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* Institute of Biotechnology, University of Helsinki, Helsinki, Finland; and Heat stress is an obvious hazard, and mechanisms to recover from thermal damage, largely unknown as of yet, have evolved in all organisms. We
have recently shown that a marker protein in the ER of
Saccharomyces cerevisiae, denatured by exposure of
cells to 50°C after preconditioning at 37°C, was reactivated by an ATP-dependent machinery, when the cells
were returned to physiological temperature 24°C. Here
we show that refolding of the marker enzyme
Hsp150 School of Biological Sciences, University of
Manchester, Manchester M13 9PT, United Kingdom
-
-lactamase, inactivated and aggregated by
the 50°C treatment, required a novel ER-located homologue of the Hsp70 family, Lhs1p. In the absence of
Lhs1p, Hsp150--lactamase failed to be solubilized
and reactivated and was slowly degraded. Coimmunoprecipitation experiments suggested that Lhs1p was
somehow associated with heat-denatured Hsp150- -lactamase, whereas no association with native marker
protein molecules could be detected. Similar findings
were obtained for a natural glycoprotein of S. cerevisiae, pro-carboxypeptidase Y (pro-CPY). Lhs1p had no
significant role in folding or secretion of newly synthesized Hsp150--lactamase or pro-CPY, suggesting that the machinery repairing heat-damaged proteins
may have specific features as compared to chaperones
assisting de novo folding. After preconditioning and
50°C treatment, cells lacking Lhs1p remained capable
of protein synthesis and secretion for several hours at
24°C, but only 10% were able to form colonies, as compared to wild-type cells. We suggest that Lhs1p is involved in a novel function operating in the yeast ER,
refolding and stabilization against proteolysis of heatdenatured protein. Lhs1p may be part of a fundamental
heat-resistant survival machinery needed for recovery
of yeast cells from severe heat stress.
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