Abstract

Since the 1980s, 70 kDa heat shock proteins (HSP70s) have been recognized as central regulators of proteostasis, with diverse roles in cellular physiology and pathology. Recent research has significantly expanded our understanding of these molecular chaperones, revealing functions that extend beyond their classical roles in proteostasis. In this review, we integrate these emerging insights with foundational knowledge by outlining the biology of HSP70s, with particular emphasis on recent discoveries, such as new data on the substrate specificity and molecular dynamics of HSP70–client interactions. In addition, increasing evidence highlights their noncanonical anti-inflammatory properties, as well as other nonimmune functions, including the promotion of adipose tissue browning and the enhancement of angiogenesis through extracellular HSP70 activity. Finally, although HSP70s have long been known to regulate mRNA degradation in a transcript-specific manner, new findings demonstrate their ability to bind double-stranded RNA, further broadening their functional repertoire.

Introduction

Molecular chaperones are cellular components that maintain the integrity of proteome and facilitate proper folding, maturation, and recycling of proteins (Zuppini et al., 2025). Many molecular chaperones are heat shock proteins (HSPs), as they are essential in stress conditions, including temperature-related (Hagymasi et al., 2022). Within this group, the 70-kDa HSP family (HSP70 or HSPA) is crucial in safeguarding protein homeostasis or proteostasis (Mecha et al., 2022; Binder and Pedley, 2023).

The HSP70 family comprises the largest number of members whose functions differ, although with shared patterns of proteostatic activity (Kampinga et al., 2009; Soldatov et al., 2024). Although distinct HSP70s are localized in the mitochondria and endoplasmic reticulum (ER), the principal HSP70 family members are located in the cytosol. These include HSPA8, also known as heat shock cognate protein (HSC70), which is constitutively expressed in the cytosol, and HSPA1, which has two stress-inducible isoforms, HSPA1A and HSPA1B (Kampinga et al., 2009; Venediktov et al., 2023). The structures of HSC70 and HSPA1 are highly similar, and they share most co-chaperones; however, their functional machinery and client protein repertoires differ (Ryu et al., 2020). By managing these client proteins, HSP70s function in two principal modes: facilitating the proper folding of nascent and partially denatured proteins or targeting abnormal and irreversibly damaged proteins for degradation and clearance, a process known as protein quality control (PQC) (Yan et al., 2020).

HSP70s deploy chaperone functions either through an ATP-dependent mechanism, known as foldase activity, which facilitates protein folding (Mayer and Bukau, 2005), or via an ATP-independent mechanism that prevents protein misfolding, referred to as holdase activity (Karunanayake and Page, 2021). In addition to their molecular chaperone roles, HSP70s are involved in various other cellular processes, including the regulation of programmed cell death (Venediktov et al., 2023). Owing to their high intracellular abundance, they also serve as indicators of compromised cellular integrity: once released extracellularly, HSP70s act as damage-associated molecular patterns (DAMPs), triggering immune responses through receptors such as the toll-like receptors TLR2 and TLR4 (Theivanthiran et al., 2022; Wang H. et al., 2025; Hulina et al., 2018). This ubiquitous role reflects their profound integration into multiple signaling and metabolic pathways, making HSP70s a frequent hot spot in pathological conditions, including neurodegeneration, cancer, and inflammation (Craig and Marszalek, 2017; Martinková et al., 2018). Recent evidence shows that HSP70s are involved into transmembrane protein translocation via entropic pulling (Rukes et al., 2024), into the protection of damaged muscle fibers via calcium reuptake from the sarcoplasm to the reticulum (Barfoot et al., 2025), and into the regulation of enzymes that mark proteins with ultraviolet-dependent damage (Zeng et al., 2025). New data have emerged on the role of HSP70s in the regulation of inflammation (Borges et al., 2025), amyloid accumulation (Ruggiero et al., 2025), vascular endothelial function (Pinto-Martinez et al., 2026) and adipose tissue (Zhuang et al., 2025), as well as its effect on viral replication by binding double-stranded RNA (Fletcher et al., 2025). Overall, we aim to synthesize the expanding body of recent research with classical studies (Kao et al., 1985; Subjeck et al., 1985) on HSP70s, providing an integrated perspective that highlights both foundational discoveries and emerging insights.

