German cockroach extract prevents IL-13-induced CCL26 expression in airway epithelial cells through IL-13 degradation.

Inhaled aeroallergens can directly activate airway epithelial cells (AECs). Exposure to cockroach allergens is a strong risk factor for asthma. Cockroach allergens mediate some of their effects through their serine protease activity; protease activity is also a major contributor to allergenicity. The Th2 cytokine interleukin-13 (IL-13) induces upregulation of the eosinophil chemotactic factor CCL26. CCL26 induces eosinophil migration in allergic inflammation. In this work, we studied the effect of cockroach proteases on IL-13-induced effects. Immersed cultures of the human bronchial epithelial cell line BEAS-2B and air-liquid interface (ALI) cultures of primary normal human bronchial epithelial (NHBE) cells were stimulated with IL-13, Blattella Germanica cockroach extract (CE), or both. IL-13-induced genes were analyzed with qRT-PCR. IL-13 induced upregulation of CCL26, periostin, and IL-13Rα2 in bronchial epithelial cells which were decreased by CE. CE was heat-inactivated (HICE) or pre-incubated with protease inhibitors. HICE and CE preincubated with serine protease inhibitors did not prevent IL-13-induced CCL26 upregulation. CE-degraded IL-13 and specific cleavage sites were identified. CE also decreased IL-4-induced CCL26 upregulation and degraded IL-4. Other serine proteases such as bovine trypsin and house dust mite (HDM) serine proteases did not have the same effects on IL-13-induced CCL26. We conclude that CE serine proteases antagonize IL-13-induced effects in AECs, and this CE effect is mediated primarily through proteolytic cleavage of IL-13. IL-13 cleavage by cockroach serine proteases may modulate CCL26-mediated effects in allergic airway inflammation by interfering directly with the pro-inflammatory effects of IL-13 in vivo.

Monkeypox virus infection of human astrocytes causes gasdermin B cleavage and pyroptosis.

Monkeypox virus (MPXV) infections in humans cause neurological disorders while studies of MPXV-infected animals indicate that the virus penetrates the brain. Pyroptosis is an inflammatory type of regulated cell death, resulting from plasma membrane rupture (PMR) due to oligomerization of cleaved gasdermins to cause membrane pore formation. Herein, we investigated the human neural cell tropism of MPXV compared to another orthopoxvirus, vaccinia virus (VACV), as well as its effects on immune responses and cell death. Astrocytes were most permissive to MPXV (and VACV) infections, followed by microglia and oligodendrocytes, with minimal infection of neurons based on plaque assays. Aberrant morphological changes were evident in MPXV-infected astrocytes that were accompanied with viral protein (I3) immunolabelling and detection of over 125 MPXV-encoded proteins in cell lysates by mass spectrometry. MPXV- and VACV-infected astrocytes showed increased expression of immune gene transcripts (IL12, IRF3, IL1B, TNFA, CASP1, and GSDMB). However, MPXV infection of astrocytes specifically induced proteolytic cleavage of gasdermin B (GSDMB) (50 kDa), evident by the appearance of cleaved N-terminal-GSDMB (30 kDa) and C-terminal- GSDMB (18 kDa) fragments. GSDMB cleavage was associated with release of lactate dehydrogenase and increased cellular nucleic acid staining, indicative of PMR. Pre-treatment with dimethyl fumarate reduced cleavage of GSDMB and associated PMR in MPXV-infected astrocytes. Human astrocytes support productive MPXV infection, resulting in inflammatory gene induction with accompanying GSDMB-mediated pyroptosis. These findings clarify the recently recognized neuropathogenic effects of MPXV in humans while also offering potential therapeutic options.

Data-Independent Acquisition Proteomics and N-Terminomics Methods Reveal Alterations in Mitochondrial Function and Metabolism in Ischemic-Reperfused Hearts.

