A Rubisco-binding protein is required for normal pyrenoid number and starch sheath morphology in Chlamydomonas reinhardtii

Alan K Itakura, Kher Xing Chan, Nicky Atkinson, Leif Pallesen, Lianyong Wang, Gregory Reeves, Weronika Patena, Oliver Caspari, Robyn Roth, Ursula Goodenough, Alistair J McCormick, Howard Griffiths, Martin C Jonikas

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

A phase-separated, liquid-like organelle called the pyrenoid mediates CO2 fixation in the chloroplasts of nearly all eukaryotic algae. While most algae have 1 pyrenoid per chloroplast, here we describe a mutant in the model alga Chlamydomonas that has on average 10 pyrenoids per chloroplast. Characterization of the mutant leads us to propose a model where multiple pyrenoids are favored by an increase in the surface area of the starch sheath that surrounds and binds to the liquid-like pyrenoid matrix. We find that the mutant's phenotypes are due to disruption of a gene, which we call StArch Granules Abnormal 1 (SAGA1) because starch sheath granules, or plates, in mutants lacking SAGA1 are more elongated and thinner than those of wild type. SAGA1 contains a starch binding motif, suggesting that it may directly regulate starch sheath morphology. SAGA1 localizes to multiple puncta and streaks in the pyrenoid and physically interacts with the small and large subunits of the carbon-fixing enzyme Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), a major component of the liquid-like pyrenoid matrix. Our findings suggest a biophysical mechanism by which starch sheath morphology affects pyrenoid number and CO2-concentrating mechanism function, advancing our understanding of the structure and function of this biogeochemically important organelle. More broadly, we propose that the number of phase-separated organelles can be regulated by imposing constraints on their surface area.

Original languageEnglish
Pages (from-to)18445-18454
Number of pages10
JournalProceedings of the National Academy of Sciences (PNAS)
Issue number37
Early online date27 Aug 2019
Publication statusPublished - 10 Sept 2019

Keywords / Materials (for Non-textual outputs)

  • carbon fixation
  • phase separation
  • starch
  • Rubisco
  • pyrenoid


Dive into the research topics of 'A Rubisco-binding protein is required for normal pyrenoid number and starch sheath morphology in Chlamydomonas reinhardtii'. Together they form a unique fingerprint.

Cite this