[SWI+], the Prion Formed by the Chromatin Remodeling Factor Swi1, Is Highly Sensitive to Alterations in Hsp70 Chaperone System Activity

Abstract

The yeast prion [SWI+], formed of heritable amyloid aggregates of the Swi1 protein, results in a partial loss of function of the SWI/SNF chromatin-remodeling complex, required for the regulation of a diverse set of genes. Our genetic analysis revealed that [SWI+] propagation is highly dependent upon the action of members of the Hsp70 molecular chaperone system, specifically the Hsp70 Ssa, two of its J-protein co-chaperones, Sis1 and Ydj1, and the nucleotide exchange factors of the Hsp110 family (Sse1/2). Notably, while all yeast prions tested thus far require Sis1, [SWI+] is the only one known to require the activity of Ydj1, the most abundant J-protein in yeast. The C-terminal region of Ydj1, which contains the client protein interaction domain, is required for [SWI+] propagation. However, Ydj1 is not unique in this regard, as another, closely related J-protein, Apj1, can substitute for it when expressed at a level approaching that of Ydj1. While dependent upon Ydj1 and Sis1 for propagation, [SWI+] is also highly sensitive to overexpression of both J-proteins. However, this increased prion-loss requires only the highly conserved 70 amino acid J-domain, which serves to stimulate the ATPase activity of Hsp70 and thus to stabilize its interaction with client protein. Overexpression of the J-domain from Sis1, Ydj1, or Apj1 is sufficient to destabilize [SWI+]. In addition, [SWI+] is lost upon overexpression of Sse nucleotide exchange factors, which act to destabilize Hsp70's interaction with client proteins. Given the plethora of genes affected by the activity of the SWI/SNF chromatin-remodeling complex, it is possible that this sensitivity of [SWI+] to the activity of Hsp70 chaperone machinery may serve a regulatory role, keeping this prion in an easily-lost, meta-stable state. Such sensitivity may provide a means to reach an optimal balance of phenotypic diversity within a cell population to better adapt to stressful environments.

The sensitive [SWI+] prion

“…The amino acid composition of the putative Prion-forming Domains (PrDs) of Ure2 and Swi1 are dissimilar from those of Sup35 and Rnq1, as recently pointed out by Crow and Li and reaffirmed in our recent study. 1,20 The PrDs of Sup35 and Rnq1 are enriched in glutamine (Q) compared to asparagine (N), rich in glycine, and sparse in certain large hydrophobic residues (F, W, L, I, V, & M), compared to other prions ( propagation, and thus strengthening the argument that this machinery may be universally required among amyloid-forming yeast prions. 1 However the degree to which [SWI + ] appears to rely upon moderate Hsp70 cycling is striking and currently unprecedented among the prions studied thus far.…”

“…1,20 The PrDs of Sup35 and Rnq1 are enriched in glutamine (Q) compared to asparagine (N), rich in glycine, and sparse in certain large hydrophobic residues (F, W, L, I, V, & M), compared to other prions ( propagation, and thus strengthening the argument that this machinery may be universally required among amyloid-forming yeast prions. 1 However the degree to which [SWI + ] appears to rely upon moderate Hsp70 cycling is striking and currently unprecedented among the prions studied thus far. These results imply that [SWI + ] serves as an example of a meta-stable element, sitting precariously on a razor's edge of precisely balanced chaperone activities.…”

“…We recently reported that one such prion, [SWI + ], differs from the best studied, archetypal prion [PSI + ] in several significant ways. 1 Notably, [SWI + ] is highly sensitive to alterations in Hsp70 system chaperone activity and is lost upon growth at elevated temperatures. In that report we briefly noted a correlation amongst prions regarding amino acid composition, seed number and sensitivity to the activity of the Hsp70 chaperone system.…”

“…[4][5][6] The currently accepted model of prion fragmentation posits that components of the Hsp70 chaperone system work in congress with the disaggregase Hsp104. 1,[7][8][9] Hsp70-type chaperones function by repeatedly binding and releasing client polypeptides in an ATP-dependent manner, a cycle that is tightly regulated by co-chaperone proteins (Fig. 1).…”

“…We found no evidence for [SWI + ] loss in response to acute stresses like heat-, ethanol-or oxidative-shock. 1 1,19 What structural features might underlie the observed differences in chaperone sensitivity? One plausible answer is amino acid composition.…”