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The exceptional mechanical properties of the load-bearing connection of tendon to bone rely on an intricate interplay of its biomolecular composition, microstructure and micromechanics. Here we identify that the Achilles tendon–bone insertion is characterized by an interface region of ∼500 μm with a distinct fibre organization and biomolecular composition. Within this region, we identify a heterogeneous mechanical response by micromechanical testing coupled with multiscale confocal microscopy. This leads to localized strains that can be larger than the remotely applied strain. The subset of fibres that sustain the majority of loading in the interface area changes with the angle of force application. Proteomic analysis detects enrichment of 22 proteins in the interfacial region that are predominantly involved in cartilage and skeletal development as well as proteoglycan metabolism. The presented mechanisms mark a guideline for further biomimetic strategies to rationally design hard–soft interfaces. High-resolution imaging, composition analysis and mechanical testing provide a new insight into the structure and function of the Achilles enthesis.

Fungal diseases impact the lives of a millions of people across the globe, and with our current repertoire of therapeutic options dwindling, effective treatment strategies are urgently needed. Critically, the emergence of azole-resistant isolates in the clinic following prolonged treatment regimes, environmental fungicide exposure, and fungal evolution, threatens the outcome of current therapeutics, further endangering the survival of infected individuals. Here, we investigate the underpinnings of antifungal resistance using quantitative proteomics to discover protein-level signatures of fluconazole (FLC) resistance in the opportunistic human fungal pathogen, Cryptococcus neoformans . We explore ClpX, an ATP-dependent unfoldase, as a target to overcome FLC resistance and explore target efficacy through macrophage and murine models of cryptococcal infection. Here we show that disruption of ClpX, following gene deletion or targeted inhibition, re-introduces FLC susceptibility into resistant strains, rendering FLC treatment effective once again. Further, we identify and experimentally confirm mechanisms by which ClpX influences susceptibility to FLC, through association with both heme biosynthesis and ergosterol production. Overall, our results contribute to the understanding of mechanisms driving FLC resistance in a globally important fungal pathogen, and we provide avenues for targeting proteins as a therapeutic strategy to reverse antifungal resistance. Here, the authors reveal that the ATP-dependent unfoldase ClpX is a key driver of fluconazole resistance in Cryptococcus neoformans , and show that targeting ClpX restores drug susceptibility, offering a potential therapeutic strategy to reverse antifungal resistance and to render current drugs effective.

Natural compounds are a valuable source of highly active biomolecules for the discovery of innovative drug targets as well as drug leads. The natural compound neocarzilin A (NCA) exhibits pronounced antiproliferative and antimigratory activity, which we previously ascribed to the target proteins vesicle amine transporter protein 1 (VAT-1) and bone marrow stromal antigen 2 (BST-2). We here additionally demonstrate the perturbation of mitochondrial functions (fragmentation of mitochondrial networks, ultrastructural changes, increased Opa1 splicing, loss of mitochondrial membrane potential, and excessive ROS generation) upon treatment with NCA. We observe impairment of the electron transfer chain and diminished ATP synthesis. Furthermore, NCA triggers apoptosis via activation of caspase-8, enhanced Bid processing, and cytochrome c release from mitochondria into the cytosol, leading to the activation of caspase-3 and -9 and, finally, PARP cleavage and DNA fragmentation. Endoplasmic reticulum (ER) stress is induced by treatment with NCA, and subsequently, the unfolded protein response (UPR) via the protein kinase r-like ER kinase (PERK) branch is prompted. Proteomic ABPP data indicate reticulon 4 (Rtn4, Nogo), an ER-located protein mainly involved in shaping ER tubules and maintaining proper ER function, as a promising hit to explain those effects. This novel molecular target was verified by co-staining of the target probe NC-4 and Rtn4, as well as RNA interference experiments, which resulted in reduced responsiveness of HeLa cells to NCA treatment. We propose NCA as a powerful tool to study the biology of Rtn4, and to develop more specific modulators of reticulons in the future. Furthermore, we introduce—to our knowledge—the first small molecular modulator of reticulon proteins.

The ClpXP machinery is a two-component protease complex that performs targeted protein degradation in bacteria and mitochondria. The complex consists of the AAA+ chaperone ClpX and the peptidase ClpP. The hexameric ClpX utilizes the energy of ATP binding and hydrolysis to engage, unfold and translocate substrates into the catalytic chamber of tetradecameric ClpP, where they are degraded. Formation of the complex involves a symmetry mismatch, because hexameric AAA+ rings bind axially to the opposing stacked heptameric rings of the tetradecameric ClpP. Here we present the cryo-EM structure of ClpXP from Listeria monocytogenes. We unravel the heptamer-hexamer binding interface and provide novel insight into the ClpX-ClpP cross-talk and activation mechanism. Comparison with available crystal structures of ClpP and ClpX in different states allows us to understand important aspects of the complex mode of action of ClpXP and provides a structural framework for future pharmacological applications.

A fundamental step in membrane protein biogenesis is their integration into the lipid bilayer with a defined orientation of each transmembrane segment. Despite this, it remains unclear how cells detect and handle failures in this process. Here we show that single point mutations in the membrane protein connexin 32 (Cx32), which cause Charcot-Marie-Tooth disease, can cause failures in membrane integration. This leads to Cx32 transport defects and rapid degradation. Our data show that multiple chaperones detect and remedy this aberrant behavior: the ER–membrane complex (EMC) aids in membrane integration of low-hydrophobicity transmembrane segments. If they fail to integrate, these are recognized by the ER–lumenal chaperone BiP. Ultimately, the E3 ligase gp78 ubiquitinates Cx32 proteins, targeting them for degradation. Thus, cells use a coordinated system of chaperones for the complex task of membrane protein biogenesis, which can be compromised by single point mutations, causing human disease. A fundamental step in membrane protein biogenesis is their integration into the lipid bilayer with a defined orientation of each transmembrane segment. Here, the authors show that mutations in connexin 32 can cause failures in membrane integration which is detected by the ER chaperone machinery.

Metalloproteases are a large, diverse class of enzymes involved in many physiological and disease processes. Metalloproteases are regulated by post-translational mechanisms that diminish the effectiveness of conventional genomic and proteomic methods for their functional characterization. Chemical probes directed at active sites offer a potential way to measure metalloprotease activities in biological systems; however, large variations in structure limit the scope of any single small-molecule probe aimed at profiling this enzyme class. Here, we address this problem by creating a library of metalloprotease-directed probes that show complementary target selectivity. These probes were applied as a 'cocktail' to proteomes and their labeling profiles were analyzed collectively using an advanced liquid chromatography–mass spectrometry platform. More than 20 metalloproteases were identified, including members from nearly all of the major branches of this enzyme class. These findings suggest that chemical proteomic methods can serve as a universal strategy to profile the activity of the metalloprotease superfamily in complex biological systems.