Calcium-dependent autophosphorylation of the glucose-regulated protein, Grp78

The characteristics of phosphorylation of the 78-kDa glucose-regulated protein (Grp78), also known as the immunoglobulin heavy chain binding protein, were studied in vitro and in vivo. The purified protein from either calf liver or bovine kidney cells (MDBK) could be phosphorylated in vitro with [γ-³²P]ATP, in a reaction that is stimulated by Ca²⁺ and inhibited by the Ca²⁺-chelator ethylene glycol bis(β-aminoethyl ether)N,N′-tetraacetic acid (EGTA). In the presence of EGTA, excess Ca²⁺ increased the rate of phosphorylation about 18-fold. Based on EGTA Ca²⁺ titrations, the optimal Ca²⁺ concentration for phosphorylation was estimated to be 10-50 mm. Other divalent cations such as Mg²⁺, Mn²⁺, and Zn²⁺ were found to be inhibitory as was the Ca²⁺ antagonist lanthanum (La³⁺). The in vivo phosphorylation of Grp78 was studied in MDBK cells labeled with ³²P_i. In the presence of inducers of Grp78 synthesis, such as ionomycin, tunicamycin, or 2-deoxyglucose, there was a large increase in the level of Grp78 in the cells but a decrease in the amount of phosphorylated protein. Two-dimensional gel analysis of Grp78 purified from bovine liver and MDBK cells identified at least four isoforms. After in vivo and in vitro phosphorylation of Grp78 all the acidic isoforms contained radioactivity but not the most basic isoform. Phosphoamino acid analysis of Grp78 showed that serine and threonine were phosphorylated in vivo and only threonine was phosphorylated in vitro.