Characterization of cooperative bicarbonate uptake into chloroplast stroma in the green alga Chlamydomonas reinhardtii

Abstract

SignificanceThe entry of inorganic carbon (Ci; CO2 and HCO3–) into cells involves many biological processes in both animals and plants, and aquaporins as well as bicarbonate transporters play roles in Ci transport. Although transporting external HCO3– into the stroma through the chloroplast envelope is one of the rate-limiting factors for aquatic photosynthetic organisms, specific molecular components in this process have not yet been identified experimentally. Molecular identification of proteins essential for Ci uptake located in the chloroplast envelope and in the plasma membrane documented in this study helps in understanding how aquatic photosynthetic organisms developed machinery to acclimate to CO2-limiting environment and to maintain adequate levels of photosynthesis for survival or growth.

Chloroplast-mediated regulation of CO 2 -concentrating mechanism by Ca 2+ -binding protein CAS in the green alga Chlamydomonas reinhardtii

“…Because it was reported that CEF activity increases in LC conditions possibly for increased ATP demand for ABC-type bicarbonate transporter HLA3 in CCM (12,28) and that CAS is associated with the CEF complex formation in anaerobic conditions (26), it was possible that CAS-dependent CEF might contribute to CCM. However, CEF activity was not affected by the lack of CAS protein in the LC conditions at 120 μmol photons·m -2 ·s -1 that we examined ( Fig.…”

“…Of those genes regulated by CAS, HLA3 and LCIA, whose accumulation was decreased in H82 cells (Fig. 1D), are involved in HCO − 3 uptake for operation of the CCM (11)(12)(13)(14). However, simultaneous expression of HLA3 and LCIA could only partially rescue the decreased Ci affinity of H82 cells (Fig.…”

“…CCM1/CIA5 was identified as a zinc-finger-type regulatory factor for the induction of most limiting-CO 2 -induced genes, including HLA3 (high-light activated 3), LCIA (low-CO 2 -inducible gene A), and LCIB (low-CO 2 -inducible gene B) (6)(7)(8)(9). HLA3 is an ATP-binding cassette transporter localized to the plasma membrane and associated with HCO 3 -transport from the outside of cells into the cytosol (10)(11)(12)(13). LCIA is a possible anion channel localized to the chloroplast envelope and is associated with inorganic carbon (Ci) (CO 2 and HCO 3 -) uptake into the chloroplast stroma in cooperation with HLA3 (12,14).…”

“…HLA3 is an ATP-binding cassette transporter localized to the plasma membrane and associated with HCO 3 -transport from the outside of cells into the cytosol (10)(11)(12)(13). LCIA is a possible anion channel localized to the chloroplast envelope and is associated with inorganic carbon (Ci) (CO 2 and HCO 3 -) uptake into the chloroplast stroma in cooperation with HLA3 (12,14). LCIB is a chloroplast soluble protein whose localization is associated with distinct CO 2 -acclimation states, including high-CO 2 (HC) (5 to 0.5%), low-CO 2 (LC) (0.03 to 0.5%), and very-low-CO 2 (VLC) (<0.03%) (15).…”

“…To examine the localization of CAS, total protein, soluble/insoluble, chloroplast envelope, and thylakoid membrane fractions were isolated and probed with antibodies against CAS, thylakoid membrane-localized D1, soluble protein LCIB, plasma membrane-localized H + -ATPase (12, 32), and chloroplast enveloplocalized LCIA (12) (Fig. S3 A-C).…”