Title

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

Document Type

Article

Publication Date

8-20-2019

Abstract

The green alga possesses a CO concentrating mechanism (CCM) that helps in successful acclimation to low CO conditions. Current models of the CCM postulate that a series of ion transporters bring HCO from outside the cell to the thylakoid lumen, where the carbonic anhydrase 3 (CAH3) dehydrates accumulated HCO to CO, raising the CO concentration for Ribulose bisphosphate carboxylase/oxygenase (Rubisco). Previously, HCO 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 (, , and ) belonging to the bestrophin family have been found to be up-regulated in low CO 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 has near-normal growth on low CO, 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 - were unable to grow at low CO concentrations, exhibited a reduced affinity to inorganic carbon (C) compared with the wild-type cells, and showed reduced C uptake. We propose that these bestrophin-like proteins are essential components of the CCM that deliver HCO accumulated in the chloroplast stroma to CAH3 inside the thylakoid lumen.

Publication Source (Journal or Book title)

Proceedings of the National Academy of Sciences of the United States of America

First Page

16915

Last Page

16920

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