Condensate size control in the algal pyrenoid

Biomolecular condensates spatially organize cellular functions, but the regulation of their size, number, and phase transitions is poorly understood. The pyrenoid, an algal biomolecular condensate that mediates one-third of global CO₂fixation, typically exists as one condensate per chloroplast, but during cell division it transiently dissolves and reconfigures into multiple smaller condensates. We identified a kinase, KEY1, in the model alga Chlamydomonas reinhardtii that regulates pyrenoid condensate size, number, dissolution, and function. We show that KEY1 localizes to the condensate and dissolves it by disrupting interactions between its core constituents, the CO₂-fixing enzyme Rubisco and its linker protein EPYC1, through EPYC1 phosphorylation. We develop a minimal mathematical model of kinase activity that recapitulates pyrenoid size and number regulation. These results provide a foundation for mechanistic understanding of the regulation of size, number, localization, and phase transitions in pyrenoids and other biomolecular condensates.