Comprehensive identification of HSP70/HSC70 Chaperone Clients in Human Cells
The HSP70 family of chaperones are the front-line of protection from stress-induced misfolding and aggregation of polypeptides in most organisms and are responsible for promoting the stability, folding, and degradation of clients to maintain cellular protein homeostasis. Here we demonstrate quantitative identification of HSP70 and HSC70 clients using an ubiquitin-mediated proximity tagging strategy and show that, despite their high degree of similarity, these enzymes have largely non-overlapping specificities. Both proteins show a preference for association with newly synthesized polypeptides but each responds differently to changes in the stoichiometry of proteins in obligate multi-subunit complexes. In addition, expression of an ALS-associated SOD1 mutant protein induces changes in HSP70 and HSC70 client association and aggregation toward polypeptides with predicted disorder, indicating that there are global effects from a single misfolded protein that extend to many clients within chaperone networks. Together these findings show that the ubiquitin-mediated UBAIT fusion system can efficiently isolate the complex interactome of HSP chaperone family proteins under normal and stress conditions.