Me. Grimaldi et al., DELETION OF AMINO-ACID-RESIDUES-18-75 INACTIVATES THE PLASMA-MEMBRANECA2+ PUMP, The Journal of biological chemistry, 271(43), 1996, pp. 26995-26997
A mutant of the plasma membrane Ca2+ pump hPMCA4b(d18-75)(ct120) conta
ining a deletion of the N-terminal amino acid residues 18-75 and lacki
ng the C-terminal 120 amino acid residues was expressed in COS-1 cells
, The deletion in the N-terminal region did not significantly affect t
he level of expression of the Ca2+ pump. Tryptic digestion of the hPMC
A4b(d18-75)(ct120) mutant resulted in the appearance of the same fragm
ents obtained by proteolysis of the hPMCA4b(ct120) enzyme, suggesting
that deletion of residues 18-75 neither impeded the insertion in the m
embrane nor extensively affected the folding of the mutant protein. Th
e functional competence of the hPMCA4b(d18-75)(ct120) enzyme was exami
ned by measuring the Ca2+ transport and the Ca2+ ATPase activity of CO
S-1 cell microsomes expressing the mutant protein. Both tests proved t
he mutant to be inactive, Under conditions in which hPMCA4b(ct120) bec
omes phosphorylated, hPMCA4b(d18-75)(ct120) was incapable of reacting
with ATP and Ca2+ to form the phosphoenzyme. Taken together these resu
lts suggest that the segment of amino acids 18-75 is essential for the
activity of the plasma membrane Ca2+ pump.