General overview of HSP70sMembers

The term HSP70 refers to a family of chaperones with a molecular weight of approximately 70 kDa, consisting of 13 members in humans with distinct functions and intracellular localizations (Kampinga et al., 2009), derived from corresponding genes, whereas HSPA7 is usually considered as a pseudogene (Ding et al., 2021), with ambiguous data on HSPA7 role (Li et al., 2022). HSPA4, despite its traditional name, actually belongs to the HSP110 family (Kaneko et al., 1997). Together with the HSP110 family, the HSP70 family is sometimes referred to as the HSP70 superfamily (Kampinga et al., 2009). A general overview of each member is provided in Table 1 and Figure 3, whereas new insights about certain members are discussed in Section Distinct HSP70s.

Family memberLocalizationKey features & functionsReferencesHSPA1A & HSPA1BCytoplasm, nucleus, plasma membrane, exosomesInducible chaperone with two similar isoforms to prevent misfolding under stress conditions, multifaceted interactions with other chaperones in proteostasis, early recompartmentalization to nucleolus in response to heat stress, immune signaling in cell damageCalderwood et al. (2016), Deane and Brown (2017), Shevtsov et al. (2018), Xiao et al. (2025), Zuo et al. (2025)HSPA1LCytoplasm, nucleusNon-inducible and low abundant HSPA1 isoform, promotes translocation of certain damaged proteins from organellesHasson et al. (2013)HSPA2Cytoplasm, nucleus, exosomes, extracellular vesiclesCellular differentiation as well as signaling during differentiation and response to cell damageSojka et al. (2023), Gogler et al. (2025)HSPA3ExcludedHSPA4, HSPA4LBelongs to the HSP110 familyHSPA5/GRP78/BiPEndoplasmic reticulum, exosomesEndoplasmic reticulum–stress and cell cycle controlHetz et al. (2020); Du et al. (2025)HSPA6Cytoplasm, perinuclear zone, exosomesInducible chaperone with late relocalization to nucleolus in response to heat stressDeane and Brown (2017)HSPA7PseudogeneHSPA8/HSC70Cytoplasm, nucleus, cell membranesKey actor in protein quality control, especially in chaperone-mediated autophagy and chaperone-assisted selective autophagyCalderwood et al. (2016), Qiao et al. (2023), Ulbricht et al. (2013)HSPA9/mtHSP70/GRP75/mortalinMitochondriaMaintenance of mitochondrial proteostasis and electron transporting chain components assembly, especially proteostasis in damage by reactive oxygen speciesSong et al. (2023), Bakovic et al. (2025), Acquarone et al. (2025)HSPA10Belongs to the HSP110 familyHSPA11Non-existentHSPA12ACytoplasmProteostasis in proteins participating in metabolic regulationHan et al. (2003), Yu et al. (2024)HSPA12BCytoplasm, exosomesEndothelial isoform, immune signaling in endothelial damageRadons (2016), Fan et al. (2020)HSPA13Endoplasmic reticulum, exosomesModulation of protein translocation, control of nascent proteinsEspinoza et al. (2022)HSPA14Cytoplasm, plasma membraneInducible chaperone with assistance in proper folding during protein translationVenediktov et al. (2023), Radons (2016)

Members of the HSP70 family. HSP70s may be stress inducible or not inducible and reside in different cell sites at different concentrations, thereby affecting their function.

Structure and mode of action

A molecule of HSP70 consists of an N-terminal nucleotide-binding domain (NBD), a substrate-binding domain (SBD), a linker region connecting the two, and variable C-terminal motifs such as EEVD (Flaherty et al., 1990) (Figure 1). In turn, the SBD includes two functional parts: a β subdomain of 8 β chains and an α subdomain of 4–5 α chains (Zhu et al., 1996; Stevens et al., 2003). The substrate binding by the SBD is implemented in two steps: after binding to the client protein via SBD-ß, HSP70s trap it by closing SBD-α, serving as a cap or lid (Wang et al., 1998; Zhang et al., 2014).