Myocardial ischemia-reperfusion (IR) (stunning) injury triggers changes in the proteome and degradome of the heart. Here, we utilize quantitative proteomics and comprehensive degradomics to investigate the molecular mechanisms of IR injury in isolated rat hearts. The control group underwent aerobic perfusion, while the IR injury group underwent 20 min of ischemia and 30 min of reperfusion to induce a stunning injury. As MMP-2 activation has been shown to contribute to myocardial injury, hearts also underwent IR injury with ARP-100, an MMP-2-preferring inhibitor, to dissect the contribution of MMP-2 to IR injury. Using data-independent acquisition (DIA) and mass spectroscopy, we quantified 4468 proteins in ventricular extracts, whereby 447 proteins showed significant alterations among the three groups. We then used subtiligase-mediated N-terminomic labeling to identify more than a hundred specific cleavage sites. Among these protease substrates, 15 were identified following IR injury. We identified alterations in numerous proteins involved in mitochondrial function and metabolism following IR injury. Our findings provide valuable insights into the biochemical mechanisms of myocardial IR injury, suggesting alterations in reactive oxygen/nitrogen species handling and generation, fatty acid metabolism, mitochondrial function and metabolism, and cardiomyocyte contraction.

Application of N-Terminal Labeling Methods Provide Novel Insights into Endoproteolysis of the Prion Protein in Vivo.

Alternative α- and ß-cleavage events in the cellular prion protein (PrPC) central region generate fragments with distinct biochemical features that affect prion disease pathogenesis, but the assignment of precise cleavage positions has proven challenging. Exploiting mouse transgenic models expressing wild-type (WT) PrPC and an octarepeat region mutant allele (S3) with increased ß-fragmentation, cleavage sites were defined using LC-MS/MS in conjunction with N-terminal enzymatic labeling and chemical in-gel acetylation. Our studies profile the net proteolytic repertoire of the adult brain, as deduced from defining hundreds of proteolytic events in other proteins, and position individual cleavage events in PrPC α- and ß-target areas imputed from earlier, lower resolution methods; these latter analyses established site heterogeneity, with six cleavage sites positioned in the ß-cleavage region of WT PrPC and nine positions for S3 PrPC. Regarding α-cleavage, aside from reported N-termini at His110 and Val111, we identified a total of five shorter fragments in the brain of both mice lines. We infer that aminopeptidase activity in the brain could contribute to the ragged N-termini observed around PrPC's α- and ß-cleavage sites, with this work providing a point of departure for further in vivo studies of brain proteases.

Gasdermin D activation in oligodendrocytes and microglia drives inflammatory demyelination in progressive multiple sclerosis.

Neuroinflammation coupled with demyelination and neuro-axonal damage in the central nervous system (CNS) contribute to disease advancement in progressive multiple sclerosis (P-MS). Inflammasome activation accompanied by proteolytic cleavage of gasdermin D (GSDMD) results in cellular hyperactivation and lytic death. Using multiple experimental platforms, we investigated the actions of GSDMD within the CNS and its contributions to P-MS. Brain tissues from persons with P-MS showed significantly increased expression of GSDMD, NINJ1, IL-1ß, and -18 within chronic active demyelinating lesions compared to MS normal appearing white matter and nonMS (control) white matter. Conditioned media (CM) from stimulated GSDMD+/+ human macrophages caused significantly greater cytotoxicity of oligodendroglial and neuronal cells, compared to CM from GSDMD-/- macrophages. Oligodendrocytes and CNS macrophages displayed increased Gsdmd immunoreactivity in the central corpus callosum (CCC) of cuprizone (CPZ)-exposed Gsdmd+/+ mice, associated with greater demyelination and reduced oligodendrocyte precursor cell proliferation, compared to CPZ-exposed Gsdmd-/- animals. CPZ-exposed Gsdmd+/+ mice exhibited significantly increased G-ratios and reduced axonal densities in the CCC compared to CPZ-exposed Gsdmd-/- mice. Proteomic analyses revealed increased brain complement C1q proteins and hexokinases in CPZ-exposed Gsdmd-/- animals. [18F]FDG PET imaging showed increased glucose metabolism in the hippocampus and whole brain with intact neurobehavioral performance in Gsdmd-/- animals after CPZ exposure. GSDMD activation in CNS macrophages and oligodendrocytes contributes to inflammatory demyelination and neuroaxonal injury, offering mechanistic and potential therapeutic insights into P-MS pathogenesis.

Disease-Associated Mutations in Tau Encode for Changes in Aggregate Structure Conformation.

