Structural studies of the limiting CO2 inducible B protein family and the yeast cleavage factor I complex

Abstract

Aquatic photosynthetic microorganisms use a variety of CO2 concentrating mechanisms (CCM) to ensure a sufficient supply of CO2 to the carbon fixation enzyme rubisco. In the green algae Chlamydomonas reinhardtii, the limiting CO2-inducible protein B (LCIB) is an essential component of the CCM and is critical for survival in air levels of CO2. To elucidate the functions of the LCIB proteins, we solved the crystal structures of several LCIB homologs in C. reinhardtii and P. tricornutum and characterized them biochemically. For the first time, the LCIB proteins were revealed to resemble β-type carbonic anhydrase (CA) closely in the overall fold, active site, and oligomerization. The LCIB homologs in P. tricornutum are also active CAs. Furthermore, the proton shuttle may play an important role in regulating the activity of the LCIB proteins. The structural and biochemical data demonstrated that the existence of a potential alternative protein exit tunnel could rationalize the different CA activity levels in LCIB homologs, with the size and electrostatic landscape of the tunnel playing a role in activity regulation.