Structure and function of LCI1: a plasma membrane CO 2 channel in the Chlamydomonas CO 2 concentrating mechanism.

Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO ₂ in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO ₂ concentrating mechanism (CCM) to accumulate high internal concentrations of inorganic carbon (C _i ) to maintain photosynthetic performance. For eukaryotic algae, a combination of molecular, genetic and physiological studies using the model organism Chlamydomonas reinhardtii, have revealed the function and molecular characteristics of many CCM components, including active C _i uptake systems. Fundamental to eukaryotic C _i uptake systems are C _i transporters/channels located in membranes of various cell compartments, which together facilitate the movement of C _i from the environment into the chloroplast, where primary CO ₂ assimilation occurs. Two putative plasma membrane C _i transporters, HLA3 and LCI1, are reportedly involved in active C _i uptake. Based on previous studies, HLA3 clearly plays a meaningful role in HCO ₃ ^- transport, but the function of LCI1 has not yet been thoroughly investigated so remains somewhat obscure. Here we report a crystal structure of the full-length LCI1 membrane protein to reveal LCI1 structural characteristics, as well as in vivo physiological studies in an LCI1 loss-of-function mutant to reveal the C _i species preference for LCI1. Together, these new studies demonstrate LCI1 plays an important role in active CO ₂ uptake and that LCI1 likely functions as a plasma membrane CO ₂ channel, possibly a gated channel.
(© 2020 Society for Experimental Biology and John Wiley & Sons Ltd.)