Molecular structure of HSP70 and its cycle. The structure of HSP70 (on the right) includes a nucleotide-binding domain (NBD) for binding to NEF, a substrate-binding domain (SBD) for binding to polypeptides, a linker for binding to HSP40, and a C-terminal domain for interaction with co-chaperones HOP, CHIP or HSP40. Classical protein folding (on the left) involves the interaction of HSP40 and HSP70 with the participation of ATP. However, protein formation in ribosomes without the participation of HSP40 is also possible. Folding proteins with a more complex structure require the participation of NEFs, co-chaperones, HSP90, or HSP110. If protein folding is impossible, the aberrant molecule is destroyed by UPS or HSP100-associated disaggregation. Created with BioRender.

Generally, HSP70 binding sites in polypeptide chains repeat every ∼36 residues, mainly in β-sheets with four to five residues, such as leucine, isoleucine, valine, phenylalanine, and tyrosine (Rüdiger et al., 1997). The following ATP‒ADP transition in the NBD “closes” the flexible double-hinged lid of the SBD, preventing the client protein from leaving (Marszalek, 2022; Kumar et al., 2023). After the binding of a new ATP molecule in the NBD, the client protein leaves the substrate binding pocket (Qi et al., 2013). The cycles of binding and release can be repeated multiple times, after which the substrate is either released into the cytoplasm to exert its functions or transferred to other chaperone machines, such as Hsp90 (Lang et al., 2021).

HSP70s exert three major activities on client proteins: 1) preventing nascent proteins from misfolding and facilitating their proper folding; 2) preventing aggregation of mature proteins; and 3) solubilizing or refolding aggregated proteins (Mayer and Bukau, 2005). When interacting with nascent and mature proteins, HSP70s bind hydrophobic patches via the SBD, thus preventing spontaneous lipophilic cross-interactions. When acting on aggregated proteins, HSP70s perform the same mechanism of binding to exposed hydrophobic patches, allowing for reassembly of the compromised structure (although details have yet to be understood mechanistically). For this mechanism, HSP70s recruit different co-chaperones for specific activities: e.g., in mammals, HSP40 is involved in folding, and HSP40/HSP110 are involved in refolding (Mauthe et al., 2025a; Mauthe et al., 2025b). These interactions are probably species-specific. Thus, human HSP70s, apparently, fail to recognize client proteins of other species, for example, of Escherichia coli (Ambrose et al., 2024).

New details of molecular HSP70 action have recently been observed by an in silico research (Mahto et al., 2024). Previously, it had been reported about the structures that allow lid opening in SBD when releasing the substrate (PDB 4JN4) (Qi et al., 2013). Mahto and colleagues have revealed the lid opening to be greater than anticipated. In addition, recent data elucidate the physical nature of the binding of client proteins to HSP70 during their translocation across biological membranes. Essentially, proteins synthesized in the cytoplasm must unfold to pass through compact membrane channels to enter organelles, and the process of subsequent refolding requires the assistance of HSP70s. The conventional explanation of how HSP70s bind to unfolded client proteins that escape channels involves three disputing theories: 1) the Brownian ratchet theory, which suggests that a passive HSP70 plays a role in enveloping client proteins and limiting their movement; 2) the power stroke theory, which proposes that a strong transformation of an HSP70 molecule to a client protein occurs; and 3) the entropic pulling theory, which postulates that HSP70 increases entropy via client protein binding and therefore moves forward to obtain a more thermodynamically appropriate conformation. A recent study (Rukes et al., 2024) provided unambiguous evidence supporting the Entropic Pulling theory. Using an elegant approach based on biological nanopore sensors incorporated into artificial lipid membranes, the authors monitored the escape of various substrates from the pore. Despite opposing electric forces that hinder substrate escape, the presence of HSP70 significantly facilitates translocation by pulling the substrate to the opposite side of the membrane (Rukes et al., 2024).

