期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2016
卷号：113
期号：51
页码：14716-14721
DOI：10.1073/pnas.1616294113
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceMany aquatic microorganisms have evolved CO2-concentration mechanisms (CCMs) to boost photosynthesis. The green algae Chlamydomonas reinhardtii has the best-characterized eukaryotic CCM model. Mutants lacking the limiting CO2-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 {beta}-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. Aquatic microalgae have evolved diverse CO2-concentrating mechanisms (CCMs) to saturate the carboxylase with its substrate, to compensate for the slow kinetics and competing oxygenation reaction of the key photosynthetic CO2-fixing enzyme rubisco. The limiting CO2-inducible B protein (LCIB) is known to be essential for CCM function in Chlamydomonas reinhardtii. To assign a function to this previously uncharacterized protein family, we purified and characterized a phylogenetically diverse set of LCIB homologs. Three of the six homologs are functional carbonic anhydrases (CAs). We determined the crystal structures of LCIB and limiting CO2-inducible C protein (LCIC) from C. reinhardtii and a CA-functional homolog from Phaeodactylum tricornutum, all of which harbor motifs bearing close resemblance to the active site of canonical {beta}-CAs. Our results identify the LCIB family as a previously unidentified group of {beta}-CAs, and provide a biochemical foundation for their function in the microalgal CCMs.
