Biogenesis, engineering and function of membranes in the CO 2 -fixing pyrenoid.

Approximately one-third of global CO ₂ assimilation is performed by the pyrenoid ¹ , a liquid-like organelle found in most algae and some plants ² . Specialized membranes are hypothesized to drive CO ₂ assimilation in the pyrenoid by delivering concentrated CO ₂ ^3,4 , but their biogenesis and function have not been experimentally characterized. Here, we show that homologous proteins SAGA1 and MITH1 mediate the biogenesis of the pyrenoid membrane tubules in the model alga Chlamydomonas reinhardtii and are sufficient to reconstitute pyrenoid-traversing membranes in a heterologous system, the plant Arabidopsis thaliana . SAGA1 localizes to the regions where thylakoid membranes transition into tubules and is necessary to initiate tubule formation. MITH1 localizes to the tubules and is necessary for their extension through the pyrenoid. Tubule-deficient mutants exhibit growth defects under CO ₂ -limiting conditions, providing evidence for the function of membrane tubules in CO ₂ delivery to the pyrenoid. Furthermore, these mutants form multiple aberrant condensates of pyrenoid matrix, indicating that a normal tubule network promotes the coalescence of a single pyrenoid. The reconstitution of pyrenoid-traversing membranes in a plant represents a key milestone toward engineering a functional pyrenoid into crops for improving crop yields. More broadly, our study demonstrates the functional importance of pyrenoid membranes, identifies key biogenesis factors, and paves the way for the molecular characterization of pyrenoid membranes across the tree of life.