The function of BAG5 (BAG cochaperone 5, ENSG00000166170) is as follows. 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 (PubMed:20223214). 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 (By similarity). Inhibits both auto-ubiquitination of PRKN and ubiquitination of target proteins by PRKN (By similarity). {ECO:0000250|UniProtKB:Q5QJC9, ECO:0000250|UniProtKB:Q8CI32, ECO:0000269|PubMed:20223214}.