell contacts such as in epithelial cell tight junctions (Reymond et al. 2005). PAR-6 also appears to be a scaffold protein, but it binds aPKC through a specific sequence motif in the N-terminal PB1 domain, such that PAR-6 and PAR-3 can bind to aPKC simultaneously. PAR-6 contains interaction sites for the small G-proteins cdc42 and rac1, which regulate actin cytoskeletal dynamics and may link the cell polarity machinery to cellular transformation by ras, or to modulation by E-cadherin-medi-ated cell-cell contacts (Moscat and Diaz-Meco 2000; Hirai and Chida 2003). In mouse models, germ-line-targeted knockout of aPKC^ is embryonically lethal (Suzuki et al. 2003), suggesting essential functions in early development, whereas aPKCZ knockouts survive, but display extensive defects in cell-cell junctions and cell polarity (Suzuki et al. 2003).

This brief overview of aPKCs barely scratches the surface of a rapidly developing literature on aPKC signaling in mammalian tissues. The emphasis here has been on illustrating the involvement of the critical membrane lipids PIP2 and PIP3, as well as on providing examples of scaffolding proteins that organize large signaling complexes in which aPKCs function to regulate cell shape, cell polarity, cell survival and many other cellular processes. It is also well established that proteinprotein interactions involving aPKCs directly impact other critical downstream signaling pathways, including MAP kinase cascades and src (Moscat and Diaz-Meco 2000; Hirai and Chida 2003).

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