Protein-protein interactions are the machinery of cellular life. Enzymes pair with substrates, scaffolds anchor signalling cascades, and chaperones shepherd newly minted proteins into their correct folds. Cataloguing these interactions at scale has been a major goal of proteomics for two decades, but most existing methods carry significant limitations. Affinity purification-mass spectrometry requires targeting individual proteins and can lose weak or transient partners during sample preparation. Protein correlation profiling, which infers interactions from the co-migration of proteins through a separation column, works at proteome scale and avoids tagging, but it cannot confirm that two proteins bind directly. Cross-linking mass spectrometry solves the directness problem by covalently stitching together proteins that are in close physical proximity, but capturing a comprehensive interactome from intact tissue has proven exceptionally difficult because cross-linkers penetrate tissue poorly and cross-linked peptides are vanishingly rare in complex mixtures.

The Melbourne team attacked the problem with a dual strategy. In cell culture, they induced insulin resistance in differentiated C2C12 mouse myotubes by exposing them to the fatty acid palmitate for 24 hours, a manipulation that faithfully blunted insulin-stimulated phosphorylation of Akt and its substrate AS160, canonical markers of impaired insulin signalling. They then applied two orthogonal technologies. The first, protein correlation profiling mass spectrometry, involved lysing cells in gentle native buffer, separating intact protein complexes by blue native polyacrylamide gel electrophoresis, slicing each gel lane into thirty fractions, and analysing all 360 resulting fractions by data-independent acquisition mass spectrometry. Computational analysis with the CCProfiler algorithm mapped how thousands of protein complexes reorganise between healthy, insulin-resistant, and acutely insulin-stimulated states.

The second technology was quantitative cross-linking mass spectrometry using a membrane-permeable, phosphate-enrichable cross-linker called t-butyl-PhoX, combined with sixteen-plex tandem mass tag labelling for multiplexed relative quantification. After cross-linking, the researchers subjected samples to subcellular fractionation into cytoplasmic, membrane, nuclear, and cytoskeletal compartments, digested the proteins, depleted phosphopeptides, enriched PhoX-modified cross-linked peptides by immobilised metal-ion affinity chromatography, and then passed them through size-exclusion and high-pH reversed-phase chromatography before mass spectrometric analysis. This deep fractionation cascade was essential: cross-linked peptides are dramatically outnumbered by ordinary peptides, and without such enrichment they would be lost in the noise. Data were searched with the pLink2 engine against a skeletal muscle-specific mouse proteome database and filtered with xiFDR to stringent false discovery thresholds at the peptide, residue-pair, and protein-interaction levels.

The scale of the resulting datasets is striking. Across the cell-based experiments, the integrated workflows quantified more than 7,000 unique protein-protein interactions among 5,346 proteins. In the cross-linking analysis of myotubes, the team identified 11,295 cross-linked peptides, including 4,944 that linked two different proteins, corresponding to 3,757 unique protein pairs. Comparing the cross-link abundances between healthy and insulin-resistant cells revealed that 6,704 cross-linked peptides were significantly regulated, and crucially, 5,214 of those changed without any accompanying change in the abundance of either interacting partner — direct evidence that the interaction network itself, not merely protein expression, is being remodelled by the disease process.

Pathway analysis of the regulated cross-links pointed overwhelmingly toward one cellular compartment: the endoplasmic reticulum, the organelle where secreted and membrane proteins are folded, quality-controlled, and dispatched. Interactions among ER-resident chaperones and folding catalysts — including multiple protein disulfide isomerases, the calcium-binding chaperone calreticulin, the stress-induced chaperone HYOU1, and BiP, the master regulator of the unfolded protein response — were systematically perturbed. This converges with a substantial literature linking ER stress to insulin resistance, most famously the demonstration that ER stress activates the JNK kinase, which inhibits insulin receptor substrate-1 and thereby suppresses insulin signalling. What the new study adds is a systems-level picture of how the chaperone network’s physical architecture is dismantled in the insulin-resistant cell, suggesting the muscle loses capacity to resolve folding stress precisely when it is most needed.

