p185erbB2 binds to GRP94 in vivo. Dissociation of the p185erbB2/GRP94 heterocomplex by benzoquinone ansamycins precedes depletion of p185erbB2.

Treatment of SKBr3 cells with benzoquinone ansamycins, such as geldanamycin (GA), depletes p185erbB2, the receptor tyrosine kinase encoded by the erbB2 gene. In the same cells, a biologically active benzoquinone photoaffinity label specifically binds a protein of about 100 kDa, and the ability of various GA derivatives to reduce the intracellular level of p185erbB2 correlates with their ability to compete with the photoaffinity label for binding to this protein. In this report, we present evidence that the approximately 100-kDa ansamycin-binding protein is GRP94. Membrane-associated p185erbB2 exists in a stable complex with GRP94. GA binding to GRP94 disrupts this complex, leading to degradation of pre-existing p185erbB2 protein, and resulting in an altered subcellular distribution of newly synthesized p185erbB2.