Place, S.P.*; Hofmann, G.E.: Temperature-protein interactions of the molecular chaperone Hsc70 purified from a eurythermal goby

Many heat shock proteins (Hsps) are known molecular chaperones that are fundamental to protein folding. Although a great deal is known about Hsps as molecular chaperones in model systems (e.g., E. coli and cultured mammalian cells), very little is known about their function in organisms from natural populations. Consequently, we have little insight into the temperature interactions of Hsps under ecologically relevant conditions. In this study we examined the temperature-protein interactions of a molecular chaperone, Hsc70, from Gillichthys mirabilis, a goby that encounters a broad range of temperatures in its estuarine habitat. Seasonal temperatures experienced by this fish range from 7 – 37� C annually. Our goal was to address the question of how the biochemical function of Hsc70 mapped onto the range of temperatures encountered by G. mirabilis in its natural environment. For these studies, we examined the activity and biochemical properties of Hsc70 purified from white muscle of G. mirabilis. Since Hsc70 binds and hydrolyzes ATP during a chaperoning cycle, ATPase activity was used as an indicator of chaperone function. Using an in vitro ³²P-based ATPase assay, the results showed that (1) Hsc70 has a generally low intrinsic ATPase activity that can be stimulated by the addition of an unfolded protein, (2) Q₁₀ calculations for the chaperone range from 1.2 – 2.2 over the environmental temperature range for G. mirabilis, and (3) the thermal stability of Hsc70 greatly exceeds environmental temperatures for this species. The results indicate that Hsc70 from G. mirabilis is a thermally stable protein with an ATPase activity that is functional over a broad range of environmentally relevant temperatures.