The platelet Ca2+ transport ATPase system

The Ca2+ signal accompanying cell function involves the activities of plasma membrane Ca2+ transport ATPases (PMCA) which transport Ca2+ ions out of the cell and those of sarco/endoplasmic reticulum Ca2+ transport ATPases (SERCA), which pump Ca2+ ions into intracellular Ca2+ pools. Although a platelet Ca2+ transport ATPase was described three decades ago, for a long time it remained poorly understood in terms of its cellular localization and identity. By integrating data obtained during recent years, including newly available information in the literature for the PMCAs and aspects of our work concerning the SERCAs, the present review will show how the overall view of the platelet Ca2+ATPase system has to be modified due to the presence of a number of Ca2+ATPases in these cells. These Ca2+ATPases include a typical 144 kDa PMCA protein, although its molecular identity still remains to be established, expressed together with a multi-SERCA system constituted by the ubiquitous 100 kDa SERCA 2b isoform, the 97 kDa SERCA 3 isoform and a new 97 kDa SERCA isoform recognized by the monoclonal antibody termed PL/IM 430 which also remains to be identified. The new paradigm of the platelet multi-Ca2+ATPase system will be discussed including: (i) the problems solved, as it has now become possible to reconciliate previous contradictory observations and (ii) those which still remain due to the fact that the platelet Ca2+ATPase system is more complex than previously assumed. Finally, to put this complexity of the platelet Ca2+ transport ATPase system into perspective, the biological significance of the multi-SERCA system in the context of Ca2+ signalling will be tentatively discussed in an attempt to produce a model of the organization of the intracellular Ca2+ pools in platelets.