The role of BST4 in the pyrenoid of .

In many eukaryotic algae, CO fixation by Rubisco is enhanced by a CO-concentrating mechanism, which utilizes a Rubisco-rich organelle called the pyrenoid. The pyrenoid is traversed by a network of thylakoid-membranes called pyrenoid tubules, proposed to deliver CO. In the model alga (), the pyrenoid tubules have been proposed to be tethered to the Rubisco matrix by a bestrophin-like transmembrane protein, BST4. Here, we show that BST4 forms a complex that localizes to the pyrenoid tubules. A Chlamydomonas mutant impaired in the accumulation of BST4 () formed normal pyrenoid tubules and heterologous expression of BST4 in did not lead to the incorporation of thylakoids into a reconstituted Rubisco condensate. Chlamydomonas mutant did not show impaired growth at air level CO. By quantifying the non-photochemical quenching () of chlorophyll fluorescence, we show that displays a transiently lower thylakoid lumenal pH during dark to light transition compared to control strains. When acclimated to high light, had sustained higher NPQ and elevated levels of light-induced HO production. We conclude that BST4 is not a tethering protein, but rather is an ion channel involved in lumenal pH regulation possibly by mediating bicarbonate transport across the pyrenoid tubules.