Abstract
Antibodies 5F10 and JA3 (raised against the erythrocyte Ca 2+ pump) were used to identify hPMCA4b as the major form of the plasma membrane Ca 2+ pump in human platelets and in three human megakaryoblastoid cell lines, MEG 01, DAMI and CHRF 288-11. 5F10 was used because it has been shown to recognize all known isoforms of the hPMCA and JA3 because it reacts exclusively with hPMCA4b [Caride A.J., Filoteo A.G., Enyedi A., Verma A.K., Penniston J.T Detection of isoform 4 of the plasma membrane calcium pump in human tissues by using isoform-specific monoclonal antibodies. Biochem J 1996; 316: 353–359]. In addition to hPMCA4b, hPMCA1b was also detected in the megakaryoblastoid cells by using isoform-specific polyclonal antibodies. The apparent size of this isoform, however, was smaller than that seen in HeLa and COS-7 cell membranes indicating the presence of a modified form of hPMCA1 b. In platelets, no evidence of the expression of hPMCA1 b could be found. The amount of PMCA in these cells was compared with that of the constitutive form of the sarco/endoplasmic reticulum Ca 2+ pump in non-muscle cells (SERCA2b) and also with the amount of PMCA in human erythrocytes. A very low level of the plasma membrane Ca 2+ pump was found in platelets while in their precursor cells the expression of this Ca 2+ pump was much more abundant. Whereas the expression level of PMCA decreased dramatically in mature human platelets, the expression of SERCA2b did not change substantially upon megakaryocytic differentiation.
Published Version
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