The function of the yeast molecular chaperone Sse1 is mechanistically distinct from the closely related Hsp70 family.
Lance Shaner (1), Amy Trott (1), Jennifer Goeckeler (2), Jeffrey Brodsky (2), Kevin Morano (1)
(1) Micro. and Mol. Genet., Univ. Texas Med. Schl. Houston, 6431 Fannin, Houston, TX, 77030, USA;
(2) Dept. Biological Sciences, Univ. Pittsburgh, Pittsburgh, PA 15260
Sse1 is a member of the eukaryotic Hsp110 family of molecular chaperones in yeast. Sse1/Hsp110s are related to Hsp70, which can actively refold denatured polypeptides via a carboxyl-terminal peptide binding domain (PBD). Unlike Hsp70, both Sse1 and mammalian Hsp110 bind unfolded peptide substrates but cannot refold them. A series of residues conserved in Hsp70s and involved in ATP binding and hydrolysis were mutagenized and the mutant SSE1 alleles assayed for their ability to complement sse1 yeast. Surprisingly, all mutants predicted to abolish ATP hydrolysis were functional, whereas mutations of predicted ATP binding residues were not. The carboxyl-terminus of Hsp70s is required for substrate targeting and heterocomplex formation with other chaperones, but truncation of up to 44 carboxyl-terminal residues not included in the PBD were active. Remarkably, the two domains of Sse1 when expressed in trans functionally complement the sse1 growth phenotype and interact by coimmunoprecipitation analysis. Taken together, these data demonstrate that nucleotide occupancy of the ATPase domain may be required to foster a conformation capable of interacting with unfolded or nascent proteins in vivo. The inability to fold substrates is consistent with both the lack of a requirement for critical ATPase residues and the novel interactions between the ATPase and PB domains. We predict that Sse1 may collaborate with other foldases in the cell, such as Hsp70s, to effect protein refolding.
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