Endoplasmic reticulum stress-inducible protein GRP94 is associated with an Mg2+-dependent serine kinase activity modulated by Ca2+ and GRP78/BiP

The 94-kDa glucose-regulated protein (GRP94) is a glycoprotein in the endoplasmic reticulum (ER). It has been characterized as a Ca²⁺-binding protein and a molecular chaperone. In this report we show that highly purified GRP94 exhibits an active Mg²⁺-dependent serine kinase activity (termed 94-kinase). The 94-kinase can be recovered from ER membrane fractions and is able to phosphorylate both the constitutive and stress-induced forms of GRP94, correlating with their induction kinetics. The 94-kinase activity is distinct from casein kinase II. In contrast to the heat-stable, Ca²⁺-dependent autophosphorylation activity recently reported for GRP94, the labile 94-kinase activity is inhibited by Ca²⁺. We determined that the phosphopeptide map of in vitro phosphorylated GRP94 by the 94-kinase resembles that of the in vivo phosphorylated GRP94. Further, the 94-kinase activity can be specifically stimulated by GRP78, a coregulated protein in the ER known to interact with GRP94. J. Cell. Physiol. 170:115–129, 1997. © 1997 Wiley-Liss, Inc.