Structure and function of LCI1: a plasma membrane CO2 channel in the Chlamydomonas CO2 concentrating mechanism

Abstract

Microalgae and cyanobacteria contribute roughly half of the global photosynthetic carbon assimilation. Faced with limited access to CO2 in aquatic environments, which can vary daily or hourly, these microorganisms have evolved use of an efficient CO2 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 CO2 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 HCO3- 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 CO2 uptake and that LCI1 likely functions as a plasma membrane CO2 channel, possibly a gated channel.