Molecular characterization of the human carbonic anhydrase-related protein (HCA-RP VIII)

The very evolutionarily conserved human carbonic anhydrase-related polypeptide (CA-RP VIII) lacks the carbon-dioxide hydration-activity, characteristic of the enzymatically active carbonic anhydrases. We have expressed HCA-RP VIII as a glutathione-S-transferase fusion protein (GST-HCA-RP VIII). The purified HCA-RP VIII showed a substantially higher apparent molecular weight by gel-filtration compared to the molecular weight calculated from the amino acid sequence, indicating a larger than expected Stoke's radius. Like other studied CA's, the protein unfolds through two transitions at increasing concentrations of guanidine hydrochloride. The far-UV CD spectra of HCA-RP VIII indicates a secondary structure similar to that of the catalytically active HCA II. The very high sequence identity between human and mouse CA-RP VIII (98%), might indicate that the function of the protein involves binding of another protein. However, an attempt to use the GST-HCA-RP VIII fusion protein to affinity purify a ligand was unsuccessful.