Bestrophin-like protein 4 is involved in photosynthetic acclimation to light fluctuations in Chlamydomonas.
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BST4 is the same gene as RBMP1 (Cre06.g261750) and localizes exclusively to the pyrenoid tubules
"The protein BST4 (bestrophin-like protein 4, also known as Rubisco binding membrane protein 1, RBMP1; Cre06.g261750) localizes exclusively to the pyrenoid tubules"
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Dual tagging with RBCS1 places BST4 in the tubules, not the Rubisco matrix
"indicating that BST4 is located in the tubules and not the Rubisco-enriched pyrenoid matrix"
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BST4 has a conserved bestrophin domain related to BST1-3
"BST4 also has a well-conserved bestrophin domain similar to those in the thylakoid-localized BST1-3 proteins"
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The selectivity pore of BST4 diverges from BST1-3
"Second, BST4 has a phenylalanine residue in the first position of the putative selection pore, as opposed to valine which is conserved throughout BST1-3"
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The BST4 C-terminus interacts with Rubisco small subunit (yeast two-hybrid)
"We confirmed using a yeast-2-hybrid approach that the C-terminus of BST4 interacts with CrRBCS1"
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BST4 assembles as pentamers in vivo
"The resulting probability distribution revealed that the most common BST4 complex is made up of 5 molecules"
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The RBM-bearing C-terminus is needed for tubule localization
"BST4ΔC-term-mScarlet expressed in the bst4 mutant line did not localize to the pyrenoid tubules but was found throughout the thylakoid membrane"
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BST4 is not required for the tubule-matrix interface
"As a result, we conclude that BST4 is not necessary for the pyrenoid tubule–Rubisco matrix interface in Chlamydomonas."
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bst4 mutants grow normally at air CO2 under continuous light
"We conclude from these experiments that BST4 is not essential for growth at air levels of CO2 and might not be necessary for the functioning of the CCM."
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bst4 mutants have a growth defect under fluctuating high light
"We found that under fluctuating high light, bst4 had a growth defect compared to the WT and the line complemented with full-length BST4"
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No currents were detected for BST4 in Xenopus oocytes, so the permeant ion is unknown
"Therefore, we were unable to conclude what BST4 is permeable to."
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Authors conclude that BST4 is a pentameric tubule channel, not a tether
"Based on these findings, we conclude that BST4 is a pentameric transmembrane channel found within the pyrenoid tubules but is not crucial for Rubisco matrix tethering."
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Proposed role in lumen ion homeostasis and pH during fluctuating light
"BST4 influences the ion homeostasis and subsequently the pH of the pyrenoid tubules through its function as an ion channel, particularly during fluctuating light."