A Salt-resistant Plasma Membrane Carbonic Anhydrase Is Induced by Salt in

The mechanisms allowing proliferation of the unicellular green alga Dunaliella salina in up to saturating NaCl concentrations are only partially understood at present. Previously, the level of a plasma membrane Mr 60,000 protein, p60, was found to increase with rising external salinities. Based on cDNA cloning and enzymatic assays, it is now shown that p60 is an internally duplicated carbonic anhydrase, with each repeat homologous to animal and Chlamydomonas reinhardtii carbonic anhydrases, but exceptional in the excess of acidic over basic residues. Increasing salinities, alkaline shift, or removal of bicarbonate induced in D. salina parallel increases in the levels of p60, its mRNA, and external carbonic anhydrase activity. Moreover, purified p60 exhibited carbonic anhydrase activity comparable to other carbonic anhydrases. A p60-enriched soluble preparation showed maximal carbonic anhydrase activity at approximately 1.0 M NaCl and retained considerable activity at higher salt concentrations. In contrast, a similar preparation from C. reinhardtii was approximately 90% inhibited in 0.6 M NaCl. These results identified p60 as a structurally novel carbonic anhydrase transcriptionally regulated by CO2 availability and exhibiting halophilic-like characteristics. This enzyme is potentially suited to optimize CO2 uptake by cells growing in hypersaline media.