Material properties of Rubisco-EPYC1 condensates in an algal pyrenoid model

The photosynthetic alga, Chlamydomonas reinhardtii, concentrates carbon dioxide in the pyrenoid, a biomolecular condensate within the chloroplast, for more efficient energy collection from the sun. The pyrenoid matrix has two main components: the rigid carbon-fixing enzyme Rubisco and a linker protein Essential Pyrenoid Component 1 (EPYC1) which serves as molecular glue. In order to understand the steady-state and dynamical properties of Rubisco-EPYC1 condensates, we developed a coarse-grained model of interacting Rubisco and EPYC1, with bonding kinetics and molecular details learned from experiments. Through this model, we discovered that phase separation depends not only on the bonding stoichiometry between EPYC1 and Rubisco but also on EPYC1 properties such as the number of stickers on each molecule and the linker lengths between stickers. These results help us understand how cells regulate pyrenoid properties such as formation and dissolution via EPYC1 modifications.