Plasma membrane Ca2+ pump isoforms 2a and 2b are unusually responsive to calmodulin and Ca2+

The full-length a and b variants of the rat plasma membrane calcium pump, isoform 2 (rPMCA2a and rPMCA2b), were constructed and expressed in COS-7 cells, To characterize these isoforms, calcium transport was determined in a microsomal fraction, Both rPMCA2a and rPMCA2b had a much higher affinity for calmodulin than the corresponding forms of hPMCA4, and rPMCA2b had the highest affinity among the isoforms that have been tested so far, When analyzed at a relatively high calmodulin concentration, rPMCA2b and, to a lesser extent, rPRMCA2a showed higher apparent calcium affinity; i.e. they were more active at lower Ca2+ concentrations than hPMCA4b. This indicates that these two variants of rat isoform 2 will tend to maintain a lower free cytosolic Ca2+ level in cells where they are expressed, Both variants also showed a higher level of basal activity (in the complete absence of calmodulin) than hPMCA4b, a property which would reinforce their ability to maintain a low free cytosolic Ca2+ concentration, Experiments designed to determine the source of the higher apparent Ca2+ affinity of rPMCA2b showed that it came from the properties of the carboxyl terminus, rather than from any difference in the catalytic core.