In addition, HSP70s begin protecting proteins from misfolding as soon as the first segments of the polypeptide chain emerge from the ribosome. However, possible mistranslation, which is caused by mutations in tRNA genes, may alter the protective activity of HSP70 on nascent proteins (Lant et al., 2018). Recently, McDonald and colleagues revealed that frequent mistranslation events involving a shift from serine to either proline or arginine have distinct effects: serine–to-proline substitution reduces the ability of polypeptide chains to bind HSP70s, whereas serine–to-arginine substitution prevents misfolded nascent proteins from being denatured and degraded (McDonald et al., 2025).

Regulation of HSP70 levels

HSP70s likely bear the primary burden of cellular adaptation to various stress factors, more so than other molecular chaperones do. For example, under sustained heat stress, HSP70 levels increase more significantly than those of other heat shock proteins (Albokhadaim, 2025). However, HSP70 expression reflects adaptation not only to external environmental conditions but also to intrinsic factors, such as age. For example, human studies have shown a marked decline in HSP70 levels in older individuals (Tandara et al., 2006; Rea et al., 2001), what is recently approved in ruminants (Kaushik et al., 2022).

However, influenced by various signaling pathways, HSP70 upregulation is driven primarily by heat shock factors, particularly HSF1, which are involved in a complex regulatory network (Figure 2). HSF1 is a transcription factor that undergoes trimerization and multiple posttranslational modifications, including phosphorylation, in response to heat stress. Upon activation, HSF1 trimers translocate to the nucleus and bind to heat shock elements (HSEs) in the promoters of target genes, thereby increasing the transcription of HSP70s, which then undergo degradation or re-monomerization (Vihervaara and Sistonen, 2014). Importantly, even though HSF1-driven HSP70 upregulation is mediated by gene expression regulation, at the whole-cell level, HSP70 tends to be distributed unevenly across the cell in a demand-dependent manner. For example, a recent study revealed that oxidative damage following ischemic assault resulted in the upregulation of HSP70, specifically in astrocyte endfeet (Shim et al., 2025).

HSF1/HSP70 axis. At the center, cellular stress signalization by pathogen-, damage-, and microbial-associated molecular patterns, as well as chemical and physical factors, affects heat shock factor 1 (HSF1), which experiences trimerization and binds to heat shock elements (HSEs) on promoters for multiple chaperones. Trimerized HSF1 is destined for proteasomal degradation after ubiquitination by ligases or for monomerization. Moreover, synthesized chaperones have various biochemical structures (on the right) and functions (on the left), regulating HSF1 levels, apoptosis, and protein quality control. Created with BioRender.

HSF1 serves as a sensor of various modalities that detect disturbances in homeostasis and activate HSP expression. For example, exposure to physical stimuli of suprathreshold intensity—such as heat or mechanical forces (Gong et al., 2012) — as well as oxygen deprivation (Shim et al., 2025), can upregulate HSF1 expression. Multiple biochemical pathways that modulate HSF1 activity have also been identified. Sirtuin 1 (SIRT1) and the insulin-like growth factor receptor (IGFR) are among the most pharmacologically relevant, although not exhaustive, examples of stimulators of the HSF1–HSP70 axis (Vihervaara and Sistonen, 2014; Westerheide et al., 2009). Transient receptor potential vanilloid 1 (TRPV1) — primarily known for mediating high-temperature sensation—also directly regulates HSF1. This enables TRPV1-mediated regulation of HSP70 by capsaicin (Bevan et al., 2014) or, less classically, by cannabidiol (Ma et al., 2025). In addition, HSF1 may act directly in proteostasis without upregulating HSP70, as there is evidence of HSF1 binding to defective proteins such as amyloid oligomers (Tang et al., 2020).

Intrinsic control of HSP70 activity

In addition to the levels of HSP70s, their biochemical activity may also be finely tuned to adapt to cellular demands. In addition to the presence of ATP and natural co-chaperones such as HSP40s, this activity may be strongly activated or inhibited, principally affecting the efficiency of HSP70-driven proteostasis. Recently, new efforts have been made to design molecular constructs that mimic or replace co-chaperones, thereby increasing HSP70 activity (Zhang et al., 2025).

