The quantitative multiplexing capacity of isobaric Tandem Mass Tags (TMT) has increased the throughput of affinity purification mass spectrometry (AP-MS) to characterize protein interaction networks of immunoprecipitated bait proteins. However, variable bait levels between replicates can convolute interactor identification. We compared the Student's t-test and Pearson's R correlation as methods to generate t-statistics and assessed the significance of interactors following TMT-AP-MS. Using a simple linear model of protein recovery in immunoprecipitates to simulate reporter ion ratio distributions, we found that correlation-derived t-statistics protect against bait variance while robustly controlling Type I errors (false positives). We experimentally determined the performance of these two approaches for determining t-statistics under two experimental conditions: irreversible prey association to the Hsp40 mutant DNAJB8 H31Q followed by stringent washing, and reversible association to 14-3-3z with gentle washing. Correlation-derived t-statistics performed at least as well as Student's t-statistics for each sample, and with substantial improvement in performance for experiments with high bait level variance. Deliberately varying bait levels over a large range fails to improve selectivity but does increase robustness between runs. The use of correlation-derived t-statistics should improve identification of interactors using TMT-AP-MS. Data are available via ProteomeXchange with identifier PXD016613.

Environmental toxins and toxicants can damage proteins and threaten cellular proteostasis. Most current methodologies to identify misfolded proteins in cells survey the entire proteome for sites of changed reactivity. We describe and apply a quantitative proteomics methodology to identify destabilized proteins based on their binding to the human Hsp40 chaperone DNAJB8. These protein targets are validated by an orthogonal limited proteolysis assay using parallel reaction monitoring. We find that a brief exposure of HEK293T cells to meta-arsenite increases the affinity of two dozen proteins to DNAJB8, including known arsenite-sensitive proteins. In particular, arsenite treatment destabilizes both the pyruvate dehydrogenase complex E1 subunit and several RNA-binding proteins. This platform can be used to explore how environmental toxins impact cellular proteostasis and to identify the susceptible proteome.

Herbicides in the widely used chloroacetanilide class harbor a potent electrophilic moiety, which can damage proteins through nucleophilic substitution. In general, damaged proteins are subject to misfolding. Accumulation of misfolded proteins compromises cellular integrity by disrupting cellular proteostasis networks, which can further destabilize the cellular proteome. While direct conjugation targets can be discovered through affinity-based protein profiling, there are few approaches to probe how cellular exposure to toxicants impacts the stability of the proteome. We apply a quantitative proteomics methodology to identify chloroacetanilide-destabilized proteins in HEK293T cells based on their binding to the H31Q mutant of the human Hsp40 chaperone DNAJB8. We find that a brief cellular exposure to the chloroacetanilides acetochlor, alachlor, and propachlor induces misfolding of dozens of cellular proteins. These herbicides feature distinct but overlapping profiles of protein destabilization, highly concentrated in proteins with reactive cysteine residues. Consistent with the recent literature from the pharmacology field, reactivity is driven by neither inherent nucleophilic nor electrophilic reactivity but is idiosyncratic. We discover that propachlor induces a general increase in protein aggregation and selectively targets GAPDH and PARK7, leading to a decrease in their cellular activities. Hsp40 affinity profiling identifies a majority of propachlor targets identified by competitive activity-based protein profiling (ABPP), but ABPP can only identify about 10% of protein targets identified by Hsp40 affinity profiling. GAPDH is primarily modified by the direct conjugation of propachlor at a catalytic cysteine residue, leading to global destabilization of the protein. The Hsp40 affinity strategy is an effective technique to profile cellular proteins that are destabilized by cellular toxin exposure. Raw proteomics data is available through the PRIDE Archive at PXD030635.

Hsp40s play a central role in cellular protein homeostasis by promiscuously surveying the proteome for misfolded proteins. These misfolded client proteins are then delivered to Hsp70. Mutation of the Hsp40 J-domain blocks Hsp70 binding, inhibiting client protein release from Hsp40. We previously integrated misfolded protein recognition by Hsp40 into a platform to identify proteins that are destabilized by cellular stress. However, the dependences of Hsp40 interactions and client recovery on J-domain activity, Hsp40 identity, and crosslinking have not been addressed. Herein, we apply quantitative proteomics to systematically characterize the interactions networks of human Hsp40s DNAJB8 and DNAJB1 with intact or inactivated J-domains. We find that DNAJB8 irreversibly binds over a thousand protein interactors even in the absence of stress. Inactivation of the J-domain decreases interaction with Hsp70 family and associated proteins, but does not generally affect client binding. By contrast, J-domain inactivation and cellular crosslinking substantially increase the relative recovery of proteins from DNAJB1 co-immunoprecipitation. This advantage is completely offset by loss of DNAJB1 recovery under these conditions, making DNAJB1 a poor bait for client protein recovery as compared to DNAJB8. The J-domain inactivated DNAJB8H31Q has increased affinity to its client proteins under heat stress, while no such change in affinity is observed for the wild-type protein, despite their similar client binding profiles under basal conditions. Hence, we find that DNAJB8H31Q is an effective recognition element for the recovery of destabilized client proteins following cellular stress.

Herbicides in the popular chloroacetanilide class harbor a potent electrophilic moiety, which can damage proteins through nucleophilic substitution. In general, damaged proteins are subject to misfolding. Accumulation of misfolded proteins compromises cellular integrity by disrupting cellular proteostasis networks, which can further destabilize the cellular proteome. While direct conjugation targets can be discovered through affinity-based protein profiling, there are few approaches to probe how cellular exposure to toxicants impacts the stability of the proteome. We apply a quantitative proteomics methodology to identify chloroacetanilide-destabilized proteins in HEK293T cells based on their binding to the H31Q mutant of the human Hsp40 chaperone DNAJB8. We find that brief cellular exposure to the chloroacetanilides acetochlor, alachlor, and propachlor induces misfolding of dozens of cellular proteins. These herbicides feature distinct but overlapping profiles of protein destabilization, highly concentrated in proteins with reactive cysteine residues. Propachlor induces a general increase in protein aggregation, and selectively targets GAPDH and PARK7, leading to a decrease in their cellular activities. GAPDH is primarily modified by direct conjugation of propachlor at a catalytic cysteine residue, leading to global destabilization of the protein. The Hsp40 affinity strategy is an effective technique to profile cellular proteins that are destabilized by cellular toxin exposure. Raw proteomics data is available through the PRIDE Archive at PXD030635.