Structure of the carbonic anhydrase VI (CA6) gene: evidence for two distinct groups within the alpha-CA gene family.

The secreted carbonic anhydrase (CA VI) is believed to be one of the oldest mammalian CAs in evolutionary terms. To elucidate its gene structure and compare it with other members of the alpha-CA family, we cloned genomic fragments encoding the bovine CA6 gene and determined its exon/intron organization. The gene spans approx. 25 kb and consists of eight exons and seven introns. Exon 1 encodes the 5' untranslated region, the signal peptide and the N-terminus of the mature enzyme. Exon 8 encodes the 3' untranslated region and the C-terminal extension that is unique to CA VI. Exons 2-7 encode the CA domain, which shows significant sequence similarity to other CAs. Two distinct groups exist in the alpha-CA family on the basis of a comparison of the known gene structures. One group consists of the cytoplasmic (CA I, II, III and VII) and mitochondrial (CA V) members. The other group consists of the membrane-bound (CA IV and IX) and secreted (CA VI) members. In particular, the seven exon/intron boundaries in the CA domain of the CA6 gene are conserved in the CA9 gene, which encodes the multidomain protein CA IX that is overexpressed in tumours and has transforming potential.