Major vault protein is a substrate of endogenous protein kinases in CHO and PC12 cells

Major vault protein (MVP) is the predominant member of a large cytosolic ribonucleoprotein particle, termed vault. We have previously shown that MVP derived from electric ray electric organ becomes phosphorylated by protein kinase C in vitro and by tyrosine kinase in vivo. Here we show that MVP from two mammalian cell lines (CHO and PC12 cell) becomes highly phosphorylated by endogenous protein kinases in cell-free systems. The susceptibility, to protein kinases differs substantially from those observed in MVP derived from electric organ. Phosphorylation of MVP depends on the presence of Mg2+ and can be inhibited by the chelating agent EDTA. Inhibitors of casein kinase II attenuate the phosphorylation of MVP. In contrast to CHO cells, addition of recombinant casein kinase II enhances the phosphorylation of MVP in PC12 cells. Endogenous kinase activity is of particulate nature and copurifies with vault particles. Immuno-affinity purified vaults containing recombinant tagged MVP expressed in CHO cells reveal no autophosphorylation, suggesting that protein kinase activity is not an intrinsic property of vaults. Our results suggest that cell-specific phosphorylation of MVP may play a critical role in vault function.