Abstract

The protein Sup35 from Saccharomyces cerevisiae possesses prion properties. In vivo, a high molecular weight form of Sup35p is associated to the [PSI⁺] factor. The continued propagation of [PSI⁺] is highly dependent on the expression levels of molecular chaperones from the Hsp100, 70 and 40 families; however, so far, their role in this process is unclear. We have developed a reproducible in vitro system to study the effects of molecular chaperones on the assembly of full-length Sup35p. We show that Hsp104p greatly stimulates the assembly of Sup35p into fibrils, whereas Ydj1p has inhibitory effect. Hsp82p, Ssa1p and Sis1p, individually, do not affect assembly. In contrast, Ssa1p together with either of its Hsp40 cochaperones blocks Sup35p polymerization. Furthermore, Ssa1p and Ydj1p or Sis1p can counteract the stimulatory activity of Hsp104p, by forming complexes with Sup35p oligomers, in an ATP-dependent manner. Our observations reveal the functional differences between Hsp104p and the Hsp70–40 systems in the assembly of Sup35p into fibrils and bring new insight into the mechanism by which molecular chaperones influence the propagation of [PSI⁺].