Plasma membrane Ca2+-ATPases are key calcium exporter proteins in most tissues, and
PMCA4b is the main calcium transporter in the human red blood cells (RBCs). In order
to assess the expression level of PMCA4b, we have developed a flow cytometry and specific
antibody binding method to quantitatively detect this protein in the erythrocyte membrane.
Interestingly, we found several healthy volunteers showing significantly reduced expression
of RBC-PMCA4b. Western blot analysis of isolated RBC membranes confirmed this observation,
and indicated that there are no compensatory alterations in other PMCA isoforms. In
addition, reduced PMCA4b levels correlated with a lower calcium extrusion capacity
in these erythrocytes. When exploring the potential genetic background of the reduced
PMCA4b levels, we found no missense mutations in the ATP2B4 coding regions, while
a formerly unrecognized minor haplotype in the predicted second promoter region closely
correlated with lower erythrocyte PMCA4b protein levels. In recent GWA studies, SNPs
in this ATP2B4 haplotype have been linked to reduced mean corpuscular hemoglobin concentrations
(MCHC), and to protection against malaria infection. Our data suggest that an altered
regulation of gene expression is responsible for the reduced RBC-PMCA4b levels that
is probably linked to the development of human disease-related phenotypes.
