The HSP70 chaperone machinery: J proteins as drivers of functional specificity

Abstract

Hsp70s, ubiquitous molecular chaperones, function in a myriad of biological processes, modulating polypeptides' folding, degradation and translocation across membranes, as well as protein-protein interactions. This multitude of roles is not easily reconciled with the near conformity of biochemical activity of Hsp70s, an ATP-dependent client protein binding/release cycle. Much of the functional diversity of Hsp70s is driven by a diverse class of cofactors, Jproteins (also called Hsp40s). Often, multiple J-proteins function with a single Hsp70. Some target Hsp70 activity to clients at precise locations in cells; others bind client proteins directly, thereby delivering specific clients to Hsp70, directly determining their fate.In their native cellular environment, polypeptides are constantly at risk of attaining conformations that prevent them from functioning properly and/or cause them to aggregate into large, potentially cytotoxic complexes. Molecular chaperones guide the conformation of proteins throughout their lifetime, preventing their aggregation by protecting interactive surfaces against non-productive interactions. Through such interactions, they aid in the folding of nascent proteins as they are synthesized by ribosomes, drive protein transport across membranes, and modulate protein-protein interactions by controlling conformational changes1 , 2 . In addition to these roles under optimal conditions, stresses can exacerbate protein conformational problems (for example, heat shock causing protein unfolding; oxygen radicals causing oxidation and nitrosylation). Although in some cases chaperones can facilitate (re)folding, often such rejuvenation is not possible. In such cases, chaperones can facilitate degradation, either by simply preventing aggregation and thus keeping clients susceptible to proteolysis or by actively facilitating their transfer to proteolytic systems. These diverse functions of molecular chaperones typically involve iterative client binding and release cycles until the client has reached its final active conformation, or has entered the proteolytic system (Figure 1). Strikingly, Hsp70s, one of the most ubiquitous classes of chaperones, has been implicated in all of the biological processes mentioned above2 , 3 . This Review focuses on the means by which Hsp70 molecular chaperone machinery participates in such diverse cellular functions. Their functional diversity is remarkable considering that within and across species, Hsp70s have very high sequence identity. They share a single biochemical activity, an ATPdependent substrate binding and release cycle combined with client protein recognition, which is typically rather promiscuous. The answer to this apparent conundrum lies in the fact that Hsp70s do not work alone, but rather as "Hsp70 machines", collaborating with and h.h.kampinga@med.umcg.nl, ecraig@wisc.edu. regulated by a number of (co)chaperones and cofactors. Here, using examples from yeast and human, we discuss several such factors, particularly concentrating on how the ar...

Proteostasis in cardiac health and disease

“…HSPs are classified in six sub groups: HSPA (HSP70), HSPB (small HSPs), HSPC (HSP90), HSPD and HSPE (chaperonin families HSP60 and HSP10, respectively), HSPH (HSP110), and DNAJ (HSP40) 29 . Each subgroup comprises several family members and cofactors with distinct and/or overlapping functions, which are localized in various cellular com partments 30,31 . Members can be constitutively expressed or are induced by many different intrinsic or extrin sic stressors 30,[32][33][34] .…”

Firefly luciferase mutants as sensors of proteome stress

“…Consequently, cells possess a variety of different Hsp40 molecules with different substrate binding properties which further broadens the activity spectrum of Hsp70 (Kampinga and Craig, 2010). In addition to Hsp40, NEFs play a pivotal role in the functional cycle of Hsp70.…”

“…It has been suggested that different Hsp40s, due to their substrate recognition ability, may provide substrate specificity to Hsp70 (Qiu et al, 2006). The Hsp40 members have in common a conserved J-domain and are subdivided into three distinct groups (DnaJA, DnaJB and DnaJC) based on the presence of certain structural motifs (Cheetham and Caplan, 1998;Kampinga and Craig, 2010). However, among these different Hsp40 members, only DnaJB1 has been studied in detail.…”

“…Moreover, it has been demonstrated that overexpression of Hsp70 and DnaJB1 facilitates the degradation of polyQ expanded forms of the androgen receptor (Bailey et al, 2002). Recently, it has also been shown that less characterized members of the DnaJB family, DnaJB6 and DnaJB8, bind non-foldable clients such as polyQ proteins and keep them in a state competent for (proteasomal) degradation (Hageman et al, 2010;Kampinga and Craig, 2010). This suggests that DnaJ proteins, besides participating in folding also play an important role in preventing the accumulation of misfolded proteins by targeting them for proteasome mediated degradation.…”

Quantitative proteomics and network analysis of SSA1 and SSB1 deletion mutants reveals robustness of chaperone HSP70 network in Saccharomyces cerevisiae

“…They also have known functions in protein translocation across biological membranes and dissolution of aggregates 12. Nascent and newly synthesized polypeptides are bound by HSP70, initially stabilising the unfolded state; upon subsequent release into solution they then reach their native folded state 9.…”