STRUCTURAL ORGANIZATION OF PROKARYOTIC AND EUKARYOTIC HSP90 - INFLUENCE OF DIVALENT-CATIONS ON STRUCTURE AND FUNCTION

Hsp90 is a very abundant molecular chaperone that apparently helps to protect cellular proteins from denaturation upon temperature upshift. The unusual ability of Hsp90 to function under conditions where other proteins unfold prompted us to investigate the stability and structura l organization of Hsp90 itself. Both procaryotic and eucaryotic member s of the Hsp90 family were found to have very similar physicochemical properties: (i) they are stable against thermal unfolding up to at lea st 50 degrees C, (ii) they show biphasic, reversible unfolding transit ions in guanidinium chloride, and (iii) their oligomerization state is strongly and rapidly affected by millimolar concentrations of divalen t cations. In the presence of MnCl2 and MgCl2 defined changes in the q uaternary structure of Hsp90 could be observed which resulted in a dec rease in thermostability and an increased tendency to form larger aggr egates. The addition of divalent cations also almost completely abolis hed the chaperone function of Hsp90 and induced release of folding int ermediates of citrate synthase bound to Hsp90. These modulating effect s of divalent cations on structure and function of Hsp90 in vitro repr esent a potential mechanism for regulation of Hsp90 chaperone action i n vivo.