Regulated association of misfolded endoplasmic reticulum lumenal proteins with P58/DNAJc3

P58/DNAJc3 defends cells against endoplasmic reticulum ( ER) stress. Most P58 molecules are translocated into the ER lumen, and here we report selective and stable binding to misfolded proteins by P58's TPR-containing N-terminal domain. In vitro, too, P58 binds selectively to a model misfolded protein and challenge of that complex with physiological concentrations of the ER lumenal Hsp70-type chaperone BiP encourages disassembly. BiP-induced dissociation of P58 from its substrate depends on the presence of ATP and on interactions with P58's J-domain, which are mediated by invariant residues BiPR197 and P58(H422). A functional J-domain also accelerates dissociation of P58 from a model substrate, VSV-G(ts045), on the latter's re-folding in vivo. However, J-domain binding can be separated from the ability to promote substrate dissociation by the mutant BiPE201G and a wild-type J-domain fused ectopically to P58(H422Q) rescues the latter's inability to dissociate from substrate in response to BiP and ATP. These findings are consistent with a model whereby localized activation of the Hsp70-type partner is sufficient to promote substrate handover from the J-domain cochaperone.