Mitochondrial carbonic anhydrase CA VB: Differences in tissue distributionand pattern of evolution from those of CA VA suggest distinct physiological roles

A cDNA for a second mouse mitochondrial carbonic anhydrase (CA) called CA V B was identified by homology to the previously characterized murine CA V, n ow called CA VA. The full-length cDNA encodes a 317-aa precursor that conta ins a 33-aa classical mitochondrial leader sequence. Comparison of products expressed from cDNAs for murine CA VB and CA VA in COS cells revealed that both expressed active CAs that localized in mitochondria, and showed compa rable activities in crude extracts and in mitochondria isolated from transf ected COS cells. Northern blot analyses of total RNAs from mouse tissues an d Western blot analyses of mouse tissue homogenates showed differences in t issue-specific expression between CA VB and CA VA. CA VB was readily detect ed in most tissues, while CA VA expression was limited to liver, skeletal m uscle, and kidney. The human orthologue of murine CA VB was recently report ed also. Comparison of the CA domain sequence of human CA VB with that repo rted here shows that the CA domains of CA VB are much more highly conserved between mouse and human (95% identity) than the CA domains of mouse and hu man CA VAs (78% identity). Analysis of phylogenetic relationships between t hese and other available human and mouse CA isozyme sequences revealed that mammalian CA VB evolved much more slowly than CA VA, accepting amino acid substitutions at least 4.5 times more slowly since each evolved from its re spective human-mouse ancestral gene around 90 million years ago. Both the d ifferences in tissue distribution and the much greater evolutionary constra ints on CA VB sequences suggest that CA VB and CA VA have evolved to assume different physiological roles.