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+-bindin g protein and a molecular chaperone. In this report we show that highl y purified GRP94 exhibits an active Mg2+-dependent serine kinase activ ity (termed 94-kinase). The 94-kinase can be recovered from ER membran e fractions and is able to phosphorylate both the constitutive and str ess-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 recen tly reported for GRP94, the labile 94-kinase activity is inhibited by Ca2+ We determined that the phosphopeptide map of in vitro phosphoryla ted GRP94 by the 94-kinase resembles that of the in vivo phosphorylate d GRP94. Further, the 94-kinase activity can be specifically stimulate d by GRP78, a coregulated protein in the ER known to interact with GRP 94. (C) 1997 Wiley-iiss, inc.