HSP70s may control their own activity by changing conformation or joining into an oligomeric structure. The foldase activities of HSP70s require ATP, which binds in a Mg2+- and K+-dependent manner (Bercovich et al., 1997; Mas and Hiller, 2025). Nevertheless, the content of ionized calcium also affects the rate of ATP usage, at least for HSP70s in the ER (Mas and Hiller, 2025). Importantly, the kinetics of foldase activity depend on the type of nucleotide exchange factor (NEF) employed to provide the ATP‒ADP transition. These factors also affect the functions of certain HSP70s but have different degrees of affinity for HSP70s. For example, among the NEFs, BAG3 affinity for HSP70s is the highest, with a lower affinity for BAG1, followed by HSP110 and BAG2 (Rauch and Gestwicki, 2014). In addition to the electrolytic content and NEF involvement, the monomeric/oligomeric shift of HSP70 also affects the mode of ATP recruitment. Certain cellular activities, such as clathrin removal by HSC70, require a trimeric HSP70 (Coimbra et al., 2025).

Distinct HSP70sHSPA1A/B and HSPA8/HSC70

Although the HSP70 family generally includes many members (Table 1; Figure 3), its basic actors, constitutive HSPA8 (also known as HSC70 and HSP73) and isoforms of inducible HSPA1, carry out most of the functions of the cytosolic response to protein misfolding or aggregation. Although HSPA1A (also known as HSP72) and HSPA1B differ in several respects, they can respond in a coordinated manner to cellular damage, as recently demonstrated in the cardiac muscle tissue of C57Bl/6 mice exposed to black carbon (Zuo et al., 2025). However, our current understanding of HSPA1 remains limited—likely just the tip of the iceberg—as emerging evidence suggests that it may participate in numerous, previously unrecognized molecular pathways. For example, recent findings indicate that HSPA1B facilitates the exosomal secretion of metalloproteinases from macrophages, thereby helping to mitigate fibrosis (Xiao et al., 2025). Notably, this study did not investigate the role of HSPA1A.

HSP70 family members. At the center, HSP72 (HSPA1A, inducible HSP70) and HSP73 (HSC70/HSPA8, constitutive HSP70) account for principal HSP70s, reflecting key mechanisms to prevent misfolding and to provide protein quality control by proteasomal degradation, optionally replaced by autophagy. Lower right/left, respectively: HSPA5 (GRP78/BiP), ER-related HSP70, and mitochondrial HSP70 (mortalin/HSPA9), both of which are stress inducible and encoded by nuclear DNA, control proteostasis associated with corresponding organelles. “Minor” HSP70s are found in relatively low amounts, although they take part in vital functions such as growth regulation via the Salvador-Warts-Hippo pathway by HSPA6. Upper left: changes in the conventional HSP70 classification. Created with BioRender.

Inducible HSPA1 plays a critical role in maintaining cytosolic protein stability under stress conditions (Yenari et al., 1999). For example, in contracting skeletal muscle tissue, the sarcoplasmic reticulum Ca2+-ATPase (SERCA) facilitates calcium reuptake into the reticulum, preventing calcium overload in the sarcoplasm—likely in synergy with HSPA5 (Mázala et al., 2024). SERCA has already been shown to remain functionally intact following heat-induced damage owing to the protective activity of HSPA1 (Fu and Tupling, 2009). Recently, murine SERCA was also reported to mediate calcium reuptake from the sarcoplasm in mechanically damaged tissues after heat stress through an HSPA1-dependent mechanism (Barfoot et al., 2025).

In addition to stabilizing cytosolic proteins, HSPA1A also protects proteins of membrane organelles from damage. Recently, it was shown that the pathogenicity of mycobacteria in tuberculosis may involve the destruction of HSPA1A and its routing to proteasomal degradation. This occurs through noncatalytic stimulation by cytosolic cis-aconitate decarboxylase 1, which is upregulated in response to mycobacterial infection (Yang et al., 2025).

