Structural insights into the LCIB protein family reveals a new group of β-carbonic anhydrases

Significance Many aquatic microorganisms have evolved CO 2 -concentration mechanisms (CCMs) to boost photosynthesis. The green algae Chlamydomonas reinhardtii has the best-characterized eukaryotic CCM model. Mutants lacking the limiting CO 2 -inducible B protein (LCIB) protein are unable to survive in air. To investigate the molecular underpinnings of this effect, we biochemically and structurally characterized a number of LCIB homologues from diverse organisms, including constitutively carbonic anhydrase (CA)-active proteins. We discovered that LCIB proteins structurally resemble β-CAs in both overall fold and active site architecture. Our results provide insight into the molecular mechanism of the LCIB family involved in microalgal CCMs.