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+ ion s out of the cell and those of sarco/endoplasmic reticulum Ca2+ transp ort ATPases (SERCA), which pump Ca2+ ions into intracellular Ca2+ pool s, Although a platelet Ca2+ transport ATPase was described three decad es ago, for a long time it remained poorly understood in terms of its cellular localization and identity, By integrating data obtained durin g recent years, including newly available information in the literatur e for the PMCAs and aspects of our work concerning the SERCAs, the pre sent review will show how the overall view of the platelet Ca(2+)ATPas e system has to be modified due to the presence of a number of Ca(2+)A TPases in these cells, These Ca(2+)ATPases include a typical 144kDa PM CA protein, although its molecular identity still remains to be establ ished, expressed together with a multi-SERCA system constituted by the ubiquitous 100kDa SERCA 2b isoform, the 97kDa SERCA 3 isoform and a n ew 97kDa SERCA isoform recognized by the monoclonal antibody termed PL /IM 430 which also remains to be identified, The new paradigm of the p latelet multi-Ca(2+)ATPase system will be discussed including: (i) the problems solved, as it has now become possible to reconciliate previo us contradictory observations and (ii) those which still remain due to the fact that the platelet Ca(2+)ATPase system is more complex than p reviously assumed, Finally, to put this complexity of the platelet Ca2 + transport ATPase system into perspective, the biological significanc e of the multi-SERCA system in the context of Ca2+ signalling will be tentatively discussed in an attempt to produce a model of the organiza tion of the intracellular Ca2+ pools in platelets.