Thylakoid localized bestrophin-like proteins are essential for the CO(2) concentrating mechanism of Chlamydomonas reinhardtii.
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BST3 is Cre16.g663450, one of three paralogous bestrophin-like genes clustered on chromosome 16
"BST1 (Cre16.g662600), BST2 (Cre16.g663400), and BST3 (Cre16.g663450) (collectively BST1–3) are paralogous bestrophin-like genes"
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BST1-3 are >80% identical; BST2 and BST3 were earlier called LCI11
"BST2 and BST3 were previously reported as LCI11 by Fang et al. (16). An alignment between the 3 Chlamydomonas bestrophin-like proteins showed that the proteins are >80% identical to one another"
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BST1-3 are predicted chloroplast membrane proteins with a transit peptide
"predicted that each BST protein had a chloroplast transit peptide and was likely to be a chloroplast membrane protein"
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Homology modelling supports a pentameric bestrophin fold and an anion-selective pore
"the selective pore is positively charged, supporting the hypothesis that BST1–3 transport negatively charged ions"
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BST3 expression is strongly CIA5-dependent, like other CCM genes
"the cia5 mutant exhibited severely reduced expression of BST1 and BST3 under both CO2 conditions, a transcriptional pattern observed with other CCM genes"
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BST1-3 are induced within 2 h of transfer to low CO2
"All 3 genes had increased transcript levels within 2 h after the switch to low CO2"
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BST3 under its native promoter localizes like the PSAD-driven fusion, enriched ~1.46-fold at the pyrenoid periphery
"This line showed the same localization pattern as BST3 under the constitutive PSAD promoter (Fig. 3C), and quantification of enrichment showed a 1.46-fold enrichment"
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Venus-tagged BST1-3 localize to chloroplast thylakoid membranes extending into pyrenoid tubules
"All 3 BST-like proteins localized to the thylakoid membranes of the chloroplast (Fig. 3A), and this localization visibly extended into the thylakoid tubules of the pyrenoid"
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The bst3 CLiP mutant lacks BST3 protein but has only a weak growth phenotype
"The BST3 transcript was not detected in bst3 (SI Appendix, Fig. S4B), and the BST3 protein was absent"
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Single bst3 loss is buffered by BST1/BST2 redundancy
"This led us to think that BST1 or BST2 function might be redundant with BST3"
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Triple knockdown impairs low-CO2 growth and Ci uptake
"These results indicate that BST1–3 play an important role in Ci uptake and fixation in low CO2 conditions in Chlamydomonas."
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BST1-3 are proposed (not shown) to deliver HCO3- to lumenal CAH3
"We propose that BST1–3 are the transporters that bring HCO3− to CAH3 inside the thylakoid."
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BST1-3 function differs from Arabidopsis VCCN1 (opposite pmf effect)
"The presence of AtVCCN1 decreases pmf in Arabidopsis, but the presence of the 3 BST proteins increases pmf in Chlamydomonas."