Primary structure of a sperm cell anion exchanger and its messenger ribonucleic acid expression during spermatogenesis

Chloride/bicarbonate (Cl-/HCO3-) exchangers are a family of proteins (anion exchanger [AE] gene family) that regulate many vital cellular processes su ch as intracellular pH, cell volume, and Cl- concentration. They may also b e involved in the regulation of sperm cell motility and acrosome reaction d uring fertilization, as these two phenomena are bicarbonate dependent, and we have previously shown that a polypeptide immunologically related to eryt hrocyte band 3 is expressed in mammalian sperm cells. We have now identifie d this putative sperm cell anion exchanger as the AE2 isoform of this gene family. First, we determined its complete primary structure from the human testis lambda gt 11 cDNA library. The cloned sequence was found to consist of 3896 base pairs (bp) with an open reading frame of 3726 bp, and to be al most identical to the previously published human genomic AE2 sequence. Only four amino acid disparities were found between these two sequences. Second , our in situ hybridization analyses showed that AE2 mRNA is expressed in d eveloping sperm cells, indicating that the cloned sequence corresponds to t he sperm cell AE. Our reverse transcription-polymerase chain reaction analy ses suggested further that the expression of AE2 mRNA was variable to some extent during the epithelial cell cycle. Strongest expression was observed at stages VII-XIV except for stage X, i.e., when major structural and morph ological changes take place. These results suggest that the full-length AE2 isoform regulates HCO3- transport in mature sperm cells and thus their mot ility in vivo.