Multivalent protein–protein interactions are pivotal regulators of eukaryotic Hsp70 complexes

Abstract

Heat shock protein 70 (Hsp70) is a molecular chaperone and central regulator of protein homeostasis (proteostasis). Paramount to this role is Hsp70’s binding to client proteins and co-chaperones to produce distinct complexes, such that understanding the protein–protein interactions (PPIs) of Hsp70 is foundational to describing its function and dysfunction in disease. Mounting evidence suggests that these PPIs include both “canonical” interactions, which are universally conserved, and “non-canonical” (or “secondary”) contacts that seem to have emerged in eukaryotes. These two categories of interactions involve discrete binding surfaces, such that some clients and co-chaperones engage Hsp70 with at least two points of contact. While the contributions of canonical interactions to chaperone function are becoming increasingly clear, it can be challenging to deconvolute the roles of secondary interactions. Here, we review what is known about non-canonical contacts and highlight examples where their contributions have been parsed, giving rise to a model in which Hsp70’s secondary contacts are not simply sites of additional avidity but are necessary and sufficient to impart unique functions. From this perspective, we propose that further exploration of non-canonical contacts will generate important insights into the evolution of Hsp70 systems and inspire new approaches for developing small molecules that tune Hsp70-mediated proteostasis.

Elevated circulating Hsp70 levels are correlative for malignancies in different mammalian species

“…Since it is nearly impossible to resuspend the exosomal pellet fraction in ELISA buffer properly, ELISA data of the exosomal fraction were not provided. In a previous study, biophysical characteristics of exosomes were determined by the presence of cytosolic chaperones such as Hsc70, the nucleotide exchange factor Bag4 (Johnson and Gestwicki 2022 ) and the lysosomal marker Rab4 and the absence of ER residing chaperones such as Grp94 or calnexin in the lumen of the exosomes, a size of the microvesicles in the range of 50 to 100 nm, and a peak in the acetyl-choline esterase activity at a density of 1.17 g/mL (Gastpar et al 2005 ). Since these antibodies which are used for human exosomal markers show no cross-reactivity with the antigens of other mammalian species, these markers could not be determined on exosomes derived from other mammalian species apart from humans.…”

Cellular protection from H2O2toxicity by Fv-Hsp70. Protection via catalase and gamma-glutamyl cysteine synthase

“…The protective effect of Fv-Hsp70 may have been through a cellular protein or pathway not studied. Fv-Hsp70 may interact with many non-canonical proteins not yet described (Johnson and Gestwicki 2022). Fv-Hsp70 may be protecting catalase and GGCS prior to H 2 O 2 damage and not after damage as suggested by the activity of Hsp70 homolog DnaK in E.coli (Santra 2018).…”

Alternative ATPase domain interactions in eukaryotic Hsp70 chaperones

“…The rationale put forth was that the binding sequence contained within the BIR domain may not be in an extended and partially unfolded state to which Hsp70 could bind as a substrate, but that binding occurs in an alternative manner. Evidence to support this idea comes from the observation that Hsp70 NBD (lacking the SBD) was sufficient to disrupt binding between Hsp70 and the XIAP 120-356 and as a result of disturbing the non-canonical interaction led to degradation of XIAP ( Cesa et al, 2018 ; Johnson and Gestwicki, 2022 ). Thus, from the data XIAP binds to the NBD of Hsp70, but it is unclear as to what the biological role of this mode of binding is.…”

“…This manifests in Hsp70 interactions that do not fit with the classical client and ATP regulated binding mode and represent a specialised protein interaction that may serve a distinct biological role. The molecular details of such non-canonical interactions are yet to be clearly understood (Larburu et al, 2020;Johnson and Gestwicki, 2022). In this review, we focus on interactions mediated via the ATPase domain of Hsp70 with emphasis on recently characterised associations involving the ER Hsp70 chaperone, BiP, and discuss other adapted eukaryotic Hsp70 systems.…”