Thylakoid localized bestrophin-like proteins are essential for the CO₂ concentrating mechanism of Chlamydomonas reinhardtii.

ABSTRACT: The green alga Chlamydomonas reinhardtii possesses a CO₂ concentrating mechanism (CCM) that helps in successful acclimation to low CO₂ conditions. Current models of the CCM postulate that a series of ion transporters bring HCO₃ ^- from outside the cell to the thylakoid lumen, where the carbonic anhydrase 3 (CAH3) dehydrates accumulated HCO₃ ^- to CO₂, raising the CO₂ concentration for Ribulose bisphosphate carboxylase/oxygenase (Rubisco). Previously, HCO₃ ^- transporters have been identified at both the plasma membrane and the chloroplast envelope, but the transporter thought to be on the thylakoid membrane has not been identified. Three paralogous genes (BST1, BST2, and BST3) belonging to the bestrophin family have been found to be up-regulated in low CO₂ conditions, and their expression is controlled by CIA5, a transcription factor that controls many CCM genes. YFP fusions demonstrate that all 3 proteins are located on the thylakoid membrane, and interactome studies indicate that they might associate with chloroplast CCM components. A single mutant defective in BST3 has near-normal growth on low CO₂, indicating that the 3 bestrophin-like proteins may have redundant functions. Therefore, an RNA interference (RNAi) approach was adopted to reduce the expression of all 3 genes at once. RNAi mutants with reduced expression of BST1-3 were unable to grow at low CO₂ concentrations, exhibited a reduced affinity to inorganic carbon (C_i) compared with the wild-type cells, and showed reduced C_i uptake. We propose that these bestrophin-like proteins are essential components of the CCM that deliver HCO₃ ^- accumulated in the chloroplast stroma to CAH3 inside the thylakoid lumen.

SUBMITTER: Mukherjee A 

PROVIDER: S-EPMC6708349 | biostudies-literature | 2019 Aug

REPOSITORIES: biostudies-literature