Alternate covalent form of platelet αIIbβ3

The αIIbβ3 integrin receptor coordinates platelet adhesion, activation and mechanosensing in thrombosis and haemostasis. Using differential cysteine alkylation and mass spectrometry, we have identified a disulfide bond in the αIIb subunit linking cysteines 490 and 545 that is missing in about one in three integrin molecules on the resting and activated human platelet surface. This alternate covalent form of αIIbβ3 is pre-determined as it is also produced by human megakaryoblasts and bovine hamster kidney cells transfected with recombinant human integrin. From co-immunoprecipitation experiments, the alternate form selectively partitions into focal adhesions on the activated platelet surface. Its function was evaluated in bovine hamster kidney cells expressing a mutant integrin with an ablated C490-C545 disulfide bond. The disulfide mutant integrin has extended residency time in focal adhesions due to reduced rate of clathrin-mediated integrin internalisation and recycling that is associated with enhanced affinity of the αIIb subunit for clathrin adaptor protein-2. The alternate covalent form also has different conformational dynamics measured by monoclonal antibody binding and molecular dynamic simulations, which is reflected in reduced fibrinogen binding and outside-in signalling. These findings indicate that the αIIbβ3 integrin receptor is produced in different covalent forms that have different functions. The C490, C545 cysteine pair is conserved across all 18 integrin α subunits and the disulfide bond in αV in a cancer cell line is similarly missing, suggesting that this alternate integrin form and function is also conserved.