P75, A MEMBER OF THE HEAT-SHOCK PROTEIN FAMILY, UNDERGOES TYROSINE PHOSPHORYLATION IN RESPONSE TO OXIDATIVE STRESS

The combination of H2O2 and vanadate generates aqueous peroxovanadium (pV) species, which are effective cell-permeable oxidants, and potent inhibitors of protein-tyrosine phosphatases. As a result, treatment of intact cells with pV compounds significantly enhances protein Tyr pho sphorylation. Here we demonstrate that treatment of intact rat hepatom a Fao cells with pV markedly enhances Tyr phosphorylation of a 75-kDa protein, termed pp75. Amino-terminal sequencing of pp75 revealed that this protein is a member of the 70-75-kDa heat shock protein family, w hich includes PBP-74, glucose-related protein (GRP)-75, and mortalin. Tyr phosphorylation of pp75 is selective, because other proteins that belong to the heat shock protein 70 family, such as GRP-72, Bip (GRP-7 8), and HSC-70 fail to undergo Tyr phosphorylation when cells are trea ted with pV. Our findings suggest that heat shock proteins such as pp7 5 may undergo tyrosine phosphorylation when intact cells are subjected to oxidative stress induced by pV compounds.