Purification and phosphorylation of a M(r) 25,000 protein, an effective phosphate acceptor for casein kinase II and protein kinase C, detected in the cytosolic fraction of Xenopus laevis oocytes.

A common and effective phosphate acceptor protein for casein kinase II and Ca(2+)-phospholipid-dependent protein kinase (protein kinase C) was purified to near homogeneity from the cytosolic fraction of Xenopus laevis oocytes. Its molecular mass was estimated to be approximately 25,000 by SDS-polyacrylamide slab gel electrophoresis and gel filtration analyses. About 1 and 2 mol of phosphate were incorporated per mol of this protein with casein kinase II and protein kinase C, respectively, and the phosphorylated amino acid was identified as serine irrespective of the protein kinase employed. The Km values were calculated to be 1 and 0.5 microM for this M(r) 25,000 protein with casein kinase II and protein kinase C, respectively. However, this protein was a relatively poor substrate for casein kinase I and did not serve as one for cAMP-dependent protein kinase. The amino acid sequence of its amino-terminal region suggests that this protein is a newly identified substrate for these two protein serine/threonine kinases.