BIOPHYSICS SEMINAR<br>Bistability, Trigger Waves, and the Spatial Coordination of Mitosis

Xenopus laevis eggs are huge cells, so that even a freely diffusing protein would take hours to make it from the center of the egg to the cortex. Despite this, mitosis takes place quickly and in a spatially coordinated fashion in fertilized Xenopuseggs. Because there is positive feedback in the circuit that regulates Cdk1, the egg has the possibility of supporting trigger waves of Cdk1 activation the spread over large distances faster than diffusion alone would allow. We carried out experiments to look for these trigger waves, using cell free Xenopus egg extracts in thin Teflon tubes and a fluorescence microscopy assay for mitosis. We found that Cdk1 activation does, as hypothesized, spread linearly through these extracts at a constant speed of ~1 µm/sec, allowing Cdk1 activity to spread from the center to the cortex of an egg in about 10 min. We suspect that trigger waves may be found in other signaling systems where events need to be coordinated over long distances.