A protein kinase C delta-binding protein SRBC whose expression is induced by serum starvation

West-Western screening of a cDNA expression library using P-32-labeled, autophosphorylated protein kinase C delta (PKC delta) as a probe, led us to identify cDNA clones encoding a PKC delta-binding protein that contains a leucine zipper-like motif in its N-terminal region and two PEST sequences in its C-terminal region. This protein shows overall Sequence similarity (43.3%) to the serum deprivation response (sdr) gene product, and we named it SPEC (sdr-related gene product that binds to (c) under bar-kinase). PKC delta binds to the C-terminal half of SRBC through the regulatory domain and phosphorylates it in vitro. In COS1 cells, the phosphorylation of over-expressed SPEC is stimulated by 12-O-tetradecanoylphorbol-13-acetate and further enhanced by the over-expression of PKC delta. The mRNA for SRBC is detected in a wide variety of Cultured cell lines and tissues and is strongly induced by serum starvation. Furthermore, SRBC mRNA is induced during retinoic acid-induced differentiation of P19 cells. These results suggest that SPEC serves as a substrate and/or receptor for PHC and might be involved in the control of cell growth mediated by PKC.