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

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 of mechanisms that govern formation and intracellular targeting of aPKC·adapter protein complexes is limited. Caenorhabditis elegans protein kinase C adapter proteins (CKA1 and CKAIS) bind and target aPKCs and provide prototypes for mechanistic analysis. CKA1 binds an aPKC (PKC3) via a phosphotyrosine binding (PTB) domain. A distinct, Arg/ Lys-rich N-terminal region targets CKA1 to the cell periphery. We discovered that a short segment (²¹²GGIDNGAFHEHEI²²⁴) of the V ₂ (linker) region of PKC3 creates a binding surface that interacts with the PTB domain of CKA1/CKA1S. The docking domain of PKC3 differs from classical PTB ligands by the absence of Tyr and Pro. Substitution of Ile ²¹⁴, Asn²¹⁶, or Phe²¹⁹ with Ala abrogates binding of PKC3 with CKA1; these residues cooperatively configure a docking site that complements an apolar surface of the CKA1 PTB domain. Phosphorylation site domains (PSD1, residues 11-25; PSD2, residues 61-77) in CKA1 route the adapter (and tethered PKC3) to the cell periphery. Phosphorylation of Ser ¹⁷ and Ser⁶⁵ in PSDs 1 and 2 elicits translocation of CKA1 from the cell surface to cytoplasm. Activities of DAG-stimulated PKCs and opposing protein Ser/Thr phosphatases can dynamically regulate the distribution of adapter protein between the cell periphery and cytoplasm.