One protein emerged as a central hub of this dysregulated network: PDIA6, an ER-resident protein disulfide isomerase that catalyses the formation and rearrangement of disulfide bonds, thereby tuning the oxidation state of cysteine residues in its client proteins. The team identified eighteen direct interactions with PDIA6, several of them not annotated in the latest human interactome databases, and all thirty-four cross-linked peptides bridging PDIA6 and BiP were downregulated with insulin resistance. Protein correlation profiling independently showed PDIA6 redistributing from its monomeric molecular weight into larger assemblies under insulin-resistant conditions. This echoes recent work showing that PDIA6 forms calcium-dependent biomolecular condensates in the ER lumen that anchor other folding chaperones under homeostatic conditions, dispersing throughout the ER during stress to increase folding capacity at the cost of substrate specificity.

To test whether these interactome changes matter functionally, the researchers overexpressed PDIA6 throughout the skeletal musculature of young mice using a muscle-tropic viral vector, then fed the animals a high-fat diet for twelve weeks to induce insulin resistance. Western blotting confirmed a five-fold overexpression. Contrary to the team’s hypothesis that boosting chaperone capacity would protect insulin sensitivity, the overexpressing mice showed subtly reduced insulin-stimulated glucose uptake in isolated soleus muscle and diminished phosphorylation of Akt and AS160. Cysteine redox proteomics using isobaric iodoTMT labelling then quantified 578 cysteine-containing peptides in the overexpressing muscles, revealing that the most significantly oxidised cysteines sat on proteins physically cross-linked to PDIA6, including the chaperones MANF, calreticulin, TXNDC5, and PDIA3, alongside calcium-handling proteins such as the calcium pump ATP2A1 and sarcalumenin.

Perhaps the most technically ambitious component of the study was the extension of quantitative cross-linking mass spectrometry to intact skeletal muscle tissue. The researchers excised extensor digitorum longus muscles from chow-fed and high-fat-diet-fed mice, incubated the freshly isolated tissues with the cross-linker in oxygenated physiological buffer for two hours, and pushed them through the same deep-fractionation pipeline. This yielded 8,327 cross-linked peptides spanning 3,541 unique protein pairs, of which 1,101 were significantly regulated by the high-fat diet. Regulated interactions clustered in glycolysis, calcium signalling, and contractile machinery, including interactions among the calcium release channel RYR1, calsequestrin, triadin, and SERCA1. A striking pattern of regulated intra-protein cross-links concentrated on the light meromyosin region of myosin heavy chain 7, consistent with conformational shifts between the energy-saving super-relaxed state and the disordered-relaxed state that have recently been implicated in diabetic muscle energetics.

Validation of the workflow came from mapping cross-links onto experimentally solved structures of the immunoproteasome, mitochondrial complex I, the ARP2/3 complex, and the actomyosin complex: 97.5 percent of mapped cross-links fell within the expected thirty-five angstrom distance constraint of the cross-linker, with the few violations occurring in regions known to flex during contraction. The authors are candid about limitations — cross-linker infusion into living animals remains impractical, the ex vivo cross-linking window may itself perturb the interactome, and the extensive fractionation could bias which proteins are captured. Nevertheless, the study establishes that quantitative interactome mapping is feasible in whole tissue and delivers biological insight unattainable from abundance measurements alone. By showing that the rewiring of protein interaction networks — and specifically the disulfide-isomerase activities of PDIA6 — reshapes cysteine oxidation and insulin sensitivity in muscle, the work opens a genuinely new dimension in the search for the molecular roots of metabolic disease.

Subject of Research: Quantitative mapping of protein-protein interaction remodelling in skeletal muscle insulin resistance

Article Title: Quantitative interactome mapping of skeletal muscle insulin resistance

Article References: Ng, Y.-K., Blazev, R., Wong, J. P. H., Scott, N. E., Molendijk, J., & Parker, B. L. (2026). Quantitative interactome mapping of skeletal muscle insulin resistance. Molecular Systems Biology, 22(9), 1430-1452. https://doi.org/10.1038/s44320-026-00224-7

Image Credits: AI Generated

DOI: 10.1038/s44320-026-00224-7

Keywords: insulin resistance, skeletal muscle, protein-protein interactions, cross-linking mass spectrometry, proteomics, PDIA6, endoplasmic reticulum stress, diabetes, interactome mapping, cysteine oxidation, type 2 diabetes, molecular systems biology