Fig. 1: MITH1 and SAGA1 are necessary for pyrenoid matrix-traversing membranes in Chlamydomonas.

a, The Chlamydomonas pyrenoid is found at the base of the cell’s cup-shaped chloroplast and consists of a Rubisco matrix traversed by membrane tubules that join in a reticulated region at the centre¹². The pyrenoid is surrounded by a starch sheath thought to act as a barrier against CO₂ escape³¹. b, Flat sheets of thylakoid membrane transition into cylindrical tubules as they enter the pyrenoid matrix through gaps in the starch sheath, according to cryo-electron tomography data¹². Bicarbonate (HCO₃⁻) is thought to be concentrated in the lumen of pyrenoid matrix-traversing membranes and converted to CO₂ by carbonic anhydrase 3 (CAH3) (ref. ²²). c–f, TEM images of whole Chlamydomonas cells and pyrenoids. The strains include (c) the wild-type CMJ030 (ref. ⁴⁶), (d) a mith1 mutant³⁶, (e) a complemented strain generated by transforming MITH1-Venus into the mith1 mutant background (see Methods) and (f) a saga1 mutant¹³. Representative images from 3 biological repeats are shown. Arrowheads point to pyrenoids. t, traversing membrane tubules; m, matrix; s, starch. Images in each row are presented at the same scale.