Big1 is a cell-cycle regulator linking cell size to basal body number in Tetrahymena thermophila

Cell-size control is necessary for the optimum function and fitness of cells. In many eukaryotes, cell size is controlled through the initiation of cell division once a size threshold is achieved.¹ Cells sense their size through the relationship of cell-size-dependent and independent factors, though the mechanistic details are known for few eukaryotes.^2,3 An additional dimension of cell-size control is the subcellular localization and control of cell-cycle regulators. The cell cortex is a known site of cell-cycle regulator accumulation that maintains an established cell size based on surface area.^4,5,6 Once cells commit to division, cyclins and their associated cyclin-dependent kinases (CDKs) coordinate DNA synthesis, mitosis, and cell division.^7,8 Cyclin/CDK activity is regulated by multiple mechanisms,^{9,10,11,12,13,14} including by non-coding RNAs¹⁵ and RNA-binding proteins.^16,17 Centrosomes are also activation sites of cyclin/CDKs and other cell-cycle regulators.^{18,19,20,21,22,23,24,25,26,27,28,29,30} The cortex of the ciliate Tetrahymena thermophila is distinguished by ciliary rows comprised of basal bodies (BBs), which are similar to the centrioles within centrosomes and the site of Tetrahymena CDK localization.^31,32,33 The number of BBs per Tetrahymena cell remains consistent for each cell-cycle stage, even as the number and lengths of ciliary rows vary,³⁴ indicating that the BB number is more regulated than cell volume. Utilizing the large-cell Tetrahymena big1-1 mutant strain,³⁵ we identify Big1 to be an RNA-binding domain (RBD)-containing protein that localizes to BBs and controls cell size through its influence on cell-cycle progression, suggesting that BBs are a size-dependent measure of cell growth.