Assembly of the algal CO2-fixing organelle, the pyrenoid, is guided by a Rubisco-binding motif

Abstract

Approximately one-third of the Earths photosynthetic CO2 assimilation occurs in a pyrenoid, an organelle containing the CO2-fixing enzyme Rubisco. How constituent proteins are recruited to the pyrenoid, and how the organelles sub-compartments - membrane tubules, a surrounding phase-separated Rubisco matrix, and a peripheral starch sheath - are held together is unknown. Using the model alga Chlamydomonas reinhardtii, we discovered that pyrenoid proteins share a sequence motif. We show that the motif is sufficient to target proteins to the pyrenoid and that the motif binds to Rubisco, suggesting a mechanism for targeting. The presence of the Rubisco-binding motif on proteins that localize to the tubules and on proteins that localize to the matrix-starch sheath interface suggests that the motif holds the pyrenoids three sub-compartments together. Our findings advance our understanding of pyrenoid biogenesis and illustrate how a single protein motif can underlie the architecture of a complex multi-layered phase-separated organelle. One Sentence SummaryA ubiquitous Rubisco-binding motif targets proteins to the pyrenoid and holds together the pyrenoids three sub-compartments.