Modulation of protein phosphorylation by Mr 25,000 protein partially overlapping phosvitin and lipovitellin 2 in Xenopus laevis vitellogenin B1 protein.

A phosphorylated protein with molecular mass of 25,000 (pp25) is a component of Xenopus laevis vitellogenin B1. In an attempt to elucidate the physiological role of pp25, its effect on protein phosphorylation was studied. In vitro phosphorylation of some endogenous proteins from the cytoplasm and membrane fraction of Xenopus oocytes by casein kinase II and protein kinase C (PKC) was inhibited by increasing the concentration of pp25. By Western blot analysis using an antibody against phospho-(Ser/Thr) PKC substrate, phosphorylation of some endogenous proteins, especially in the cytoplasm, of Xenopus embryos was seen to increase when pp25 disappeared during developmental stages 35-45. These results suggest that pp25 may have a role as an inhibitory modulator of some protein phosphorylation in Xenopus oocytes and embryos.