Role of the C-terminus of Protein kinase C-related kinase in cell signalling

Abstract

Protein Kinase C-related kinases (PRK) are members of the protein kinase C (PKC) superfamily of serine/threonine protein kinases. The structure of members of the PKC superfamily is highly conserved, with an N-terminal regulator domain linked to a C-terminal catalytic domain via a hinge segment. The catalytic core of all PKC superfamily members has 40-50% sequence identity. At the end of the catalytic domain, there is a C-terminal tail consisting of approximately 70 amino acid residues called the variable region 5 (V5) domain. This V5 domain is present in all PKC/PRKs, ranging from yeast to humans, but with a much lower sequence homology compared with that of the catalytic core. This project aims to test the hypothesis that the V5 domain is a functional determinant in isozyme-specific regulation in PRK1 and PRK2. The V5 region appears attractive for specific regulation due to its great variability in terms of amino acid length and sequence among the isozymes in the PKC family. Both PRK1 and PRK2 tolerated removal of up to seven amino acids from the C-terminal (PRK1-I?940 and PRK2-I?978), where the hydrophobic motif was deleted, thus it appears that the hydrophobic motif is dispensable for PRK1 and PRK2 activity. However, PRK1 and PRK2 lacking the eight amino acids or more were catalytically inactive. Co-immunoprecipitation studies indicate that the absence of the hydrophobic motif in PRK1 and PRK2 did not affect PDK-1 binding. Thus, the dogma that the hydrophobic motif functions as a substrate docking site for PDK-1 seems to be an over-simplified generalization. The results presented here on the relationship between PDK-1 binding with PRK1 or PRK2 and kinase activities suggest that the interaction of PDK-1 with PRK1 and PRK2, and the productive phosphorylations of the activation loop are separate biochemical events. Further biochemical and enzymological studies revealed that this very C-terminal segment of PRK1 and PRK2 is necessary for optimal activation. We propose that the importance of the very C-terminus in conferring the catalytic competence in PRK1 and PRK2 represents a common feature among several other PKC isozymes. This study has discovered that the extreme C-terminal of seven amino acids in PRK1 and PRK2 are critical for the function and regulation of the kinases. The data suggest that this segment in the V5 domain in necessary to maintain critical interaction with the kinase domain to allow proper folding for catalysis.