Sequence of bovine carbonic anhydrase VI: potential recognition sites for N-acetylgalactosaminyltransferase

Carbonic anhydrases (CAs I-VII) are products of a gene family that encodes seven isoenzymes and several CA-related proteins. We report the cloning and sequencing of the cDNA clones encoding one of these isoenzymes, CA VI, from bovine submaxillary gland. The translated polypeptide consists of 319 amino acids, including a signal peptide (14 amino acids) typical of secreted proteins. The predicted mature protein contains 305 amino acids including a 13-amino-acid C-terminal sequence that is also present in the sheep but absent in human CA VI. The deduced mature bovine protein is 87% and 68% identical to that of sheep and human CA VI, respectively. Active-site residues of the enzyme, as well as the three zinc-binding histidines and the two cysteines involved in an intra-chain disulphide bond, are all conserved in the three species. Two potential Asn-glycosylation sites are also conserved, both of which appear to be glycosylated in sheep and bovine CA VI. Two potential peptide recognition sequences are present in bovine CA VI for the glycoprotein hormone: N-acetylgalactosaminyltransferase (GalNAc-transferase), which is one of the two transferases required to form GalNAc-4-SO4 in bovine CA VI-linked oligosaccharides. Specifically, these two sequences are Asp-Leu-Lys-Met-Lys-Lys and Ile-Thr-Lys-Arg-Lys-Lys. Comparison of these sequences with sheep and human CA VI sequences indicates that distinct glycoforms of CA VI could exist in submaxillary gland from different species.