Burton, M. I. | bioRxiv Science

bioRxiv2026

SBE1 drives the circumferential growth of the starch sheath around the Chlamydomonas reinhardtii pyrenoid

Pyrenoids are CO2-fixing organelles responsible for approximately one-third of global CO2 fixation. The pyrenoids of many algae are surrounded by a starch sheath proposed to perform the critical function of slowing leakage of concentrated CO2 out of the pyrenoid. How the cell shapes starch granules into curved starch plates that encase the pyrenoid to enable efficient CO2 fixation is currently unknown. Here, we elucidate how starch transitions from granules into a fully formed pyrenoid starch sheath using confocal microscopy of the model green alga Chlamydomonas reinhardtii. We observe that after initiation, starch granules grow circumferentially along the surface of the pyrenoid matrix to encapsulate it. We show that the starch branching enzyme SBE1 localizes to the pyrenoid and is essential for this circumferential starch granule growth. Our data suggest that SBE1 promotes the circumferential growth of pyrenoid-associated starch granules by branching starch at the matrix-granule-stroma interface. Our findings advance the understanding of pyrenoid starch sheath assembly and, more broadly, starch-shaping mechanisms. Significance StatementThe shape of the starch granules that surround the pyrenoid is critical for efficient carbon fixation, but how these granules are shaped into curved plates is currently not understood. We find that starch granules spread along the pyrenoid surface rather than growing equally in all directions, and that this bias requires SBE1, a branching enzyme localized to the surface of the spherical pyrenoid matrix condensate. The findings support a model in which localizing a metabolic enzyme directs polymer growth to create curvature. This mechanism connects branching enzyme placement to starch granule geometry and contributes to the understanding of how cells convert local biosynthetic activity into large-scale organelle architecture.

molecular biology↗

bioRxiv2026

SAGA1 and SAGA2 localize the starch sheath to the pyrenoid in Chlamydomonas reinhardtii

Most algae enhance their CO2 assimilation by concentrating CO2 within the pyrenoid, a biomolecular condensate that contains the CO2-fixing enzyme Rubisco. Many pyrenoids are surrounded by a starch sheath that is thought to slow the escape of CO2 from the pyrenoid, but how the starch sheath is localized to the pyrenoid remains poorly understood. Here, in the leading model alga Chlamydomonas reinhardtii, we find that the protein SAGA2 is necessary for early pyrenoid starch sheath biogenesis and works redundantly with its homolog, SAGA1, to localize the starch sheath to the pyrenoid. SAGA2 and SAGA1 were enriched in different regions of the pyrenoid-starch sheath interface, suggesting that they play complementary roles. Both saga2 and saga1 mutants showed defects in starch sheath coverage early during pyrenoid formation that were improved at a later timepoint. Strikingly, a saga1;saga2 double mutant did not have a starch sheath around the pyrenoid and showed decreased overall starch content. SAGA1 and SAGA2 starch-binding domains bound to starch, the starch mimic molecule {beta}-cyclodextrin, and the starch precursor molecule maltoheptaose, suggesting a role for SAGA1 and SAGA2 in starch granule initiation. We propose a model where SAGA1 and SAGA2 each locally prime starch sheath initiation in a distinct region of the pyrenoid surface by enriching starch precursor molecules around the pyrenoid. These findings advance the understanding of algal starch sheath biogenesis and provide insights into the associations between biomolecular condensates and other cellular structures. Significance StatementEukaryotic algae enhance their carbon assimilation using an organelle called the pyrenoid, where concentrated CO2 is supplied to the CO2-fixing enzyme Rubisco. In many algae, a starch sheath surrounding the pyrenoid is thought to enhance CO2 fixation, but how starch is localized to pyrenoids is unknown. Here, we show that two proteins, SAGA1 and SAGA2, each bind to starch precursor molecules and redundantly localize starch to the pyrenoid in the alga Chlamydomonas reinhardtii. Our results suggest that SAGA1 and SAGA2 promote starch sheath initiation at the pyrenoid, rather than merely tethering starch, as previously thought. This work advances the understanding of the proteins and molecular mechanisms involved in pyrenoid starch sheath biogenesis and lays the foundations for their further study.

molecular biology↗