Acute stimulation of corpus luteum cells by gonadotrophin or adenosine 3',5'-monophosphate causes accumulation of a phosphoprotein concurrent with acceleration of steroid synthesis.

A protein (ib), which we have detected previously in peptide hormone or cAMP-stimulated corpus luteum cells, is shown to be a phosphoprotein and to be posttranslationally converted into a more acidic phosphoprotein (ia). Phosphorylation is demonstrated by two types of experiments, both using two-dimensional gel electrophoresis. In the first type, gels from [35S]methionine-labeled solubilized cell extracts are compared to gels from such extracts treated with alkaline phosphatase. This in vitro phosphatase treatment converts protein ib quantitatively into protein pb, which is synthesized in vivo only in unstimulated cells. Similarly, ia is converted into pa, the posttranslational product of pb. The second type of experiment demonstrates 32P label incorporation into proteins with the same electrophoretic mobilities as proteins ib and ia. Limited proteolytic digestion of all four proteins from phosphatase-treated and untreated corpus luteum cells shows that the newly detected acidic products, ia and pa, give rise to cleavage patterns similar to those of ib and pb. Further, these patterns resemble those produced by all four such proteins from the adrenal. These findings suggest that in both stimulated corpus luteum and adrenal, a similar protein ib, which accumulates with kinetics and stimulant dose response paralleling those of steroid hormone biosynthesis, is phosphorylated during its synthesis and is degraded by conversion to another phosphoprotein (ia).