Thylakoid localized bestrophin-like proteins are essential for the CO 2 concentrating mechanism of Chlamydomonas reinhardtii

Significance Models of the CO 2 concentrating mechanism (CCM) of green algae and diatoms postulate that chloroplast CO 2 is generated from HCO 3 − brought into the acidic thylakoid lumen and converted to CO 2 by specific thylakoid carbonic anhydrases. However, the identity of the transporter required for thylakoid HCO 3 − uptake has remained elusive. In this work, 3 bestrophin-like proteins, BST1–3, located on the thylakoid membrane have been found to be essential to the CCM of Chlamydomonas . Reduction in expression of BST1–3 markedly reduced the inorganic carbon affinity of the alga. These proteins are prime candidates to be thylakoid HCO 3 − transporters, a critical currently missing step of the CCM required for future engineering efforts of the Chlamydomonas CCM into plants to improve photosynthesis.