Polypeptide transfer from Hsp40 to Hsp70 molecular chaperones

Abstract

Heat shock protein 40 (Hsp40) co-chaperones assist in cellular protein folding and degradation through the binding and delivery of non-native proteins to heat shock protein 70 (Hsp70). The mechanism for substrate transfer from Hsp40s to Hsp70 is unknown. Two recent studies provide new details that shed light on novel mechanisms for substrate recognition by Hsp40s and a common mechanism for polypeptide transfer to Hsp70.

Figure 1

(a) Hsp70 polypeptide binding and release is regulated through cycles of ATP hydrolysis and exchange. Protein misfolding is managed, in part, through the action of Hsp70 molecular chaperones in coordination with various cellular cofactors. Initially, a non-native polypeptide is bound by an Hsp40 co-chaperone (light blue). (b) The Hsp40 J-domain then binds Hsp70 (blue-green) in an ATP-bound state. (c) J-domain-stimulated ATP hydrolysis in the nucleotide-binding domain (NBD) induces a conformational shift in the Hsp70 substrate-binding domain (SBD), increasing affinity for the non-native polypeptide that is released from the Hsp40. Additional cofactors such as the E3 ubiquitin ligase CHIP (brown) with a ubiquitin-conjugating E2 (blue) can bind to Hsp70 and the non-native polypeptide, targeting the substrate for degradation by the ubiquitin-proteasome system (Poly-Ub). (d) Nucleotide exchange factors (NEFs) such as the Hsp110s (green) replace the ADP with ATP, releasing the polypeptide from Hsp70. (e) If the polypeptide remains in a non-native conformation, the cycle can be repeated until folding is complete.

Figure 2

Domain structures of several human Hsp40s. Type I Hsp40s such as Hdj-2 (also called DnaJA1) possess a J-domain, G/F-rich region (green) and a ZFLR (blue). Hdj-2 is distinct from many Hsp40s because it is farnesylated at a C-terminal CAAX motif (CQTS). Type II Hsp40s, such as Hdj-1 (also called DnaJB1), lack the ZFLR. HSJ1a and HSJ1b (also called DnaJB2) are neuron-specific type II Hsp40 isoforms. Both isoforms possess two ubiquitin interaction motifs (UIMs; brown); however, Hsj1b also contains a CAAX motif that is geranylgeranylated (CLIL). ER-localized Hsp40s have also evolved specialized domains. For example, ERdj3 (also called DnaJB11) contains a cysteine-rich motif (purple) in the C terminus [21]. P58 (also called DnaJC3) contains nine tetratricopeptide repeats (TPR; red) that might bind non-native polypeptides, whereas ERdj5 contains three thioredoxin (TRX; yellow) domains, which are responsible for disulfide reductase activity. The length of each Hsp40 represents its relative size. The intracellular location of each Hsp40 is noted.