LCIB functions as a carbonic anhydrase: evidence from yeast and Arabidopsis carbonic anhydrase knockout mutants.

Chlamydomonas reinhardtii evolved a CO-concentrating mechanism (CCM) because of the limited CO in its natural environment. One critical component of the C. reinhardtii CCM is the limiting CO inducible B (LCIB) protein. LCIB is required for acclimation to air levels of CO. C. reinhardtii cells with a mutated LCIB protein have an 'air-dier' phenotype when grown in low CO conditions, meaning they die in air levels of CO but can grow in high and very low CO conditions. The LCIB protein functions together with its close homolog in C. reinhardtii, limiting CO inducible C protein (LCIC), in a hexameric LCIB-LCIC complex. LCIB has been proposed to act as a vectoral carbonic anhydrase (CA) that helps to recapture CO that would otherwise leak out of the chloroplast. Although both LCIB and LCIC are structurally similar to βCAs, their CA activity has not been demonstrated to date. We provide evidence that LCIB is an active CA using a Saccharomyces cerevisiae CA knockout mutant (∆NCE103) and an Arabidopsis thaliana βCA5 knockout mutant (βca5). We show that different truncated versions of the LCIB protein complement ∆NCE103, while the full length LCIB protein complements βca5 plants, so that both the yeast and plant mutants can grow in low CO conditions.