Platelet Glycoproteins tIb and lila as a Calcium Channel in Liposomes

Human platelet membrane glycoproteins llb and lIla (GPllb and lIla) were incorporated into phospholipid vesicles by the reverse-phase technique to assess the ability of GPIIb and lIla to function as a Ca2 channel. Movement of Ca2 across the lipid bilayer was quantitated by injection of proteoliposomes with encapsulated Fura-2 into Ca2 buffers and measurement of Fura-2 fluorescence as an mdicator of Ca2 influx. Reciprocally, to assess the function of proteins in an inside-out orientation. Ca2 -loaded vesicles were injected into Ca2 -free buffer and Ca2 efflux monitored by a calcium electrode. Incorporation of the llb-Illa complex produced significant facilitation of Ca2 movement across the lipid bilayer. No net transmembrane Ca2 movement was seen with dissociated lIb and lIla. Movement of Ca2 was proportional to the transmembrane Ca2