The nucleotide exchange factors of Hsp70 molecular chaperones

Abstract

Molecular chaperones of the Hsp70 family form an important hub in the cellular protein folding networks in bacteria and eukaryotes, connecting translation with the downstream machineries of protein folding and degradation. The Hsp70 folding cycle is driven by two types of cochaperones: J-domain proteins stimulate ATP hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) promote replacement of Hsp70-bound ADP with ATP. Bacteria and organelles of bacterial origin have only one known NEF type for Hsp70, GrpE. In contrast, a large diversity of Hsp70 NEFs has been discovered in the eukaryotic cell. These NEFs belong to the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families. In this short review we compare the structures and molecular mechanisms of nucleotide exchange factors for Hsp70 and discuss how these cochaperones contribute to protein folding and quality control in the cell.

Keywords: BAG domain; GrpE; Hsp110/Grp170; HspBP1/Sil1; cochaperone; protein folding; proteostasis.

Figure 1

Domain compositions of human NEF homologs. Human cells express two mitochondrial GrpE isoforms (mtGrpE1/GRPEL1 and mtGrpE2/GRPEL2), three Hsp110 homologs (Hsp105/HSPH1, Apg-1/HSPA4L, and Apg-2/HSPA4—the codes designate the gene names) and five BAG-domain proteins. There is only one form of Grp170/HYOU1, HspBP1, and Sil1, respectively. Isoforms arising from alternative initiation sites and splicing were described for Hsp105, HspBP1, and Bag1 (not shown). mtGrpE isoforms contain mitochondrial signal sequences (ss). The α- and β-domains (orange and green, respectively) are conserved with GrpE from E. coli. The Hsp110/Grp170 family proteins consist of an N-terminal nucleotide binding domain (NBD, blue), a β-sandwich (β-Dom, green) and a α-helix bundle domain (α-Dom, pale yellow). All isoforms contain long variable insertions in the β-sandwich and at the C-terminus. SS indicates signal sequences for ER import of Grp170 and Sil1. HspBP1 and Sil have characteristic Armadillo repeat folds (orange). All members of the BAG family in humans, Bag1-5, contain C-terminal Hsp70-binding BAG domains (red), but have otherwise divergent domain composition. Bag1 contains an Ubiquitin-like domain (Ubl, dark blue), which might associate with the regulatory particle of the 26S proteasome, and a NLS sequences (purple) for nuclear targeting. Bag2 contains a coiled-coil dimerization domain (CC, orange) (Page et al., 2012). Bag3 comprises multiple N-terminal sequence motifs including WW domains (WW, yellow), IPV sequence motifs (brown) and PXXP repeats (pink), which mediate interactions with proline-rich motifs, HspB8 and SH3 domains, respectively (Doong et al., ; Fuchs et al., ; Iwasaki et al., ; Ulbricht et al., 2013). Bag5 has four additional 3-helix bundle domains of unknown function (Arakawa et al., 2010). Bag6 is not shown because the original assignment as an NEF of Hsp70 was incorrect (Mock et al., 2015).

Figure 2

Structure and mechanism of nucleotide exchange factors. Exemplary structures of the four NEF families are shown together with respective Hsp70 complexes. The NEF is always shown in blue; the Hsp70 NBD in orange with subdomain IIB highlighted in red. On the right the structure of the NBD in the complex is superposed with the ADP-bound conformation, and the putative nucleotide exchange mechanism indicated. The drawings are based on the PDB coordinate sets 1DKG (GrpE•DnaK, Harrison et al., 1997), 2V7Y (DnaK•ADP, Chang et al., 2008), 3D2F (Sse1p•Hsp70, Polier et al., 2008), 1HPM (Hsc70•ADP, Wilbanks and Mckay, 1995), 1XQS (HspBP1•Hsp70-lobeII, Shomura et al., 2005) and 1HX1 (Bag1•Hsc70, Sondermann et al., 2001).