TNF receptor‐associated factor 4 (TRAF4) is a novel binding partner of glycoprotein Ib and glycoprotein VI in human platelets

Background: Reactive oxygen species generation is one consequence of ligand engagement of platelet glycoprotein (GP) receptors GPIb‐IX‐V and GPVI, which bind VWF/collagen and initiate thrombosis at arterial shear; however, the precise molecular mechanism coupling redox pathway activation to engagement of these receptors is unknown. Objective: The objective of this study was to identify novel binding partners for GPIb‐IX‐V and GPVI that could provide a potential link between redox pathways and early platelet signaling events. Methods and Results: Using protein array analysis and affinity‐binding assays, we demonstrated that the orphan TNF receptor‐associated factor (TRAF) family member, TRAF4, selectively binds cytoplasmic sequences of GPIbβ and GPVI. TRAF4, p47phox [of the NADPH oxidase (Nox2) enzyme complex] and other redox relevant signaling proteins such as Hic‐5, co‐immunoprecipitate with GPIb/GPVI from human platelet lysates whilst MBP‐TRAF4 or MBP‐p47phox fusion proteins specifically pull‐down GPIb/GPVI. GPIb‐ or GPVI‐selective agonists induce phosphorylation of the TRAF4‐associated proteins, Hic‐5 and Pyk2, with phosphorylation attenuated by Nox2 inhibition. Conclusion: These results describe the first direct association of TRAF4 with a receptor, and identify a novel binding partner for GPIb‐IX‐V and GPVI, providing a potential link between these platelet receptors and downstream TRAF4/Nox2‐dependent redox pathways.