| Cell polarity refers to asymmetric organization of cell structure and differential distribution of cellular materials. It is implicated in a variety of processes including early patterning of organisms, development of organs, stem cell self-renew, junction formation in epithelial cells, cell migration and axonal specification in neurons. In multicellular organisms, the Par complex which is composed of scaffold proteins PAR3 and PAR6 (PARtitioning-defective 3 and 6) and the serine/threonine protein kinase aPKC (atypical Protein Kinase C) is involved in all known cell polarity events. This protein complex is thought to receive upstream polarization signals, amplify and relay them downstream to induce cellular responses including cytoskeleton rearrangements and protein localization changes.;aPKC is the only enzyme in the Par complex, thus it is a good candidate to amplify polarity signals and to transduce them to downstream. However, neither the substrate specificity nor the regulation mechanism is clear for the enzyme. The catalytic domain of aPKC has been reported to bind to and phosphorylate the aPKC binding domain of PAR3. To study these questions, biochemical studies were carried out to map the minimal aPKC binding region of PAR3 and to identify the phosphorylation site(s) of the kinase. The complex structure of aPKC catalytic domain-PAR3 peptide was solved by X-ray crystallography. The structure, together with biochemistry studies, reveals a set of substrate recognition sites within the aPKC catalytic domain, and allows the consensus substrate sequence of aPKC to be discerned. An unexpected finding from the structure of the aPKC catalytic domain-PAR3 peptide complex was that the kinase domain assumes an active conformation without the activation loop phosphorylation. This finding suggests that the activity regulation of aPKC originates from a region(s) outside the kinase's catalytic domain. I further found that the pseudosubstrate sequence of aPKC can directly regulate the kinase activity by inhibiting substrate from binding. |
