Characterization of a human carbonic anhydrase-like gene with novel chromosomal location.

Abstract

Carbonic anhydrase (CA) is present in amniotes as at least three separately encoded isozymes, CA I, CA II, and CA III, each with distinct tissue distribution patterns, levels of expression, specific activities, and susceptibilities to inhibition. Other reported tissue specific CAs including membrane-bound forms, mitochondrial forms, and a secreted form, also appear to be encoded by separate genes. The lack of sequence data for these additional forms, however, has limited our understanding of this gene family. While screening a human genomic library with a mouse CA II cDNA probe, clones were isolated that contained a cross-hybridizing open reading frame homologous to the sixth exon of the mouse CA II gene. The predicted amino acid sequence of this region is similar to the corresponding regions of human CA I, II, or III, indicating the probable discovery of a gene (tentatively designated CA Z) encoding an uncharacterized member of the CA gene family. Additional exons were located by reduced stringency cross-hybridization to CA II probes or directly by sequencing upstream and downstream from previously defined exons. The 10 kb gene consists of seven exons and 6 introns that interrupt the coding region where introns are found in other CA isozyme genes. These putative exons predict a protein sequence of 263 amino acids which is 50%, 56% and 49% homologous to human CA I, II, and III, respectively. Residues conserved in all carbonic anhydrases, as well as active site residues, are highly conserved in CA Z, whereas most substitutions have occurred at positions not conserved between the various CA isozymes. This pattern of conservation and substitution suggests that this gene encodes a functional enzyme. Hybridization of a CA Z probe to somatic cell hybrid DNA shows a correlation between the gene and human chromosome 16. In situ hybridization localizes the gene to the 16q21-23 region, unlike the CA I, II, and III genes which are clustered on chromosome 8. Preliminary evidence from Northern blot analysis of baboon RNAs indicate expression in the salivary gland. Since protein sequence and molecular weight data suggest that CA Z gene does not encode the CA IV, CA V, or CA VI isozyme, we propose that it be designated the CA VII gene.