Three-dimensional structure of a halotolerant algal carbonic anhydrase predicts halotolerance of a mammalian homolog

摘要

Protein molecular adaptation to drastically shifting salinities was studied in dCA Ⅱ, an α-type carbonic anhydrase (EC 4.2.1.1) from the exceptionally salt-tolerant unicellular green alga Dunaliella salina. The salt-inducible, extracellular dCA Ⅱ is highly salt-tolerant and thus differs from its mesophilic homologs. The crystal structure of dCA Ⅱ, determined at 1.86-A resolution, is globally similar to other α-type carbonic carbonic anhydrases except for two extended α-helices and an added Na-binding loop. Its unusual electrostatic properties include a uniformly negative surface electrostatic potential of lower magnitude than that observed in the highly acidic halophilic proteins and an exceptionally low positive potential at a site adjoining the catalytic Zn~(2+) compared with mesophilic homologs. The halotolerant dCA Ⅱ also differs from typical halophilic proteins in retaining conformational stability and solubility in low to high salt concentrations. The crucial role of electrostatic features in dCA Ⅱ halotolerance is strongly supported by the ability to predict the unanticipated halotolerance of the murine CA XIV isozyme, which was confirmed biochemically. A proposal for the functional significance of the halotolerance of CA XIV in the kidney is presented.