Big1 is a cell cycle regulator linking cell size to basal body number

Cell size control in dividing cells coordinates cell growth with cell division. In the ciliated protozoan, Tetrahymena, there is a tight link between cell size and the cytoskeletal assemblies at the cell cortex organized around basal bodies (BBs). BBs dictate the distribution of ciliary units governing cell motility and are organized into 18-22 ciliary rows. The number of BBs per cell remains remarkably consistent even when the number and lengths of ciliary rows vary. big1-1 mutant cells are large and have elevated numbers of BBs, providing a system to investigate links between BB number and cell size control. We discovered BIG1 encodes a protein with an RRM3 RNA-binding domain similar to the fission yeast meiotic entry gene, mei2. The big1-1 mutation is a predicted null allele. By extending the duration of specific cell cycle stages conducive to new BB assembly, big1-1 promotes cell size increases through BB amplification. In contrast, excess Big1 protein localizes to BBs and drives cells into premature cell division, resulting in small cells with fewer BBs. Thus, Tetrahymena Big1 localizes to BBs and controls cell cycle progression, indicating BBs and Big1 link cell growth to the cell division cycle.