Some proteins are stabilized by both HSPA1 and HSC70. Initially, these chaperones together provide a proper folding of nascent proteins closer to 80S ribosomal subunit in eukaryotes (Han B. et al., 2025). Further, in cytosol, they also maintain the stability of protein kinase B (AKT), a key regulator of cell survival (Koren et al., 2010). However, because AKT is often overactivated in cancers, this chaperone-mediated stability can contribute to pathological processes. Recently, the circadian clock gene Period2 was shown to be overexpressed, and its protein product inhibits the binding of HSP70 to AKT (Yu et al., 2025). Thus, the PER2-dependent mechanism disrupts AKT proteostasis, potentially altering cell fate.

Despite the prominent role of HSPA1 in protecting proteins from damage, many autophagic processes require the suppression of HSPA1 activity. Earlier studies proposed that macroautophagy, regulated by p62, is accompanied by increased activity of most HSP70s (Sheng et al., 2012). However, heat stress-induced upregulation of HSF1 and HSP70 silences macroautophagy stimulators such as mitogen-activated protein kinase (MAPK) through mechanisms involving the mammalian target of rapamycin (mTOR) pathway (Alhasan et al., 2024), as demonstrated in cell culture models. Consistently, activation of macroautophagy is accompanied by decreases in HSPA1 and HSPA5 (ER-associated HSP70) levels (Sattari et al., 2025).

In contrast to HSPA1, HSC70 is more prone to provide PQC and is capable of maintaining proteome stability via both protein routing to proteasomes and autophagy. Generally, ensuring solubilization and preventing misfolding is a primary event of HSP70 activity, whereas the ubiquitin–proteasome system (UPS) is a compensatory mechanism to degrade proteins that are irreversibly damaged or become damaged at excessive levels unable to be refolded; autophagy is the next line of compensation active when the UPS cannot degrade proteins, especially during senescence (Feleciano et al., 2019). Thus, the UPS is a vital first-line component of PQC, and HSC70 is well known to recruit it when it joins its co-chaperone, CHIP (Shimura et al., 2004; Zhang et al., 2020). Importantly, HSPA1 can be degraded by the UPS, and CHIP blockade prevents this degradation, which is a useful tool for slowing the pace of cell death (e.g., in cardiovascular pathology) (Lin et al., 2025).

HSC70 has been recently shown to ensure proper interaction between S-phase kinase-associated protein 1/cullin 1/F-box protein (SCF) and constitutive photomorphogenesis 9 signalosomes (CSNs) (Nishimura et al., 2025). This interaction marks regulatory enzymes for ubiquitination if they are damaged by physical factors, especially ultraviolet light or radiation (Lyapina et al., 2001). For example, the stability of HSP90 molecules is provided by the SCF–CSN machinery (Zeng et al., 2025). This role of HSC70 clearly contributes to the overall functioning of the UPS.

Despite this “multifaceted hiring” in PQC, HSP70s, which represent the most common group of diseases associated with gradually worsening proteostasis, have long been considered promising tools for hindering neurodegenerative pathology. Unfortunately, the efficacy of elevated HSP70 levels and/or activity is much more evident in vitro and in vivo than in clinical studies, as we noted earlier (Venediktov et al., 2023). For example, in amyotrophic lateral sclerosis (ALS), HSP70s may modulate mutant proteins such as SOD1, FUS, C9orf72, and TARDBP by preventing their solidification or facilitating their disaggregation and/or clearance via autophagy. Recently, Takeda and colleagues reported that mutant HSC70 — nominally beneficial for modifying the SOD1–ALS phenotype—paradoxically exacerbated symptoms in mice despite reducing SOD1 content (Takeda et al., 2025). However, our recent research revealed another mode of HSP70 involvement. Briefly, mice exhibiting the FUS–ALS phenotype (characterized by FUS translocation from the nucleus to the cytoplasm) demonstrated longer lifespan, reduced disease severity and improved histological patterns when intracellular HSPA1A was overexpressed (Piavchenko et al., 2024; Piavchenko et al., 2025a; Piavchenko et al., 2025b). These findings suggest that, compared with HSC70, HSPA1 may have a stronger protective effect in this context, although differences in the affinities of SOD1 and FUS for certain co-chaperones—and thus distinct PQC strategies—may also play a role.

