The conversion of carbon dioxide and water into sugar is one of the most important biological processes for life on earth. The majority of carbon dioxide exists in the form of bicarbonate (HCO3) in the ocean. To improve carbon assimilation, cyanobacteria, the most common bacteria in the ocean, evolved the CO2 concentration mechanism. It contains specific transport proteins in cellular membranes to pump HCO3？/CO2 into the cell; the enriched HCO3？ inside the cyanobacterial cell is then used by an enzyme known as RuBisCO, which is encapsulated within a protein-based organelle called the carboxysome.Therefore, understanding how HCO3？ transporters fulfil their functions in nature is important in the field.

　　Using state-of-the-art protein crystallization and cryo-electron microscopy techniques, we obtained the three-dimensional structure of a key HCO3？ transporter-BicA in CCM. BicA is a low-affinity, high-flux SLC26-family bicarbonate transporter. It contains a transmembrane domain (BicATM) and a cytoplasmic STAS domain (BicASTAS). The structure revealed the BicATMin an inward-facingHCO3—boundconformation, and the BicASTASessential for the functional dimer formation. The residues involved in substrate binding and dimer formation were further verified by fluorescence microscopy and physiological analysis. The structure information allows a proposal of an elevator transport mechanism that is applicable to the SLC26/4 family of sodium-dependent bicarbonate transporters.This work represents a step towards installing functional carbon assimilation systems in crop plants to improve photosynthesis and crop yields in the long-term. In addition, it provides further insights into the generic principles that bicarbonate transporters use to work, as bicarbonate transporters are also essential for human health and disease.

　　This work entitled “Structural mechanism of the active bicarbonate transporter from cyanobacteria”has been published in Nat Plants 2019. 5,1184-1193.This work was supported by grants from the National Natural Science Foundation of China, Royal Society Newton Advanced Fellowship, and Chinese academy of Sciences.

　　Link: https://www.nature.com/articles/s41477-019-0538-1 

　　Contact:

　　Dr. Peng Zhang, Professor

　　National Key Laboratory of Plant Molecular Genetics, CAS Center for Excellence in Molecular Plant Sciences/Shanghai Institute of Plant Physiology and Ecology (SIPPE), Chinese Academic of Sciences

　　Email: Pengzhang01@sibs.ac.cn