Aims: We addressed the question of whether a previously uncharacterized platelet dysfunction contributes to bleeding diathesis in 2B-VWDMPS patients.Methods: We studied the 2 members (mother and daughter) of the MPS kindred with severe bleeding phenotype and addressed this question by coupling quantitative platelet shotgun proteomics and validating biochemical assays, with the systematic analysis of platelet procoagulant membrane dynamics (PMD).Results: Previously, we showed that membrane ballooning, a principal part of the platelet PMD after collagen stimulation, is driven by the influx of Na+ and Cl-, followed by the entry of water.Here, we report in 2B-VWDMPS platelets, loss of the transmembrane chloride channel-1 (CLIC1), and reduced chloride ion influx after collagen stimulation.This was associated with diminished membrane ballooning, phosphatidylserine externalisation and membrane thrombin formation, as well as a distinct phenotypic composition of platelets over fibrillar collagen.Using N-terminomics/ TAILS (terminal amine isotopic labeling of substrates), we compared changes in proteolysis between healthy and 2B-VWDMPS platelets and identified processing differences of von Willebrand factor (VWF), fibronectin (FN1) and Crk-like protein (CRKL).Conclusion(s): 2B-VWDMPS platelets are basally activated, partially degranulated, and have marked loss of regulatory, cytoskeletal, and contractile proteins that may account for structural disorganization, giant platelet formation and the reduction in the haemostatic response.