The phosphoprotein that regulates platelet Ca2+ transport is located on the plasma membrane, controls membrane-associated Ca2(+)-ATPase and is not glycoprotein Ib beta-subunit.

The localization and identity of the human platelet 24 kDa cyclic AMP (cAMP)-dependent phosphoprotein, previously reported to regulate Ca2+ transport, was investigated. It was found to be located on plasma membranes after isolation of these membranes from microsomes. Thus cAMP-dependent regulation of Ca2+ transport was associated with the plasma membrane fraction. Time course studies showed that the catalytic subunit of cAMP-dependent protein kinase (c-sub) induced a maximal 2-fold stimulation of Ca2+ uptake by the plasma membrane vesicles. This stimulation was dose-dependent up to 15 micrograms of c-sub/ml. The increase in Ca2+ uptake also depended upon the outside Ca2+ concentration, and was maximal at 1 microM. As regards the identity of the phosphoprotein, it was clearly distinct from the beta-subunit of glycoprotein Ib, as after electrophoresis under reduced conditions it appeared as a 24 kDa protein, but under non-reduced conditions it appeared as a 22 kDa and not as a 170 kDa protein. Nevertheless, glycoprotein Ib was certainly present, because it was detected with two polyclonal antibodies raised against its two subunits. Furthermore, the 24 kDa phosphoprotein was also present in membranes isolated from platelets obtained from patients with Bernard Soulier Syndrome; these membranes contain no glycoprotein Ib.