Signalling Pathways Regulating the Dephosphorylation of Ser729 in the Hydrophobic Domain of Protein Kinase Cε upon Cell Passage

Abstract

We have recently demonstrated that in quiescent fibroblasts protein kinase C (PKC) ε95 is phosphorylated at Ser729, Ser 703, and Thr566 and that upon passage of quiescent cells phosphorylation at Ser729 is lost, giving rise to PKCε 87. Ser729 may be rephosphorylated later, suggesting cycling between PKCε87 and PKCε95. Here we show that the dephosphorylation at Ser729 is insensitive to okadaic acid, calyculin, ascomycin C, and cyclosporin A, suggesting that dephosphorylation at this site is not mediated through protein phosphatases 1, 2A or 2B. We demonstrate that this dephosphorylation at Ser729 requires serum and cell readhesion and is sensitive to rapamycin, PD98059, chelerythrine, and Ro-31-8220. These results suggest that the phosphorylation status of Ser 729 in the hydrophobic domain at Ser729 is regulated independently of the phosphorylation status of other sites in PKCE, by a mTOR-sensitive phosphatase. The mitogen-activated protein kinase pathway and PKC are also implicated in regulating the dephosphorylation at Ser 729.