Evidence that (a) serine specific protein kinase(s) different from protein kinase C is responsible for the insulin-stimulated actin phosphorylation by placental membrane.

Partially purified phospholipid- and Ca2+-dependent protein kinase C from human placenta catalyzes the Mg-ATP-dependent phosphorylation of serine residues of purified rabbit muscle actin. Two tryptic [32P]-phosphopeptides were found on HPLC separation. Confirming the previous report by Machicao and Wieland [(1985) Curr. Top. Cell. Regul. 27, 95-105], actin is phosphorylated at serine residues by human placental membranes, and this is stimulated by insulin. In the absence of insulin trypsin treatment yielded eight [32P]phosphopeptides, two of which coincided with the ones due to protein kinase C. Insulin led to the appearance of three new [32P]phosphopeptides. These results suggest that insulin stimulates (a) serine protein kinase(s) which, like protein kinase C, is present in placental membranes.