AE1 and carbonic anhydrase II: are they colocalized and do they exhibit direct functional interaction in cell membranes?

Abstract

Previous studies have reported that intracellular carbonic anhydrase (CA) II binds to the C‐terminal domain of AE1 and it has been suggested that this interaction directly increases the transport rate of AE1. The goal of our study was a) to detect the intracellular distribution and possible colocalization of CAII and AE 1 in living cells and b) to measure bicarbonate transport rate of AE1 in the absence and presence of CAII in HEK 293 cells und human red blood cells. We expressed fusion proteins of AE1 and yellow fluorescent protein and of CAII and cyan fluorescent protein in HEK 293 cells and observed the distribution of CAII and AE1 with a confocal laser scanning microscope. AE1 was exclusively associated with the plasma membrane, while CAII was homogeneously distributed in the cytoplasm. There was no apparent enrichment of CAII in the plasma membrane. In mass spectrometric measurements of bicarbonate permeability (PHCO3−) we observed a marked increase of PHCO3− over controls upon expression of AE1. Coexpression of AE1 and CAII did not lead to a further increase in PHCO3−. In another study, we measured the bicarbonate permeability of normal and CAII‐ deficient human red blood cells and found no difference in PHCO3−. Both results suggest that there is no direct functional interaction between AE1 and CAII in HEK 293 and human red blood cells.