Tyrosine phosphorylation of VCP, the mammalian homologue of the Saccharomyces cerevisiae CDC48 protein, is unusually sensitive to stimulation by sodium vanadate and hydrogen peroxide.

A mAb produced by immunization of mice with tyrosine-phosphorylated proteins from activated B lymphocytes was found to recognize valosin-containing protein (VCP). VCP is the mammalian homologue of the Saccharomyces cerevisiae CDC48 protein and has localized regions of sequence identity with the yeast Sec18 and Pas1 proteins and the mammalian NSF protein, all of which are important in intracellular vesicular traffic or formation. VCP was found to be constitutively phosphorylated on tyrosine in Rous sarcoma virus-transformed fibroblasts. Phosphorylation of VCP on tyrosine was stimulated only modestly during activation of B lymphocytes by ligation of membrane Ig. In contrast, treatment of B cells with either H2O2 or a combination of H2O2 and Na3VO4 greatly increased tyrosine phosphorylation of VCP. These results may suggest that under normal conditions tyrosine phosphorylation of VCP has a rapid turnover and that it can be detected easily only when dephosphorylation is inhibited by artificial means.