Unique cytoplasmic phosphoproteins are associated with cell growth arrest

Abstract

THE intracellular molecular processes responsible for the regulation of animal cell growth are not understood. Cyclic AMP has been implicated in the arrest of cell growth (see ref. 1 for review) and it has been postulated that all cyclic AMP effects are mediated through protein kinase². Insel et al.³ have demonstrated with mutants that a cyclic AMP-dependent protein kinase mediates the growth inhibitory effect of cyclic AMP in S49 cells. Maller and Krebs⁴ have established that the catalytic subunit of cyclic AMP-dependent protein kinase was both necessary and sufficient to block progesterone induced meiotic cell division in Xenopus oocytes. The above observations suggest that a high steady-state level of a phosphoprotein which is subject to control by cyclic AMP-dependent protein kinase functions in a regulatory manner to arrest cell growth. Since cyclic AMP-dependent protein kinases are found predominantly in the cytoplasm, we investigated the phosphorylation of cytoplasmic phosphoproteins and report here that quiescent cell cytosol contains several unique phosphoproteins.