HSPA2

HSPA2, previously considered a relatively minor member of the HSP70 family, is now recognized as playing key roles in cell growth and mitosis within epithelial tissues, as well as participating in extracellular signaling (Sojka et al., 2023). Recently, Gogler and colleagues demonstrated that HSPA2 is a crucial factor in keratinocyte differentiation and migration to the strata spinosum and granulosum (Gogler et al., 2025). Moreover, their research revealed that HSPA2 knockout (KO) induces a proinflammatory cytokine secretion profile, accompanied by increased expression of receptors involved in antigen presentation.

HSPA5/GRP78/BiP

The accumulation of unfolded or misfolded proteins in the ER activates a signaling pathway known as the unfolded protein response (UPRER), which is regulated by three main sensors: protein kinase RNA-like ER kinase (PERK), inositol-requiring enzyme 1α (IRE1α), and activating transcription factor 6 (ATF6). These three regulators, especially PERK and IRE1α, closely interact with HSPA5, or glucose-regulated protein 78 (GRP78) (Hetz et al., 2020), which is a crucial ER-associated HSP70 family member. Thus, GRP78 is extremely important for the export of proteins from the cell. For example, GRP78 has been recently reported as a key chaperone preventing misfolding of coagulation factor VIII; therefore, its stability is pivotal in hemophilia type A molecular pathology (Srivastava et al., 2025).

However, GRP78 can be transferred to mitochondria and lysosomes, especially via ER-adjacent portions of their membranes, and can be transported to the cytosol to be secreted from cells. In addition, GRP78 is normally located in the ER at low levels, and its expression (but not its functional rate) may be upregulated by calcium ionophores, calcium depletors or chelators, and inhibitors of the protein secretory pathway (Casas, 2017). Selective inhibitors of GRP78 found in silico with possible benefits in ER stress-related tumor treatment (Ambrose et al., 2023). Nanobodies with targeted immunotoxin delivery have recently been reported to successfully suppress GRP78, too (Wang H. et al., 2025).

Human GRP78 activity has also been shown in vitro to be upregulated by its posttranslational modification with cell filamentation protein (FIC) (Sanyal et al., 2015). Consistent data were obtained by Truttmann and colleagues for orthologs of GRP78 (HSP3 and HSP4) and FIC (FIC-1) in Caenorhabditis elegans (Truttmann et al., 2016). However, the same team has recently reported a FIC KO to improve the PQC in the ER of C. elegans (Van Pelt and Truttmann, 2025). Moreover, in this work, Van Pelt and Truttmann mentioned an HSP70 member of the nematodes, F44E5.4 (initially cytosolic), to manage PQC in the ER in depletion of GRP78 orthologs, at least for the clearance of mutant polyglutamine proteins.

Thus, cytosolic HSP70 may affect proteostasis in the ER in the absence of active GRP78 isoforms, although the distinct mechanisms involved remain to be elucidated. However, the aforementioned involvement of cytosolic F44E5.4 in ER-related proteostasis in C. elegans may be not applicable to humans, as the ER–HSP70 systems of the two species differ greatly, at least because GRP78 is the only ER-associated HSP70 in humans, although it has two orthologs in C. elegans. Moreover, some points of the overall machinery are similar. Both C. elegans and Homo sapiens are able to translocate ER proteins for lysosomal eradication via macroautophagy via GRP78-IRE1α mediation of the UPRER and further recruitment of translocon Sec-62 (in worms, an orthologous C18E19.2) (Fumagalli et al., 2016; Urban et al., 2025).

