Purification and characterisation of the platelet-activating GPVI/FcRγ complex in SMALPs.

ABSTRACT

The collagen/fibrin(ogen) receptor, glycoprotein VI (GPVI), is a platelet activating receptor and a promising anti-thrombotic drug target. However, while agonist-induced GPVI clustering on platelet membranes has been shown to be essential for its activation, it is unknown if GPVI dimerisation represents a unique conformation for ligand binding. Current GPVI structures all contain only the two immunoglobulin superfamily (IgSF) domains in the GPVI extracellular region, so lacking the mucin-like stalk, transmembrane, cytoplasmic tail of GPVI and its associated Fc receptor γ (FcRγ) homodimer signalling chain, and provide contradictory insights into the mechanisms of GPVI dimerisation. Here, we utilised styrene maleic-acid lipid particles (SMALPs) to extract GPVI in complex with its two associated FcRγ chains from transfected HEK-293T cells, together with the adjacent lipid bilayer, then purified and characterised the GPVI/FcRγ-containing SMALPs, to enable structural insights into the full-length GPVI/FcRγ complex. Using size exclusion chromatography followed by a native polyacrylamide gel electrophoresis (PAGE) method, SMA-PAGE, we revealed multiple sizes of the purified GPVI/FcRγ SMALPs, suggesting the potential existence of GPVI oligomers. Importantly, GPVI/FcRγ SMALPs were functional as they could bind collagen. Mono-dispersed GPVI/FcRγ SMALPs could be observed under negative stain electron microscopy. These results pave the way for the future investigation of GPVI stoichiometry and structure, while also validating SMALPs as a promising tool for the investigation of human membrane protein interactions, stoichiometry and structure.