The hsp110 and Grp1 70 stress proteins: newly recognized relatives of the Hsp70s

Abstract

Both the Grp170 and Hsp110 families represent relatively conserved and distinct sets of stress proteins, within a more diverse category that also includes the Hsp70s. All of these families are found in a wide variety of organisms from yeasts to humans. Although Hsp110s or Grp170s are not Hsp70s any more than Hsp70s are Hsp110s or Grp170s, it is still reasonable to refer to this combination of related families as the Hsp70 superfamily based on arguments discussed above and since no obvious prokaryotic Hsp110 or Grp170 has yet been identified. These proteins are related to their counterparts in the Hsp70/Grp78 family of eukaryotic stress proteins but are characterized by significantly larger molecular weights. The members of the Grp170 family are characterized by C-terminal ER retention sequences and are ER localized in yeasts and mammals. As a Grp, Grp170 is recognized to be coregulated with other major Grps by a well-known set of stress conditions, sometimes referred to as the unfolded protein response (Kozutsumi et al 1988; Nakaki et al 1989). The Hsp110 family members are localized in the nucleus and cytoplasm and, with other major Hsps, are also coregulated by a specific set of stress conditions, most notably including hyperthermic exposures. Hsp110 is sometimes called Hsp105, although it would be preferable to have a uniform term. The large Hsp70-like proteins are structurally similar to the Hsp70s but differ from them in important ways. In both the Grp170 and Hspl10 families, there is a long loop structure that is interposed between the peptide-binding ,-domain and the alpha-helical lid. In the Hsp110 family and Grp170, there are differing degrees of expansion in the alpha-helical domain and the addition of a C-terminal loop. This gives the appearance of much larger lid domains for Hsp110 and Grp170 compared with Hsp70. Both Hsp110 and Grp170 families have relatively conserved short sequences in the alpha-helical domain in the lid, which are conserved motifs in numerous proteins (we termed these motifs Magic and TedWylee as discussed earlier). The structural differences detailed in this review result in functional differences between the large (Grp170 and Hspl10) members of the Hsp70 superfamily, the most distinctive being an increased ability of these proteins to bind (hold) denatured polypeptides compared with Hsc70, perhaps related to the enlarged C-terminal helical domain. However, there is also a major difference between these large stress proteins; Hsp110 does not bind ATP in vitro, whereas Grp170 binds ATP avidly. The role of the Grp170 and Hsp110 stress proteins in cellular physiology is not well understood. Overexpression of Hsp110 in cultured mammalian cells increases thermal tolerance. Grp170 binds to secreted proteins in the ER and may be cooperatively involved in folding these proteins appropriately. These roles are similar to those of the Hsp70 family members, and, therefore, the question arises as to the differential roles played by the larger members of the superfamily. We have discussed evidence that the large members of the superfamily cooperate with members of the Hsp70 family, and these chaperones probably interact with a large number of chaperones and cochaperones in their functional activities. The fundamental point is that Hsp110 is found in conjunction with Hsp70 in the cytoplasm (and nucleus) and Grp170 is found in conjunction with78 in tha ER in every eucaryotic cell examined from yeast to humans. This would strongly argue that Hsp110 Grp170 exhibit functions in eucaryotes not effectively performed by Hsp70s or Grp78, respectively. Of interest in this respect is the observation that all Hsp110s loss of function or deletion mutants listed in the Drosophila deletion project database are lethal. The important task for the future is to determine the roles these conserved molecular chaperones play in normal and physiologically stressed cells.

Fig 1.

Models for the fold of Hsp110 and Grp170 based on the structure of DnaK (modified from Oh et al 1999). (A) The secondary structure and fold of the DnaK peptide-binding domains based on the x-ray crystallographic structure reported by Zhu et al (1996). The predicted secondary structure and proposed fold for (B) Hsp110 and (C) Grp170, beginning at residue 390, are diagrammatically represented. The domains (A, B, L, and H), as defined in the text, are indicated on the diagram. The β-strands in DnaK and Hsp110 β-domains, sharing both structural (PHD) and significant sequence similarity by Matchbox alignment (Depiereux et al 1997), appear in yellow. Although there is sequence similarity between the β-domains of Hsp110 and Grp170, there is not enough structural and sequence similarity to allow us to identify homologous strands. The predicted helical segments in Hsp110 and Grp170 are shown arranged in a helix-turn-helix structure, as suggested by the helical domain in DnaK. The positions of the conserved sequence motifs (Magic and TedWylee) are indicated by green and orange lines

Fig 2.

Overexpression of Hsp110 in cultured mammalian cells increases thermal tolerance. Hamster Hsp110 was placed under the control of a tet-off inducible promoter. When Hsp110 was expressed at heat shock levels (induced), partial resistance to 45°C heat shocks of varying duration was observed relative to uninduced control cells. Full thermotolerance is induced by preheating. Modified from Oh et al (1997)

Fig 3.

Analysis of Hsp110 deletion mutants. Schematic diagram illustrating the deletion mutants of Hsp110. The domains examined are as follows: A, ATP-binding domain; B, β-sheet (peptide-binding) domain; L, acidic loop; H, α-helical domain; H′, first 2 α-helices of H domain. ND, not determined. Modified from Oh et al (1997)

Fig 4.

Phylogenetic tree for Hsp70 superfamily. Eleven members of the Hsp110/SSE family and 9 members of the Grp170 family were aligned with 5 members of the Hsp70/DnaK family using the CLUSTAL W program, and the alignments were analyzed by programs in PHYLIP (Phylogeny Inference Package) version 3.57c. The alignments were bootstrapped with SEQBOOT to generate 100 sets of alignment data. The data sets were used to generate 100 phylogenetic trees by use of the POTOPARS (maximum parsimony) program. A consensus tree was determined by use of the program CONSENSE. The tree is displayed as unrooted, with the italicized numbers indicating the number of trees in which the consensus bifurcation distal to the number shown was represented in the total set of 100 trees. Abbreviations and sequences used are the same as in Tables 2 and 3, except the following additional sequences were used in the tree: HSP110CHO (Q60446) Chinese hamster, APG-1 MOUS (D49482) mouse, APG-2 MOUS (Q61316) mouse, YAM6 S. POM (Q10061) S pombe, 14G8.3 C.E. (CAA91809) C elegans, SSA1 S.C. (6319314), S cerevisiae Grp78S.C.; KAR2 (6322426) S cerevisiae, HSP70BOV (P34933) bovine