Thiol-exchange in DTSSP crosslinked peptides is proportional to cysteine content and precisely controlled in crosslink detection by two-step LC-MALDI MSMS

Pseudomonas aeruginosa quorum-sensing metabolite induces host immune cell death through cell surface lipid domain dissolution

Bacterial quorum-sensing autoinducers are small chemicals released to control microbial community behaviours. N-(3-oxo-dodecanoyl) homoserine lactone, the autoinducer of the Pseudomonas aeruginosa LasI-LasR circuitry, triggers significant cell death in lymphocytes. We found that this molecule is incorporated into the mammalian plasma membrane and induces dissolution of eukaryotic lipid domains. This event expels tumour necrosis factor receptor 1 into the disordered lipid phase for its spontaneous trimerization without its ligand and drives caspase 3-caspase 8-mediated apoptosis. In vivo, P. aeruginosa releases N-(3-oxo-dodecanoyl) homoserine lactone to suppress host immunity for its own better survival; conversely, blockage of caspases strongly reduces the severity of the infection. This work reveals an unknown communication method between microorganisms and the mammalian host and suggests interventions of bacterial infections by intercepting quorum-sensing signalling.

High-purity capture and release of circulating exosomes using an exosome-specific dual-patterned immunofiltration (ExoDIF) device

We present a microfluidic device for the capture and release of circulating exosomes from human blood. The exosome-specific dual-patterned immunofiltration (ExoDIF) device is composed of two distinct immuno-patterned layers, and is capable of enhancing the chance of binding between the antibody and exosomes by generating mechanical whirling, thus achieving high-throughput exosome isolation with high specificity. Moreover, follow-up recovery after the immuno-affinity based isolation, via cleavage of a linker, enables further downstream analysis. We verified the performance of the present device using MCF-7 secreted exosomes and found that both the concentration and proportion of exosome-sized vesicles were higher than in the samples obtained from the conventional exosome isolation kit. We then isolated exosomes from the human blood samples with our device to compare the exosome level between cancer patients and healthy donors. Cancer patients show a significantly higher exosome level with higher selectivity when validating the exosome-sized vesicles using both electron microscopy and nanoparticle tracking analysis. The captured exosomes from cancer patients also express abundant cancer-associated antigens, the epithelial cell adhesion molecule (EpCAM) on their surface. Our simple and rapid exosome recovery technique has huge potential to elucidate the function of exosomes in cancer patients and can thus be applied for various exosome-based cancer research studies.

Probing the transient interaction between the small heat-shock protein Hsp21 and a model substrate protein using crosslinking mass spectrometry

Small heat-shock protein chaperones are important players in the protein quality control system of the cell, because they can immediately respond to partially unfolded proteins, thereby protecting the cell from harmful aggregates. The small heat-shock proteins can form large polydisperse oligomers that are exceptionally dynamic, which is implicated in their function of protecting substrate proteins from aggregation. Yet the mechanism of substrate recognition remains poorly understood, and little is known about what parts of the small heat-shock proteins interact with substrates and what parts of a partially unfolded substrate protein interact with the small heat-shock proteins. The transient nature of the interactions that prevent substrate aggregation rationalize probing this interaction by crosslinking mass spectrometry. Here, we used a workflow with lysine-specific crosslinking and offline nano-liquid chromatography matrix-assisted laser desorption/ionization tandem time-of-flight mass spectrometry to explore the interaction between the plant small heat-shock protein Hsp21 and a thermosensitive model substrate protein, malate dehydrogenase. The identified crosslinks point at an interaction between the disordered N-terminal region of Hsp21 and the C-terminal presumably unfolding part of the substrate protein.

Detection of crosslinks within and between proteins by LC-MALDI-TOFTOF and the software FINDX to reduce the MSMS-data to acquire for validation

Lysine-specific chemical crosslinking in combination with mass spectrometry is emerging as a tool for the structural characterization of protein complexes and protein-protein interactions. After tryptic digestion of crosslinked proteins there are thousands of peptides amenable to MSMS, of which only very few are crosslinked peptides of interest. Here we describe how the advantage offered by off-line LC-MALDI-TOF/TOF mass spectrometry is exploited in a two-step workflow to focus the MSMS-acquisition on crosslinks mainly. In a first step, MS-data are acquired and all the peak list files from the LC-separated fractions are merged by the FINDX software and screened for presence of crosslinks which are recognized as isotope-labeled doublet peaks. Information on the isotope doublet peak mass and intensity can be used as search constraints to reduce the number of false positives that match randomly to the observed peak masses. Based on the MS-data a precursor ion inclusion list is generated and used in a second step, where a restricted number of MSMS-spectra are acquired for crosslink validation. The decoupling of MS and MSMS and the peptide sorting with FINDX based on MS-data has the advantage that MSMS can be restricted to and focused on crosslinks of Type 2, which are of highest biological interest but often lowest in abundance. The LC-MALDI TOF/TOF workflow here described is applicable to protein multisubunit complexes and using (14)N/(15)N mixed isotope strategy for the detection of inter-protein crosslinks within protein oligomers.