The accumulation of tau fibrils is associated with neurodegenerative diseases, which are collectively termed tauopathies. Cryo-EM studies have shown that the packed fibril core of tau adopts distinct structures in different tauopathies, such as Alzheimer's disease, corticobasal degeneration, and progressive supranuclear palsy. A subset of tauopathies are linked to missense mutations in the tau protein, but it is not clear whether these mutations impact the structure of tau fibrils. To answer this question, we developed a high-throughput protein purification platform and purified a panel of 37 tau variants using the full-length 0N4R splice isoform. Each of these variants was used to create fibrils in vitro, and their relative structures were studied using a high-throughput protease sensitivity platform. We find that a subset of the disease-associated mutations form fibrils that resemble wild-type tau, while others are strikingly different. The impact of mutations on tau structure was not clearly associated with either the location of the mutation or the relative kinetics of fibril assembly, suggesting that tau mutations alter the packed core structures through a complex molecular mechanism. Together, these studies show that single-point mutations can impact the assembly of tau into fibrils, providing insight into its association with pathology and disease.

The roles of intracellular proteolysis in cardiac ischemia-reperfusion injury.

Ischemic heart disease remains a leading cause of human mortality worldwide. One form of ischemic heart disease is ischemia-reperfusion injury caused by the reintroduction of blood supply to ischemic cardiac muscle. The short and long-term damage that occurs due to ischemia-reperfusion injury is partly due to the proteolysis of diverse protein substrates inside and outside of cardiomyocytes. Ischemia-reperfusion activates several diverse intracellular proteases, including, but not limited to, matrix metalloproteinases, calpains, cathepsins, and caspases. This review will focus on the biological roles, intracellular localization, proteolytic targets, and inhibitors of these proteases in cardiomyocytes following ischemia-reperfusion injury. Recognition of the intracellular function of each of these proteases includes defining their activation, proteolytic targets, and their inhibitors during myocardial ischemia-reperfusion injury. This review is a step toward a better understanding of protease activation and involvement in ischemic heart disease and developing new therapeutic strategies for its treatment.

SARS-CoV-2 Mpro Protease Variants of Concern Display Altered Viral Substrate and Cell Host Target Galectin-8 Processing but Retain Sensitivity toward Antivirals.

The main protease of SARS-CoV-2 (Mpro) is the most promising drug target against coronaviruses due to its essential role in virus replication. With newly emerging variants there is a concern that mutations in Mpro may alter the structural and functional properties of protease and subsequently the potency of existing and potential antivirals. We explored the effect of 31 mutations belonging to 5 variants of concern (VOCs) on catalytic parameters and substrate specificity, which revealed changes in substrate binding and the rate of cleavage of a viral peptide. Crystal structures of 11 Mpro mutants provided structural insight into their altered functionality. Additionally, we show Mpro mutations influence proteolysis of an immunomodulatory host protein Galectin-8 (Gal-8) and a subsequent significant decrease in cytokine secretion, providing evidence for alterations in the escape of host-antiviral mechanisms. Accordingly, mutations associated with the Gamma VOC and highly virulent Delta VOC resulted in a significant increase in Gal-8 cleavage. Importantly, IC50s of nirmatrelvir (Pfizer) and our irreversible inhibitor AVI-8053 demonstrated no changes in potency for both drugs for all mutants, suggesting Mpro will remain a high-priority antiviral drug candidate as SARS-CoV-2 evolves.

Identification of Human Host Substrates of the SARS-CoV-2 Mpro and PLpro Using Subtiligase N-Terminomics.

The recent emergence of SARS-CoV-2 in the human population has caused a global pandemic. The virus encodes two proteases, Mpro and PLpro, that are thought to play key roles in the suppression of host protein synthesis and immune response evasion during infection. To identify the specific host cell substrates of these proteases, active recombinant SARS-CoV-2 Mpro and PLpro were added to A549 and Jurkat human cell lysates, and subtiligase-mediated N-terminomics was used to capture and enrich protease substrate fragments. The precise location of each cleavage site was identified using mass spectrometry. Here, we report the identification of over 200 human host proteins that are potential substrates for SARS-CoV-2 Mpro and PLpro and provide a global mapping of proteolysis for these two viral proteases in vitro. Modulating proteolysis of these substrates will increase our understanding of SARS-CoV-2 pathobiology and COVID-19.

CaspSites: A Database and Web Application for Experimentally Observed Human Caspase Substrates Using N-Terminomics.

