Properties of the protein kinase that phosphorylates prothymosin alpha

The prothymosin a kinase (ProT alpha K) is an apparently novel enzyme that is responsible for the phosphorylation of prothymosin alpha (ProT alpha), i nvolved in the proliferation of mammalian cells. The present study investig ated the properties of this enzyme. ProT alpha K is more effectively activa ted by Mn2+ than by other divalent cations, and its activity is unaffected by RNA. Its principal substrate in proliferating cells appears to be ProT a lpha. Both in vivo and in vitro, it is unable to phosphorylate the peptides thymosin alpha(1) and thymosin alpha(11), derived from the amino terminus of ProT alpha, despite the fact that the sites of phosphorylation of ProT a lpha are contained within this part of its sequence. In trials in vivo, inh ibition of gene expression abolished both phosphorylation of ProT alpha and ProT alpha K activity. ProT alpha K is located in the cytosolic fractions throughout the cell cycle. Its activity, which is dependent on cell prolife ration, increases markedly during S phase and begins to decline as the cell enters G2. Studies of the effects of activators and inhibitors of protein kinases involved in signal transduction pathways suggest that ProT alpha K is activated by phosphorylation in a mitogen-initiated pathway that is depe ndent on PKC; however, PKC does not itself phosphorylate ProT alpha K, whic h is therefore presumably phosphorylated by another kinase.