Co-chaperone for HSP/HSP70 proteins. It functions as a nucleotide-exchange factor promoting the release of ADP from HSP70, thereby activating HSP70-mediated protein refolding.Has an essential role in maintaining proteostasis at junctional membrane complexes (JMC), where it may function as a scaffold between the HSPA8 chaperone and JMC proteins enabling correct, HSPA8-dependent JMC protein folding.Inhibits both auto-ubiquitination of PRKN and ubiquitination of target proteins by PRKN.