TY - JOUR
T1 - Thylakoid localized bestrophin-like proteins are essential for the CO2 concentrating mechanism of Chlamydomonas reinhardtii
AU - Mukherjee, Ananya
AU - Lau, Chun Sing
AU - Walker, Charlotte Elizabeth
AU - Rai, Ashwani
AU - Prejean, Camille I.
AU - Yates, Gary
AU - Emrich-Mills, Thomas
AU - Lemoine, Spencer G.
AU - Vinyard, David J.
AU - Mackinder, Luke
AU - Moroney, James V.
PY - 2019/8/20
Y1 - 2019/8/20
N2 - The green alga Chlamydomonas reinhardtii possesses a CO2 concentratingmechanism (CCM) which helps in successful acclimationto low CO2 conditions. Current models of the CCM postulate that aseries of ion transporters bring HCO3- from outside the cell to thethylakoid lumen, where the carbonic anhydrase CAH3 dehydratesaccumulated HCO3- to CO2, raising the CO2 concentration forRubisco. Previously, HCO3- transporters have been identified atboth the plasma membrane and the chloroplast envelope, butthe transporter thought to be on the thylakoid membrane hasnot been identified. Three paralogous genes (BST1, BST2, BST3)belonging to the bestrophin family have been found to be upregulatedin low CO2 conditions, and their expression is controlledby CIA5, a transcription factor that controls many CCM genes.YFP fusions demonstrate that all three proteins are located onthe thylakoid membrane, and interactome studies indicate thatthey might associate with chloroplast CCM components. A singlemutant defective in BST3 still grows nearly normally on low CO2,indicating that the three bestrophin-like proteins may have redundantfunctions. Therefore, an RNAi approach was adopted to reducethe expression of all three genes at once. RNAi mutants withreduced expression of BST1-3 were unable to grow at low CO2concentrations, exhibited a reduced affinity to inorganic carboncompared to the wild type cells, and showed reduced inorganiccarbon uptake. We propose that these bestrophin-like proteins areessential components of the CCM that deliver HCO3- accumulatedin the chloroplast stroma to CAH3 inside the thylakoid lumen.
AB - The green alga Chlamydomonas reinhardtii possesses a CO2 concentratingmechanism (CCM) which helps in successful acclimationto low CO2 conditions. Current models of the CCM postulate that aseries of ion transporters bring HCO3- from outside the cell to thethylakoid lumen, where the carbonic anhydrase CAH3 dehydratesaccumulated HCO3- to CO2, raising the CO2 concentration forRubisco. Previously, HCO3- transporters have been identified atboth the plasma membrane and the chloroplast envelope, butthe transporter thought to be on the thylakoid membrane hasnot been identified. Three paralogous genes (BST1, BST2, BST3)belonging to the bestrophin family have been found to be upregulatedin low CO2 conditions, and their expression is controlledby CIA5, a transcription factor that controls many CCM genes.YFP fusions demonstrate that all three proteins are located onthe thylakoid membrane, and interactome studies indicate thatthey might associate with chloroplast CCM components. A singlemutant defective in BST3 still grows nearly normally on low CO2,indicating that the three bestrophin-like proteins may have redundantfunctions. Therefore, an RNAi approach was adopted to reducethe expression of all three genes at once. RNAi mutants withreduced expression of BST1-3 were unable to grow at low CO2concentrations, exhibited a reduced affinity to inorganic carboncompared to the wild type cells, and showed reduced inorganiccarbon uptake. We propose that these bestrophin-like proteins areessential components of the CCM that deliver HCO3- accumulatedin the chloroplast stroma to CAH3 inside the thylakoid lumen.
U2 - 10.1073/pnas.1909706116
DO - 10.1073/pnas.1909706116
M3 - Article
SN - 0027-8424
VL - 116
SP - 16915
EP - 16920
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 34
ER -
