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

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 PKC delta and by other PKCs, Deletion analysis and site-directed mutagenesis revealed that Ser(302) is a major K protein site phosphorylated by PKC delta 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 Ser(302) decreased the level of phosphorylation of exogenously expressed K protein in phorbol la-myristate 13-acetate-treated COS cells, suggesting that Ser(302) is also a site for PKC-mediated phosphorylation in vivo. In vitro, PKC delta binds K protein via the highly interactive KI domain, an interaction that is blocked by poly(C) RNA. Mutation of Ser(302) did not alter the K protein-PKG delta interaction in vitro, suggesting that phosphorylation of this residue alone is not sufficient to alter this interaction. Instead, binding of PKC delta to K protein in vitro and in vivo was greatly increased by K protein phosphorylation on tyrosine residues. The ability of PKC delta to bind and phosphorylate K protein may serve not only to alter the activity of K protein itself, but K protein may also bridge PKC delta to other K protein molecular partners and thus facilitate molecular cross-talk. The regulated nature of the PKC delta-K protein interaction may serve to meet cellular needs at sites of active transcription, RNA processing and translation in response to changing extracellular environment.