What controls the size of a cell? This is a tricky question and equally applicable to subcellular structures, such as flagella. For cells, the balance between growth and division is critical; for organelles like flagella, it is often the balance between assembly and disassembly that is important. Brian Bradley and Lynne Quarmby now show that the kinase Cnk2p regulates both (see ). Cnk2p is a Chlamydomonas relative of the mitotic kinase NIMA. The authors show that it is present along the length of Chlamydomonas flagella and makes cells produce short flagella when overexpressed – by contrast, knocking down Cnk2p by RNAi leads to formation of excessively long flagella. Significantly, they also observe that the Cnk2p-overexpressing cells are smaller than normal whereas the Cnk2p-lacking cells are bigger. Bradley and Quarmby then demonstrate that this is because Cnk2p can promote disassembly of flagella as well as entry into mitosis. Further experiments lead them to conclude that Cnk2p determines the size at which cells commit to divide. These results thus shed new light on control of size; they also raise the intriguing possibility that flagella/cilia play a role in the process.
