ABSTRACT
In the expanding gene family of carbonic anhydrases (CAs), three isoforms that are evolutionally well conserved but lack classical CA catalytic activity have been reported and designated as carbonic anhydrase-related proteins (CA-RPs): CA-RPs VIII, X, and XI. These CA-RPs lack one or more histidine residues required to bind the zinc ion, which is essential for CO2 hydration activity. Homologous CA-like domains without zinc-binding histidine residues have been found in extracellular parts of receptor-type protein tyrosine phosphatases, RPTPF and RPTPK. These CA-RPs are consistently expressed in the brain, and CA-RP VIII and RPTPK are additionally expressed in the peripheral tissues. Similar to active transmembrane CAs IX and XII, CA-RP VIII is reported to be overexpressed in certain types of carcinoma. A CA-RP domain of RPTPF expressed on the glial cell surface has been shown to bind to a neuronal cell-surface protein, contactin. Further, contactin forms a complex with a transmembrane protein, a contactin-associated protein (Caspr),
findings suggest a novel function of the CA-RP subfamily: they possibly bind to some other proteins and function as a protein complex.
