期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：23
页码：7315-7320
DOI：10.1073/pnas.1501659112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：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. The supply of inorganic carbon (Ci; CO2 and HCO3-) is an environmental rate-limiting factor in aquatic photosynthetic organisms. To overcome the difficulty in acquiring Ci in limiting-CO2 conditions, an active Ci uptake system called the CO2-concentrating mechanism (CCM) is induced to increase CO2 concentrations in the chloroplast stroma. An ATP-binding cassette transporter, HLA3, and a formate/nitrite transporter homolog, LCIA, are reported to be associated with HCO3- uptake [Wang and Spalding (2014) Plant Physiol 166(4):2040-2050]. However, direct evidence of the route of HCO3- uptake from the outside of cells to the chloroplast stroma remains elusive owing to a lack of information on HLA3 localization and comparative analyses of the contribution of HLA3 and LCIA to the CCM. In this study, we revealed that HLA3 and LCIA are localized to the plasma membrane and chloroplast envelope, respectively. Insertion mutants of HLA3 and/or LCIA showed decreased Ci affinities/accumulation, especially in alkaline conditions where HCO3- is the predominant form of Ci. HLA3 and LCIA formed protein complexes independently, and the absence of LCIA decreased HLA3 mRNA accumulation, suggesting the presence of unidentified retrograde signals from the chloroplast to the nucleus to maintain HLA3 mRNA expression. Furthermore, although single overexpression of HLA3 or LCIA in high CO2 conditions did not affect Ci affinity, simultaneous overexpression of HLA3 with LCIA significantly increased Ci affinity/accumulation. These results highlight the HLA3/LCIA-driven cooperative uptake of HCO3- and a key role of LCIA in the maintenance of HLA3 stability as well as Ci affinity/accumulation in the CCM.
