Polarized expression of a band 3-related protein in mammalian sperm cells

Extracellular bicarbonate is known to be essential for sperm motility. This finding indirectly suggests that sperm cells possess a specific carrier protein, allowing this impermeant anion to cross the cell's plasma membrane. In this study, we have identified a protein in both human and rat sperm cells that is immunologically related to erythrocyte bicarbonate/Cl exchanger (AE1) and its close homolog (AE2). We used antibodies raised against synthetic C-terminal peptides of either AE1 (band 3) or a related protein (AE2) expressed mainly in the stomach. Indirect immunofluorescence experiments revealed that both antibodies recognized a protein that is expressed in a highly polarized fashion in the sperm cells, being present only in the equatorial segment of the sperm head. Confocal laser scanning microscopy further showed that the human protein is arranged circularly close to or at the plasma membrane and that it forms a ring-like structure around the sperm head. In the human testis, the seminiferous tubules were also stained with the anti-AE2 antiserum, indicating that the protein is also expressed in developing sperm cells. This observation was also supported by Northern blot analysis, which confirmed the presence of a 4.5-kb AE2 mRNA in the rat testis tissue. We suggest that the sperm band 3-related protein could be the transport protein that mediates the effects of extracellular bicarbonate on sperm motility.