A monoclonal antibody (PL/IM 430) to human platelet intracellular membranes which inhibits the uptake of Ca2+ without affecting the Ca2+ +Mg2+-ATPase.

To probe the structure-function relationships of proteins present in the endoplasmic reticulum-like intracellular membranes of human blood platelets a panel of monoclonal antibodies have been raised, using as immunogen highly purified platelet intracellular membrane vesicles isolated by continuous flow electrophoresis [Menashi, Weintroub & Crawford (1981) J. Biol. Chem. 256, 4095-4101]. Four of these antibodies recognize a single 100 kDa polypeptide in the platelet membrane by immunoblotting. One antibody PL/IM 430 (of IgG1 subclass) inhibited (approximately 70%) the energy-dependent uptake of Ca2+ into the vesicles without affecting the Ca2+ +Mg2+-ATPase activity or the protein phosphorylation previously shown to proceed concomitantly with Ca2+ sequestration [Hack, Croset & Crawford (1986) Biochem. J. 233, 661-668]. The inhibition is independent of ATP concentration over a range 0-2 mM-ATP but shows dose-dependency for external [Ca2+] with maximum inhibition of Ca2+ translocation at concentrations of Ca2+ greater than 500 nM. This capacity of the antibody PL/IM 430 functionally to dislocate components of the intracellular membrane Ca2+ pump complex may have value in structural studies.