Overexpression of the erythrocyte plasma membrane Ca2+ pump in COS-1 cells.

A full-length cDNA corresponding to the hPMCA4 plasma membrane Ca2+ pump was assembled and expressed in COS-1 cells. The original sequence of hPMCA4 gave a very low expression. The mutation of the initiation translation site of this sequence to the consensus A/G-X-X-AUG-G increased the production of the protein. The Ca2+ pump activity in transfected cells was 1.5-3.5-fold higher than in controls. The Ca(2+)-dependence and the calmodulin stimulation of hPMCA4 expressed in COS-1 cells were comparable with those of the erythrocyte Ca2+ pump. Immunohistochemistry experiments showed that most of the expressed protein remained in intracellular membranes. Possible explanations for this targeting of the pump are discussed.