CaspSites is a free-to-use database and web application for experimentally observed human caspase substrates using N-terminomics. It can be accessed and used by all users at the web URL www.caspsites.org. CaspSites stores cleavage site information identified for human caspases 1-9 in lysates and apoptotic cells, collected from their corresponding published studies. The database can be queried, viewed, and exported using the search page of the web application. The main parameters offered are protein substrate, cleavage site (P4-P4') residues, and individual caspase data sets, which can be connected using OR, AND, or NOT logical operators for custom user-built queries. CaspSites will be regularly updated with new experimental findings for understudied caspases, providing researchers insight into the distinctive roles human caspases play in cellular processes by identifying their target proteins in relation to each other.

Loss of ADAM15 Exacerbates Transition to Decompensated Myocardial Hypertrophy and Dilation Through Activation of the Calcineurin Pathway.

BACKGROUND: Myocardial hypertrophy and dilation are key features of cardiomyopathies and involve several cellular and molecular events. ADAMs (a disintegrin and metalloproteinases) are membrane-bound proteinases with diverse functions whose role in heart disease remains underexplored. ADAM15 is expressed in the heart and is downregulated in the failing human heart. We investigated the role ADAM15 in pressure overload cardiomyopathy. METHODS: We assessed ADAM15 levels in myocardial specimens from patients. Its direct role in pressure overload was investigated by subjecting wildtype and Adam15-deficient mice to transverse aortic constriction (TAC). RESULTS: ADAM15 levels did not change in patients with concentric hypertrophy, but markedly decreased in eccentric hypertrophy and heart failure. Loss of ADAM15 alone did not cause cardiomyopathy in mice (1 year old). After TAC, Adam15-/- mice exhibited worsened eccentric hypertrophy and dilation with greater increase in hypertrophy markers (pJNK, pERK1/2; Nppb, Nppa, Myh7, Acta1) compared with wildtype-TAC. Expression of integrin-α7 (but not integrin ß1) increased significantly more in Adam15-/--TAC hearts, while the interaction of these integrins with basement membrane (laminin), decreased consistent with worsened left ventricle dilation. In vitro, ADAM15 knockdown increased cardiomyocyte hypertrophy in response to mechanical stretch. Adam15-/--TAC hearts exhibited increased calcineurin activity and de-phosphorylation of nuclear factor of activated T cells. Calcineurin inhibition (cyclosporin-A) blocked the excess hypertrophy and dilation in Adam15-/--TAC mice. Proteome profiling demonstrated the increased abundance of the key proteins linked to worsened DCM in Adam15-/--TAC. CONCLUSION: This is the first report demonstrating that ADAM15 can suppress hypertrophy through regulating the integrin-laminin interaction and the calcineurin pathway.

Mayaro Virus Non-Structural Protein 2 Circumvents the Induction of Interferon in Part by Depleting Host Transcription Initiation Factor IIE Subunit 2.

Mayaro virus (MAYV) is an emerging mosquito-transmitted virus that belongs to the genus Alphavirus within the family Togaviridae. Humans infected with MAYV often develop chronic and debilitating arthralgia and myalgia. The virus is primarily maintained via a sylvatic cycle, but it has the potential to adapt to urban settings, which could lead to large outbreaks. The interferon (IFN) system is a critical antiviral response that limits replication and pathogenesis of many different RNA viruses, including alphaviruses. Here, we investigated how MAYV infection affects the induction phase of the IFN response. Production of type I and III IFNs was efficiently suppressed during MAYV infection, and mapping revealed that expression of the viral non-structural protein 2 (nsP2) was sufficient for this process. Interactome analysis showed that nsP2 interacts with DNA-directed RNA polymerase II subunit A (Rpb1) and transcription initiation factor IIE subunit 2 (TFIIE2), which are host proteins required for RNA polymerase II-mediated transcription. Levels of these host proteins were reduced by nsP2 expression and during infection by MAYV and related alphaviruses, suggesting that nsP2-mediated inhibition of host cell transcription is an important aspect of how some alphaviruses block IFN induction. The findings from this study may prove useful in design of vaccines and antivirals, which are currently not available for protection against MAYV and infection by other alphaviruses.

Deorphanizing Caspase-3 and Caspase-9 Substrates In and Out of Apoptosis with Deep Substrate Profiling.

