Heat shock proteins are a group of conserved chaperone proteins whose expression rises sharply when cells meet stress — heat, oxidative damage, heavy metals, or the proteotoxic burden of rapid growth. They are named by approximate molecular weight (HSP100, HSP90, HSP70, HSP60, HSP40, and the small HSPs around 20–30 kDa), but the number is a size label, not a single gene. Each family contains multiple members resident in different compartments — cytosol, endoplasmic reticulum, mitochondria — and together they manage the folding, trafficking, and disposal of the proteome. This compartmental and family-wide coordination is what allows the cell to maintain protein homeostasis across diverse stresses.

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How the chaperone families are organized

The families cooperate as a rough assembly line rather than acting alone, and their activity is governed at the transcriptional level mainly by heat-shock factor 1 (HSF1). The core handoff is the key idea: HSP40 (the J-domain proteins) recognizes unfolded polypeptides and delivers them to HSP70, which uses ATP to bind and release substrates, giving them a chance to fold or passing them on. HSP70 hands selected clients to HSP90, which — with an army of co-chaperones — matures a specific set of client proteins. HSP60 (chaperonins) form barrel-shaped chambers that cage proteins for folding, HSP100 members disaggregate or unfold proteins in an ATP-driven spool, and the small HSPs act as holdases that bind denaturing proteins to prevent aggregation. This division of labor is why perturbing one family affects a defined slice of the proteome rather than protein folding as a whole.

Figure 1. The proteostasis network of heat shock proteins. (Source: Hu C, et al. 2022)

The heat-shock response that turns them on

At the transcriptional level, the system is governed largely by heat-shock factor 1 (HSF1). In calm conditions HSF1 is monomeric and held inactive by chaperones including HSP90; under stress, accumulating unfolded protein liberates HSF1, which trimerizes, enters the nucleus, and drives transcription of HSP genes. This negative-feedback loop — chaperones repress their own transcription factor — lets the cell scale protection to demand. In cancer and chronic disease, HSF1 is often constitutively active, keeping a high chaperone tone that helps malignant cells survive proteotoxic and metabolic stress.

The HSP families at a glance

Each family occupies a distinct mechanistic and compartmental niche. The table below summarizes the principal HSP families, their representative members, their cellular compartment, and their core function — the map before we zoom in on the members that dominate research and therapy.

The full family tree is useful as a map, but the field's energy concentrates on a handful of members that lead both drug discovery and biomarker work. The sections that follow spotlight HSP90 and HSP70 — the two families at the center of chaperone-targeted therapy — and then HSP60, HSP100, HSP40, and the small HSPs, the families that complete the folding network.

HSP90 — the cancer-relevant maturation machine

HSP90 is an ATP-dependent, usually dimeric chaperone that, together with a large cohort of co-chaperones (including immunophilins, the kinase-targeting adaptor CDC37, and p23), matures a defined set of "client" proteins. Its clientele is disproportionately made of signaling nodes — receptor tyrosine kinases, RAF, AKT, cyclin-dependent kinases, steroid receptors, and several E3 ligases — which is precisely why HSP90 is such a compelling oncology target: blocking it causes the simultaneous degradation of many clients, attacking multiple hallmarks of cancer at once rather than one pathway. HSP90 is compartmentalized into isoforms with distinct client sets (HSP90α/β in the cytosol, Grp94/endoplasmin in the endoplasmic reticulum, TRAP1 in mitochondria), so its reach spans the cell. The therapeutic catch is the heat-shock feedback: inhibition liberates HSF1 and induces more chaperones, and healthy cells also depend on HSP90, which narrows the therapeutic window and keeps selectivity an active frontier of medicinal chemistry.

HSP70 — the central folding hub

HSP70 is the most conserved and most central chaperone, powered by ATP-driven bind-and-release cycles that are tuned by J-domain (HSP40) co-chaperones and nucleotide-exchange factors. It captures nascent polypeptide chains, prevents aggregation, assists translocation into organelles, and selects which clients are handed onward to HSP90. Cytosolic members (inducible HSP70-1A/B, the constitutive HSC70, the ER-resident BiP/Grp78, and mitochondrial mortalin) give the family both housekeeping and stress roles, and its basal abundance is high across tissues. Beyond folding, HSP70 interfaces with cell death and proteostasis, which is why it is pursued both as a direct target and as a sensitizer — blocking it makes tumor cells more vulnerable to proteotoxic and therapeutic stress. Its very abundance and essentiality, however, make selective inhibition technically demanding, a reason much effort has focused on HSP90 first.

Together, HSP90 and HSP70 account for the bulk of chaperone-focused drug discovery and biomarker research, because their clientele sits at the heart of cellular signaling and because their inhibition produces broad, simultaneous effects that single-pathway agents cannot match. The remaining families are no less essential to the folding network — they simply offer more targeted, often less toxic points of intervention.

HSP60, HSP100, HSP40, and the small HSPs — the supporting cast

The other families complete the network and are increasingly attractive when a more defined node is desired. HSP60 (chaperonins) and its cytosolic cousin CCT/TRiC build barrel-shaped chambers that cage a substrate protein and fold it in isolation from the crowded cytosol — mitochondrial HSPD1 being the best-studied example. HSP100 (the Clp family) uses ATP-driven spooling to thread and disaggregate or unfold proteins, a role that becomes critical under severe proteotoxic stress. HSP40, the J-domain proteins, are the adaptors that actually recognize unfolded substrate and deliver it to HSP70; their large DNAJA, DNAJB, and DNAJC subfamilies effectively set HSP70's specificity, making them central to where HSP70 acts. The small HSPs (HSP27, αB-crystallin, HSP20) are ATP-independent holdases that bind denaturing proteins to prevent aggregation and are themselves regulated by phosphorylation; they are linked to stress resilience and are widely used as stress and injury markers. Perturbing any one of these families reshapes a specific part of proteostasis without the systemic feedback that HSP90 inhibition triggers.

HSPs as biomarkers of stress and disease

Because HSP levels rise with cellular stress, they are studied as biomarkers across cardiology, neurology, and oncology. Extracellular or circulating HSP70 and HSP90, and mitochondrial HSP60, have been examined as indicators of tissue injury and immune activation; in tumors, high HSP expression often tracks with aggressiveness and, paradoxically, with immune visibility, since some chaperones can present antigen. Small HSPs such as HSP27 are markers of certain injuries and are themselves regulated by phosphorylation. The literature is large and sometimes conflicting, but the core idea — that chaperone abundance reports on proteostatic strain — is well established.