Broadening the functionality of a J-protein/Hsp70 molecular chaperone system

Abstract

By binding to a multitude of polypeptide substrates, Hsp70-based molecular chaperone systems perform a range of cellular functions. All J-protein co-chaperones play the essential role, via action of their J-domains, of stimulating the ATPase activity of Hsp70, thereby stabilizing its interaction with substrate. In addition, J-proteins drive the functional diversity of Hsp70 chaperone systems through action of regions outside their J-domains. Targeting to specific locations within a cellular compartment and binding of specific substrates for delivery to Hsp70 have been identified as modes of J-protein specialization. To better understand J-protein specialization, we concentrated on Saccharomyces cerevisiae SIS1, which encodes an essential J-protein of the cytosol/nucleus. We selected suppressors that allowed cells lacking SIS1 to form colonies. Substitutions changing single residues in Ydj1, a J-protein, which, like Sis1, partners with Hsp70 Ssa1, were isolated. These gain-of-function substitutions were located at the end of the J-domain, suggesting that suppression was connected to interaction with its partner Hsp70, rather than substrate binding or subcellular localization. Reasoning that, if YDJ1 suppressors affect Ssa1 function, substitutions in Hsp70 itself might also be able to overcome the cellular requirement for Sis1, we carried out a selection for SSA1 suppressor mutations. Suppressing substitutions were isolated that altered sites in Ssa1 affecting the cycle of substrate interaction. Together, our results point to a third, additional means by which J-proteins can drive Hsp70’s ability to function in a wide range of cellular processes—modulating the Hsp70-substrate interaction cycle.

Variant‐specific and reciprocal Hsp40 functions in Hsp104‐mediated prion elimination

“…For example, in the case of [ PSI + ], weak variants tend to have larger amyloid fibers with fewer free ends, resulting in fewer transmissible aggregates to propagate the prion as well as a greater amount of soluble Sup35 (Liebman and Chernoff, ). Prion variants often have distinct requirements of chaperone activity, particularly with respect to J‐protein activity, for stable propagation (Derkatch et al ., ; Hines et al ., a, Prusiner, ; Stein and True, a,b; Harris et al ., ; Sporn and Hines, ; Schilke et al ., ; Killian and Hines, ). The J‐protein Sis1 is specifically required for the propagation of at least four yeast prions and has been shown to be the sole cytosolic J‐protein required for the propagation of strong [ PSI + ] variants (Sondheimer et al ., ; Higurashi et al ., ; Tipton et al ., ; Hines et al ., b; Schilke et al ., ).…”

Three J-proteins impact Hsp104-mediated variant-specific prion elimination: a new critical role for a low-complexity domain

“…Hsp70s are a versatile class of molecular chaperones that are essential for a variety of cellular functions including protein folding and transport, iron-sulfur cluster biogenesis, liquid-drop aggregation, and notably, prion propagation (Jung et al 2000;Kampinga and Craig 2010;Craig and Marszalek 2017;Simpson-Lavy and Kupiec 2018). The ability of Hsp70s to fine-tune their functions stems from their biochemical activity; they engage in a promiscuous and ATP-dependent client-binding and release cycle that is directed, enhanced, and calibrated by Hsp40s and nucleotide exchange factors (NEFs) (Kampinga and Craig 2010;Craig and Marszalek 2017;Schilke et al 2017). Cochaperone Hsp40s (also and hereafter called "J-proteins") stimulate Hsp70 ATPase activity via their characteristic J-domains (Li et al 2009;Craig and Marszalek 2017).…”

“…One of these J-proteins, Sis1, is essential for both cell survival and propagation of the four best-studied yeast prions: [PSI + ], [RNQ + ], [URE3], and [SWI + ] (Luke et al 1991;Sondheimer et al 2001;Aron et al 2007;Higurashi et al 2008;Hines et al 2011b). Specific chaperone requirements for propagation are largely differentiable among different prions and prion variants, particularly with respect to domain requirements of J-proteins (Derkatch et al 1996;Hines et al 2011a;Prusiner 2013;Stein and True 2014;Sporn and Hines 2015;Schilke et al 2017;Killian and Hines 2018;Killian et al 2019). In addition to Sis1, three other J-proteins-Ydj1, Swa2, and Apj1-have been previously implicated in prion biology.…”

Client processing is altered by novel myopathy-causing mutations in the HSP40 J domain

“…The function and interaction of the various Hsp40 domains has been a topic of interest for many years [20,21,[81][82][83][84][85][86]. Surprisingly, colleagues identified LGMDD1 patients with novel variants in the J domain of DNAJB6, a disease previously characterized by mutations in the G/ F domain [12,13].…”