The number of cytokinesis nodes in mitotic fission yeast scales with cell volume

Cytokinesis nodes are assemblies of stoichiometric ratios of proteins associated with the plasma membrane, which serve as precursors for the contractile ring during cytokinesis by fission yeast. The total number of nodes is uncertain, because of the limitations of the methods used previously. Here we used the ~140 nm resolution of Airyscan confocal microscopy to resolve a large population of dim, unitary cytokinesis nodes in 3D reconstructions of whole fission yeast cells. Wild-type fission yeast cells make about 200 unitary cytokinesis nodes. Most, but not all of these nodes condense into a contractile ring. The number of cytokinesis nodes scales with cell size in four strains tested, although wide rga4Δ mutant cells form somewhat fewer cytokinesis nodes than expected from the overall trend. The surface density of Pom1 kinase on the plasma membrane around the equators of cells is similar with a wide range of node numbers, so Pom1 does not control cytokinesis node number. However, varying protein concentrations with the nmt1 promoter showed that the numbers of nodes increase above a baseline of about 200 with the total cellular concentration of either Pom1 or the kinase Cdr2.