National Center for
5B5Z: Crystal Structure Of Ptlcib4 H88a Mutant, A Homolog Of The Limiting Co2-inducible Protein Lcib
Proc. Natl. Acad. Sci. U. S. A. (2016) 113 p.14716-14721
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.