Structural Properties and Mechanisms That Govern Association of C Kinase Adapter 1 with Protein Kinase C3 and the Cell Periphery

Abstract

Association of an atypical protein kinase C (aPKC) with an adapter protein can affect the location, activity, substrate specificity, and physiological role of the phosphotransferase. Knowledge Atypical protein kinase C (aPKC) 1 isoforms, which include mammalian PKCs and and Caenorhabditis elegans PKC3 (1-3), are involved in transmitting mitogenic, inflammatory, and anti-apoptotic signals; regulating gene expression; controlling vesicular trafficking and ion channel activities; establishing cell polarity; and mediating asymmetric cell divisions (4 -12). To exert control over such diverse aspects of cell physiology, aPKCs regulate effector proteins in cytoplasm, cytoskeleton, nucleus, and at the surfaces of plasma and internal membranes (4 -14). aPKCs lack structural features that mediate direct association with cytoskeleton or organelles. Thus, elucidation of alternative mechanisms that enable aPKCs to encounter and control effector proteins in discrete microenvironments is an important goal. Recent investigations (reviewed in our companion paper (45)) suggest that aPKC functions are diversified and specialized via interactions with adapter proteins (6, 14 -20). Candidate adapter proteins possess two fundamental features: a tethering domain that ligates an aPKC, and a distinct targeting region that routes the adapter⅐aPKC complex to intracellular sites enriched in substrate-effector proteins and/or regulatory molecules that modulate phosphotransferase activity. The aPKC "recruitment" model further suggests that systematic characterization of aPKC adapter proteins will ultimately yield novel insights into regulatory properties, substrate specificities, and precise physiological roles of atypical PKCs.The nematode C. elegans is an attractive model system for studies on aPKC adapter proteins. C. elegans physiology is regulated by signaling molecules, mechanisms and pathways that are operative in mammals (21, 22). Only one aPKC (PKC3) is encoded by the C. elegans genome (3). PKC3 is expressed and anchored at all developmental stages (3). This Ca 2ϩ -and diacylglycerol (DAG)-independent kinase is essential for the progression of embryogenesis, asymmetry in early cell divisions, and overall viability of the organism (3, 11). In 1-cell embryos, ϳ25% of PKC3 associates with Par-3, a multi-PDZ domain protein that is crucial for generating intracellular polarity (11,12). Mechanisms governing formation of PKC3⅐Par-3 complexes, the identity of targets for bound PKC3, and the precise biochemical/physiological function of the complex remain to be defined. Association of Ͼ90% of PKC3 with cytoskeleton/membranes in embryos indicates that additional adapter proteins are expressed during early phases of C. elegans development (3). In post-embryonic C. elegans, PKC3 accumulates in a highly asymmetric fashion in intestinal, pharyngeal, and other cells (3). Polarized enrichment of PKC3 in nondividing cells that will not undergo apoptosis suggests that anchored PKC3 plays distinct roles in terminally differentiated cells. By using k...

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