Caspases are a family of enzymes that regulate biological processes such as inflammation and programmed cell death, through proteolysis. For example, in the intrinsic pathway of apoptosis, cell death signaling involves cytochrome c release from the mitochondria, which leads to the activation of caspase-9 and eventually the executioners caspase-3 and -7. One key step in our understanding of these proteases is to identify their respective protein substrates. Although hundreds of substrates have been linked to caspase-3, only a small handful of substrates have been reported for caspase-9. Employing deep profiling by subtiligase N-terminomics, we present here an unbiased analysis of caspase-3 and caspase-9 substrates in native cell lysates. We identified 906 putative protein substrates associated with caspase-3 and 124 protein substrates for caspase-9. This is the most comprehensive list of caspase substrates reported for each of these proteases, revealing a pool of new substrates that could not have been discovered using other approaches. Over half of the caspase-9 substrates were also cleaved by caspase-3, but often at unique sites, suggesting an evolved functional redundancy for these two proteases. Correspondingly, nearly half of the caspase-9 cleavage sites were not recognized by caspase-3. Our results suggest that in addition to its important role in activating the executioners, the role of caspase-9 is likely broader and more complex than previously appreciated, which includes proteolysis of key apoptotic substrates other than just caspase-3 and -7 and involvement in non-apoptotic pathways. Our results are well poised to aid the discovery of new biological functions for these two caspases.

Biochemical Tools for Tracking Proteolysis.

All living organisms depend on tightly regulated cellular networks to control biological functions. Proteolysis is an important irreversible post-translational modification that regulates most, if not all, cellular processes. Proteases are a large family of enzymes that perform hydrolysis of protein substrates, leading to protein activation or degradation. The 473 known and 90 putative human proteases are divided into 5 main mechanistic groups: metalloproteases, serine proteases, cysteine proteases, threonine proteases, and aspartic acid proteases. Proteases are fundamental to all biological systems, and when dysregulated they profoundly influence disease progression. Inhibiting proteases has led to effective therapies for viral infections, cardiovascular disorders, and blood coagulation just to name a few. Between 5 and 10% of all pharmaceutical targets are proteases, despite limited knowledge about their biological roles. More than 50% of all human proteases have no known substrates. We present here a comprehensive list of all current known human proteases. We also present current and novel biochemical tools to characterize protease functions in vitro, in vivo, and ex vivo. These tools make it achievable to define both beneficial and detrimental activities of proteases in health and disease.

A conserved acetylation switch enables pharmacological control of tubby-like protein stability.

Tubby-like proteins (TULPs) are characterized by a conserved C-terminal domain that binds phosphoinositides. Collectively, mammalian TULP1-4 proteins play essential roles in intracellular transport, cell differentiation, signaling, and motility. Yet, little is known about how the function of these proteins is regulated in cells. Here, we present the protein-protein interaction network of TULP3, a protein that is responsible for the trafficking of G-protein-coupled receptors to cilia and whose aberrant expression is associated with severe developmental disorders and polycystic kidney disease. We identify several protein interaction nodes linked to TULP3 that include enzymes involved in acetylation and ubiquitination. We show that acetylation of two key lysine residues on TULP3 by p300 increases TULP3 protein abundance and that deacetylation of these sites by HDAC1 decreases protein levels. Furthermore, we show that one of these sites is ubiquitinated in the absence of acetylation and that acetylation inversely correlates with ubiquitination of TULP3. This mechanism is evidently conserved across species and is active in zebrafish during development. Finally, we identify this same regulatory module in TULP1, TULP2, and TULP4 and demonstrate that the stability of these proteins is similarly modulated by an acetylation switch. This study unveils a signaling pathway that links nuclear enzymes to ciliary membrane receptors via TULP3, describes a dynamic mechanism for the regulation of all tubby-like proteins, and explores how to exploit it pharmacologically using drugs.

Diverse, evolving conformer populations drive distinct phenotypes in frontotemporal lobar degeneration caused by the same MAPT-P301L mutation.

Tau protein accumulation is a common denominator of major dementias, but this process is inhomogeneous, even when triggered by the same germline mutation. We considered stochastic misfolding of human tau conformers followed by templated conversion of native monomers as an underlying mechanism and derived sensitive conformational assays to test this concept. Assessments of brains from aged TgTauP301L transgenic mice revealed a prodromal state and three distinct signatures for misfolded tau. Frontotemporal lobar degeneration (FTLD)-MAPT-P301L patients with different clinical phenotypes also displayed three signatures, two resembling those found in TgTauP301L mice. As physicochemical and cell bioassays confirmed diverse tau strains in the mouse and human brain series, we conclude that evolution of diverse tau conformers is intrinsic to the pathogenesis of this uni-allelic form of tauopathy. In turn, effective therapeutic interventions in FTLD will need to address evolving repertoires of misfolded tau species rather than singular, static molecular targets.