In addition, GRP78 activity is related to the regulation of the cell cycle. Recently, Du and colleagues demonstrated that cyclin-dependent kinase 1 (CDK1) inactivation at the end of mitosis enhances GRP78-mediated proteostasis, especially via the UPS, in epithelial cells from breast tumors (MCF10A line). In addition to regulating the cell cycle, the rate of autophagy also influences GRP78 activity (Du et al., 2025). Specifically, recent studies in a model of ischemia/reperfusion injury in mice demonstrated that GRP78 activity was suppressed by the overexpression of p62, a macroautophagy driver (Quan et al., 2025). Moreover, p62-related stimulation of macroautophagy prevents protein routing to the UPS (Liu et al., 2016). In addition, p62 recruits kelch-like enoyl-coenzyme A hydratase-associated protein 1 (KEAP1) for proper autophagosome formation; in suppressed p62, KEAP1 is known to increase cell growth and resistance to ROS via nuclear factor erythroid 2-related factor 2 (NRF2) activation (Tkachev et al., 2011; Ichimura et al., 2013). Therefore, a higher rate of damaged protein routing to macroautophagy is accompanied by lower productivity of the cytosolic UPS but increased GRP78 function and NRF2-mediated effects at the same time.

HSPA9/mortalin/mtHSP70/GRP75

HSPA9, also known as mortalin, is a constitutive but inducible mitochondrial chaperone involved in multiple functions related to proteostasis and apoptosis. For example, ATP synthase—a vital mitochondrial enzyme—requires HSPA9 for the proper assembly of its motor components, F0 and F1. In addition, HSPA9 helps prevent the degradation of these components (Song et al., 2023).

HSPA9 levels sharply increase in response to mitochondrial damage, such as excessive reactive oxygen species (ROS) production. Elevated HSPA9 expression has recently been confirmed in patients with heart failure, particularly in those with poorer prognoses (Bakovic et al., 2025). In contrast, age-related mitochondrial changes are associated with reduced HSPA9 expression and decreased ER–mitochondria membrane coupling, leading to impaired protein degradation and diminished mitochondrial calcium uptake (Acquarone et al., 2025).

HSPA12B

HSPA12B is an endothelium-specific isoform of the HSP70 family (Han Z. et al., 2003). Its function has been shown to be agonistic with endothelial nitric oxide synthase (eNOS) (Li J. et al., 2013). Although normally cytosolic, HSPA12B can be released from endothelial cells upon damage, where it promotes the acquisition of a pro-regenerative phenotype in macrophages via TLR4 signaling (Doan et al., 2009) and the PI3K–AKT–mTOR pathway (Zhou et al., 2020). The regenerative nature of this response has been further clarified in a recent study: Wang and colleagues reported that HSPA12B is internalized by macrophages through endocytosis, subsequently downregulating TLR4 signaling (Wang Y. et al., 2025). Thus, HSPA12B appears to act through both eNOS activation and TLR4 modulation, likely contributing to the mitigation of tissue damage.

HSP70 interactomeHSP40/DNAJ

Molecular chaperones such as HSP40s, or DNAJs, assist HSP70s in their foldase activity (Venediktov et al., 2023) (Figure 4). Traditionally, these co-chaperones are thought to interact with HSP70 molecules solely via the N-terminal J domain of HSP40s, without the involvement of other regions—particularly the C-terminal domain and the intermediate glycine/phenylalanine-rich (GF) linker region. However, recent data revealed that the GF region also contributes to HSP70 binding, influencing the kinetics of HSP70-driven reactions—at least for HSC70 (Hobbs et al., 2025). Despite this, HSP40s are highly diverse, and certain HSP40s serve distinct client proteins while cooperating with the same HSP70 isoform, typically HSC70 (Kampinga and Craig, 2010), thereby conferring functional specificity to HSP70s (Bhattacharjee et al., 2025; Wu et al., 2025). For example, routing toward protein clearance is mediated by HSP70 in cooperation with DNAJB6, as synthesized from multiple studies in a recent review (Hentze et al., 2025). Liquid-liquid phase separation is affected by HSP70/HSP40 interaction as well as protein clearance does (cytosolic translocation of TDP-43) (Yeo et al., 2025).