Protein phosphorylation in cultured rat RPE. Effects of protein kinase C activation

Incubation of confluent cultures of rat retinal pigment epithelium (RPE) with 32P-orthophosphate resulted in the incorporation of 32P into proteins, RNA and the nucleoside phosphates ADP, GDP, ATP and GTP. RPE cultures incubated with phorbol-12-myristate-13-acetate (PMA), a known activator of protein kinase C, did not significantly change the incorporation of 32P into total protein, RNA or the nucleoside phosphates ADP, GDP, ATP and GTP. However, PMA exposure specifically increased phosphorylation of five proteins with molecular weights of 80 kilodaltons (K), 56K, 35K, 33K, and 29K having isoelectric points between 4.3 and 6.5. PMA treated cultures also showed dephosphorylation of two proteins having molecular weights of about 33K. The observed increase in 80K phosphorylation suggests that protein kinase C is present and activated by PMA in the RPE.