Myocardial MMP-2 contributes to SERCA2a proteolysis during cardiac ischaemia-reperfusion injury.

AIMS: Matrix metalloproteinase-2 (MMP-2) is a zinc-dependent protease which contributes to cardiac contractile dysfunction when activated during myocardial ischaemia-reperfusion (IR) injury. MMP-2 is localized to several subcellular sites inside cardiac myocytes; however, its role in the sarcoplasmic reticulum (SR) is unknown. The Ca2+ ATPase SERCA2a, which pumps cytosolic Ca2+ into the SR to facilitate muscle relaxation, is degraded in cardiac IR injury; however, the protease responsible for this is unclear. We hypothesized that MMP-2 contributes to cardiac contractile dysfunction by proteolyzing SERCA2a, thereby impairing its activity in IR injury. METHODS AND RESULTS: Isolated rat hearts were subjected to IR injury in the presence or absence of the selective MMP inhibitor ARP-100, or perfused aerobically as a control. Inhibition of MMP activity with ARP-100 significantly improved the recovery of cardiac mechanical function and prevented the increase of a 70 kDa SERCA2a degradation fragment following IR injury, although 110 kDa SERCA2a and phospholamban levels appeared unchanged. Electrophoresis of IR heart samples followed by LC-MS/MS confirmed the presence of a SERCA2a fragment of ∼70 kDa. MMP-2 activity co-purified with SR-enriched microsomes prepared from the isolated rat hearts. Endogenous SERCA2a in SR-enriched microsomes was proteolyzed to ∼70 kDa products when incubated in vitro with exogenous MMP-2. MMP-2 also cleaved purified porcine SERCA2a in vitro. SERCA activity in SR-enriched microsomes was decreased by IR injury; however, this was not prevented with ARP-100. CONCLUSION: This study shows that MMP-2 activity is found in SR-enriched microsomes from heart muscle and that SERCA2a is proteolyzed by MMP-2. The cardioprotective actions of MMP inhibition in myocardial IR injury may include the prevention of SERCA2a degradation.

Protease Substrate Identification Using N-terminomics.

Proteases are key regulators of vital biological processes, such as apoptosis, cell differentiation, viral infection and neurodegeneration. Proteases are tightly regulated, largely because proteolysis is a unique post-translational modification (PTM) that is essentially irreversible. In order to understand the role of proteases in health and disease, the identification of protease substrates is an important step toward our understanding of their biological functions. Classic approaches for the study of proteolysis in complex mixtures employ gel electrophoresis and mass spectrometry. Such approaches typically identify a few protein substrates at a time but often fail to identify specific cleavage site locations. In contrast, modern proteomic methods using enrichment of proteolytic protein fragments can lead to the identification of hundreds of modified peptides with precise cleavage site determination in a single experiment. In this manuscript, we will review recent advances in N-terminomics methods and highlight key studies that have taken advantage of these technologies to advance our understanding of the role of proteases in cellular physiology.

Competing protein-protein interactions regulate binding of Hsp27 to its client protein tau.

Small heat shock proteins (sHSPs) are a class of oligomeric molecular chaperones that limit protein aggregation. However, it is often not clear where sHSPs bind on their client proteins or how these protein-protein interactions (PPIs) are regulated. Here, we map the PPIs between human Hsp27 and the microtubule-associated protein tau (MAPT/tau). We find that Hsp27 selectively recognizes two aggregation-prone regions of tau, using the conserved ß4-ß8 cleft of its alpha-crystallin domain. The ß4-ß8 region is also the site of Hsp27-Hsp27 interactions, suggesting that competitive PPIs may be an important regulatory paradigm. Indeed, we find that each of the individual PPIs are relatively weak and that competition for shared sites seems to control both client binding and Hsp27 oligomerization. These findings highlight the importance of multiple, competitive PPIs in the function of Hsp27 and suggest that the ß4-ß8 groove acts as a tunable sensor for clients.