Interaction of BiP with the J-domain of the Sec63p component of the endoplasmic reticulum protein translocation complex

Proteins of the Hsp70 family of ATPases interact with a conserved domain of their J-protein partners, the J-domain, to function in numerous cellular p rocesses. We have studied the interaction of BiP, an Hsp70 family member in the lumen of the endoplasmic reticulum, with the J-domain of Sec63p, a com ponent of the Sec complex involved in post-translational protein translocat ion across the endoplasmic reticulum membrane. In a real-time solid phase b inding assay, BiP binds to the immobilized Sec complex or to a fusion prote in of the J-domain and glutathione S-transferase in a reaction that require s ATP hydrolysis. In the final complex, BiP is bound in the ADP form with i ts peptide binding pocket occupied. An intact peptide binding pocket is req uired for this interaction. Our experiments suggest that the activation of BiP by the J-domain involves a transient contact between these components, and that in the absence of physiological substrates, J-activated BiP binds even to the J-proteins themselves.