On the role of cyclic AMP-independent protein kinases in the modification of yeast ribosomal proteins in vivo.

Two proteins of yeast 40S ribosome subunit and four proteins of the 60S ribosome subunit were labelled in vivo with [32P]orthophosphate. Five of these proteins were phosphorylated by protein kinase 3, an enzyme which is cyclic AMP-independent and uses ATP and GTP as phosphoryl donors. Two proteins, belonging to the 60S ribosome subunit were phosphorylated by another, highly specific, cyclic AMP-independent protein kinase 1 B. Both in vivo and in vitro the most extensively phosphorylated protein species were acidic proteins, L44, L45 (according to the nomenclature of Kruiswijk & Planta, Molec. Biol. Rep., 1, 409-415, 1974) possibly corresponding to bacterial L7 and L12 proteins. The 40S ribosomal protein, S9, analogous to mammalian S6 protein, was phosphorylated in vivo but was not phosphorylated in vitro by either of the cyclic AMP-independent protein kinases. The obtained results clearly indicate that cyclic AMP-independent yeast protein kinases might be involved in the modification in vivo of some ribosomal proteins, in particular of the strongly acidic proteins of 60S ribosome subunit.