Article

Protein folding activity of Hsp70 is modified differentially by the Hsp40 co-chaperones Sis1 and Ydj1

University of Alabama at Birmingham, Birmingham, Alabama, United States
Journal of Biological Chemistry (Impact Factor: 4.57). 11/1998; 273(43):27824-30. DOI: 10.1074/jbc.273.43.27824
Source: PubMed

ABSTRACT Specification of Hsp70 action in cellular protein metabolism may occur through the formation of specialized Hsp70:Hsp40 pairs. To test this model, we compared the ability of purified Sis1 and Ydj1 to regulate the ATPase and protein-folding activity of Hsp70 Ssa1 and Ssb1/2 proteins. Ydj1 and Sis1 could both functionally interact with Ssa1, but not the Ssb1/2 proteins, to refold luciferase. Interestingly, Ydj1:Ssa1 could promote up to four times more luciferase folding than Sis1:Ssa1. This functional difference was explored and could not be accounted for by differences in the ability of Sis1 and Ydj1 to regulate Ssa1 ATPase activity. Instead, differences in the chaperone function of Ydj1 and Sis1 were observed. Ydj1 was dramatically more effective than Sis1 at suppressing the thermally induced aggregation of luciferase. Paradoxically, Sis1 and Ydj1 could bind similar quantities of chemically denatured luciferase. The polypeptide binding domain of Sis1 was found to lie between residues 171-352 and correspond to its conserved carboxyl terminus. The conserved carboxyl terminus of Ydj1 is also known to participate in the binding of nonnative polypeptides. Thus, Ydj1 appears more efficient at assisting Ssa1 in folding luciferase because its contains a zinc finger-like region that is absent from Sis1. Ydj1 and Sis1 are structurally and functionally distinct Hsp40 proteins that can specify Ssa1 action by generating Hsp70:Hsp40 pairs that exhibit different chaperone activities.

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    • "between Sis1-driven and Ydj1/Xdj1-driven refolding by Hsp70 and Hsp70 ΔEEVD , we carried out refolding assays in which luciferase was partially denatured by guanidinium hydrochloride. This is a milder treatment than the urea denaturation method, and, as expected [21], refolding occurred when only Hsp70 and J-protein were included in the reaction (Fig. 1b). As with the assay containing Hsp104 and Sse1, Sis1 did not partner with Hsp70 ΔEEVD . "
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    • "Several studies have been made to elucidate the reason for the excess number of the co-chaperons in a cell. The researchers suggested that Hsp70 pairs with Hsp40 to form specific functions and currently details of the mechanism are under investigation [17]. "
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    • "The most abundant Hsp40s are members of the Type I and Type II sub-families who partner with Hsp70 to promote protein folding [12], [16], protein degradation [17], [18], [19], [20], translation [21], translocation across membranes [22] and assembly of amyloid-like fibers [23]. In eukaryotes such as yeast, the Type I and Type Hsp40s Ydj1 and Sis1 utilize their unique structural features, substrate specificity, post-translational modification, and localization to direct Hsp70 to function in different aspects of protein metabolism [10], [24], [25], [26], [27], [28]. Yet, it is still unclear how specialized Hsp70:Hsp40 pairs function in PQC networks to triage non-native clients for folding, degradation, or sequestration into misfolded protein handling centers. "
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