Low-CO₂inducible bestrophins in diatom thylakoid membranes sustain high photosynthetic efficacy at distant locations from the pyrenoid

Abstract

Anion transporters are important to sustain a variety of physiological states in cells. Bestrophins are a family of Cl- and/or HCO3- transporters conserved in bacteria, animals, algae, and plants. Recently, bestrophin paralogs were found in the green alga Chlamydomonas reinhardtii as up-regulated components in low CO2 conditions that play an essential role in the CO2-concentrating mechanism (CCM). Bestrophin orthologs are also conserved in diatoms, a group of secondary endosymbiotic algae harboring red-type plastids, but their physiological functions are not known yet. Here, we characterized the subcellular localization and expression profile of bestrophins in the marine diatoms Phaeodactylum tricornutum (PtBST1-4) and Thalassiosira pseudonana (TpBST1 and 2). PtBST1 and PtBST2 were localized at the stromal thylakoid membrane outside of the pyrenoid, and PtBST3 was localized in the pyrenoid. Contrarily, TpBST1 and TpBST2 were both localized in the pyrenoid. These bestrophin proteins were accumulated in cells grown in atmospheric CO2 but not in 1% CO2-grown cells. To assess the physiological functions, we generated knock-out mutants for PtBST1 by genome editing. The lack of PtBST1 decreased affinity of photosynthesis for dissolved inorganic carbon closer to that of the cells grown in 1% CO2. Additionally, non-photochemical quenching was 1.5-2.0 times higher in the mutants than that of the wild type cells. These data suggests that HCO3- transport at the stroma thylakoid membranes by PtBST1 is a critical part of the CO2 evolving system of the pyrenoid in the fully induced CCM, and simultaneously that PtBST1 modulates photoprotection in response to CO2 availability in P. tricornutum.

A Protein Blueprint of the Diatom CO2-Fixing Organelle

“…SLC4 candidates have been implicated in inorganic carbon uptake across both the plasma and thylakoid membranes. 8,27 Recently two bestrophin-like proteins were localized to the T. pseudonana pyrenoid, likely in the pyrenoid penetrating thylakoid (PPT) 28,29 and a θ-type carbonic anhydrase 2 confirmed to be located in the PPT. 8 Although, supporting functional data is missing for these proteins.…”