Actin nascent chains are substrates for cyclic AMP-dependent phosphorylation in vivo.

Two-dimensional gel electrophoresis of extracts of S49 mouse lymphoma cells labeled with [35S]methionine in the presence of inducers or analogs of cyclic AMP reveals a protein that both affinity purification and peptide mapping show to be a form of nonmuscle actin. This actin species also exhibits cyclic AMP-dependent labeling with [32P]phosphate, and, after acid hydrolysis, 32P label is found associated with phosphoserine. Phosphorylated actin does not appear when cells prelabeled with [35S]methionine are treated with an inducer of cyclic AMP in the presence of emetine, an inhibitor of protein synthesis; this suggests that only the nascent form of actin is a substrate for cyclic AMP-dependent phosphorylation. As well as differing slightly in isoelectric points, beta and gamma actins are found to yield different partial proteolytic cleavage products with staphylococcal protease. This microheterogeneity in the major cellular actin component is repeated in both the metabolically labile delta/epsilon actin and phosphorylated actin, suggesting that these three forms of actin derive from the same two gene products.