CELL-CYCLE-REGULATED EXPRESSION AND SERINE PHOSPHORYLATION OF THE MYRISTYLATED PROTEIN-KINASE-C SUBSTRATE, SSECKS - CORRELATION WITH CULTURE CONFLUENCY, CELL-CYCLE PHASE AND SERUM RESPONSE

We recently identified a novel myristylated protein kinase C (PKC) sub strate, named SSeCKS (pronounced essex), whose transcription is suppre ssed >15 fold in src-or ras-transformed rodent fibroblasts, but not in raf-transformed cells [1, 2]. SSeCKS associates with and controls the elaboration of a cortical cytoskeletal matrix in response to phorbol esters [2], and overexpression of SSeCKS causes growth arrest of untra nsformed NIH3T3 cells [3]. Our preliminary data suggested that SSeCKS functions as a negative mitogenic regulator by controlling cytoskeleta l architecture and that serine phosphorylation of SSeCKS by kinases su ch as PKC alters its interaction with cytoskeletal matrices and its ab ility to control mitogenesis. Here, we determine the effects of cultur e confluency, growth arrest and serum response on the steady-state abu ndance of SSeCKS RNA and protein and on the relative level of phosphos erine-free SSeCKS. SSeCKS transcription is initially induced by serum factors and by contact-inhibited growth rather than by cell-cycle arre st induced by serum starvation, hydroxyurea or nocodazole, and followi ng serum-induced G(1)/S progression, SSeCKS transcription is suppresse d. SSeCKS protein is hyperphosphorylated on serine residues during G(1 )/S progression but not during the G(2)/M phase. Finally, we show that the induction of SSeCKS protein expression by contact inhibition is i ndependent of SSeCKS;serum responsiveness. These data suggest that SSe CKS expression and function can be controlled at either the transcript ional or post-translational level in response to serum factors and cul ture confluency. The data strengthen the notion that SSeCKS plays an i mportant, yet transient, role in cell cycle progression from G(0) to G (1) that differs from its role in controlling contact-inhibited growth .