Regulated interaction of protein kinase Cδ with the heterogeneous nuclear ribonucleoprotein K protein

Abstract

The heterogeneous nuclear ribonucleoprotein (hnRNP) K protein recruits a diversity of molecular partners that are involved in signal transduction, transcription, RNA processing, and translation. K protein is phosphorylated in vivo and in vitro by inducible kinase(s) and contains several potential sites for protein kinase C (PKC) phosphorylation. In this study we show that K protein is phosphorylated in vitro by PKCδ and by other PKCs. Deletion analysis and site-directed mutagenesis revealed that Ser302 is a major K protein site phosphorylated by PKCδ in vitro. This residue is located in the middle of a short amino acid fragment that divides the two clusters of SH3- binding domains. Mutation of Ser302 decreased the level of phosphorylation of exogenously expressed K protein in phorbol 12-myristate 13-acetate- treated COS cells, suggesting that Ser302 is also a site for PKC-mediated phosphorylation in vivo. In vitro, PKCδ binds K protein via the highly interactive KI domain, an interaction that is blocked by poly(C) RNA. Mutation of Ser302 did not alter the K protein-PKCδ interaction in vitro, suggesting that phosphorylation of this residue alone is not sufficient to alter this interaction. Instead, binding of PKCδ to K protein in vitro and in vivo was greatly increased by K protein phosphorylation on tyrosine residues. The ability of PKCδ to bind and phosphorylate K protein may serve not only to alter the activity of K protein itself, but K protein may also bridge PKCδ to other K protein molecular partners and thus facilitate molecular cross-talk. The regulated nature of the PKCδ-K protein interaction may serve to meet cellular needs at sites of active transcription, RNA processing and translation in response to changing extracellular environment.