Light and low-CO2-dependent LCIB-LCIC complex localization in the chloroplast supports the carbon-concentrating mechanism in Chlamydomonas reinhardtii.

The carbon-concentrating mechanism (CCM) is essential to support photosynthesis under CO2-limiting conditions in aquatic photosynthetic organisms, including the green alga Chlamydomonas reinhardtii. The CCM is assumed to be comprised of inorganic carbon transport systems that, in conjunction with carbonic anhydrases, maintain high levels of CO2 around ribulose-1, 5-bisphosphate carboxylase/oxygenase in a specific compartment called the pyrenoid. A set of transcripts up-regulated during the induction of the CCM was identified previously and designated as low-CO2 (LC)-inducible genes. Although the functional importance of one of these LC-inducible genes, LciB, has been shown recently, the biochemical properties and detailed subcellular localization of its product LCIB remain to be elucidated. Here, using yeast two-hybrid, immunoprecipitation and mass spectrometry analyses we provide evidence to demonstrate that LCIB interacts with the LCIB homologous protein LCIC in yeast and in vivo. We also show that LCIB and LCIC are co-localized in the vicinity of the pyrenoid under LC conditions in the light, forming a hexamer complex of approximately 350 kDa, as estimated by gel filtration chromatography. LCIB localization around the pyrenoid was dependent on light illumination and LC conditions during active operation of the CCM. In contrast, in the dark or under high-CO2 conditions when the CCM was inactive, LCIB immediately diffused away from the pyrenoid. Based on these observations, we discuss possible functions of the LCIB-LCIC complex in the CCM.