Fig. 5: Cdc13–Cdc2(WT) localization to the SPB does not affect nuclear CDK activation.

a,b, Diagram of the SPB in a wild-type background (a) and in the cut12.s11 mutant. c, Left, wild-type (n = 96 cells; 1 of 3 biological repeats) and cut12.s11 (n = 195 cells; 1 of 3 biological repeats) cells, showing length at nuclear activation using NucCDK–mScarletI. Right, length of wild-type (n = 96 cells) and cut12.s11 (n = 195 cells) cells at cell division. The white dot represents the median value, the black rectangle represents the IQR, and the whiskers extend to the minimum and maximum, defined as data points within 1.5× IQR. d, Diagram of the SPB in the Cdc13^HPM background. e, Representative trace of NucCDK–mScarletI in cells expressing Cdc13(HPM)–sfGFP. f, Traces of mean nuclear Cdc13(HPM)–sfGFP intensity. n = 36 cells; 1 of 3 biological repeats. g, Representative trace of CytCDK–mScarletI in cells expressing Cdc13(HPM)–sfGFP. The y axis of CytCDK sensor readout was scaled on the basis of minimum and maximum values for CytCDK in Cdc13 wild-type cells. h, Model of CDK activation. During interphase, cyclin B–CDK accumulates in the nucleus, and CDK activity increases gradually, with a high threshold for activation. In late G2, CDK activation occurs and is amplified by CDK-Y15-dependent feedback loops. Subsequently, cyclin B–CDK nuclear export (dependent on centrosomal cyclin B–CDK) and cyclin B degradation occur, and high CDK activity is maintained by CDK-Y15-dependent feedback loops. Thus, the nuclear CDK system undergoes stable oscillations with respect to cyclin B–CDK concentration. By contrast, in the cytoplasm, a lower threshold for activation enables cytoplasmic CDK to activate upon import of cyclin B–CDK from the nucleus. Degradation of cytoplasmic cyclin B results in an immediate drop in CDK activity. Therefore, the cytoplasmic CDK system oscillates in an unstable manner with respect to cyclin–CDK concentration.