A serpin with M-r 43,000 is a binding protein of M-r 25,000 protein, a substrate for protein Ser/Thr kinase detected in Xenopus laevis oocytes

In an attempt to get some clue as to the function of M-r 25,000 protein, a protein Ser/Thr kinase substrate detected in Xenopus Laevis oocytes [Hashim oto, E. et al. (1995) J. Biochem. 118, 453-460], the binding protein was su rveyed using the P-32-labeled protein by casein kinase II as a screening pr obe. When the cytosolic proteins from oocytes were transferred to a polyvin ylidene fluoride membrane and incubated with the labeled protein, only one protein with M-r 43,000 was visualized on autoradiography. This protein was purified to a nearly homogeneous state through several column chromatograp hy steps. The amino acid sequence of the amino-terminal region of this prot ein identified it as a kind of serine protease inhibitor (serpin) [Holland, L.J. et al. (1992) J. BioL. Chem. 267, 7053-7059]. However, the M-r 25,000 protein did not have any effect on the inhibitory action of this serpin on alpha -chymotrypsin. In addition, several binding proteins were also detec ted in the particulate fraction of oocytes, although the exact identity of these proteins is not clear at this time. These results suggest that the M- r 25,000 protein may play some role(s) by interacting with these binding pr oteins in Xenopus oocytes.