Ultrasensitivity in the Regulation of Cdc25C by Cdk1

Abstract

Cdc25C is a critical component of the interlinked positive and double-negative feedback loops that constitute the bistable mitotic trigger. Computational studies have indicated that the trigger's bistability should be more robust if the individual legs of the loops exhibit ultrasensitive responses. Here, we show that in Xenopus extracts two measures of Cdc25C activation (hyperphosphorylation and Ser 287 dephosphorylation) are highly ultrasensitive functions of the Cdk1 activity; estimated Hill coefficients were 11 to 32. Some of Cdc25C's ultrasensitivity can be reconstituted in vitro with purified components, and the reconstituted ultrasensitivity depends upon multisite phosphorylation. The response functions determined here for Cdc25C and previously for Wee1A allow us to formulate a simple mathematical model of the transition between interphase and mitosis. The model shows how the continuously variable regulators of mitosis work collectively to generate a switch-like, hysteretic response.

Figure 1. Ultrasensitive Hyperphosphorylation of Cdc25C in Xenopus Egg Extracts

(A) Endogenous Cdc25C was phosphorylated in Xenopus extract in response to various concentrations of Δ65-cyclin B1-Cdk1AF. The AF mutation avoids the feedback regulation of Cdk1 by Cdc25C. (B, C) Steady-state responses of Cdk1 and Cdc25C to various concentrations of Δ65-cyclin B1-Cdk1AF. (B) Primary data from one experiment. (C) Plots of Cdk1 and Cdc25C responses from 11 experiments. (D) Cdc25C hyperphosphorylation as a function of Cdk1 activity. Scaled data from 11 experiments were pooled and fitted to a four-parameter Hill function. The Hill coefficient n_H was 11. The 95% confidence interval for n_H was 6 to 17. (E) Steady-state dephosphorylation of Cdc25C at Ser 287 (top) as assessed by immunoblotting and GelDoc quantitation, and steady-state Cdk1 activity (bottom) as assessed by histone H1 kinase assays and quantitative autoradiography. (F) Cdc25C Ser 287 phosphorylation as a function of Cdk1 activity. Scaled data from 7 data sets (4 independent experiments) were pooled and fitted to a four-parameter Hill function. The Hill coefficient n_H was 32 with a 95% confidence interval of 17 to 47.

Figure 2. Comparison of Cdc25C and Wee1A Responses

(A) Steady-state responses. Extracts were incubated for two hours with different concentrations of non-degradable Δ65-cyclin B1, which was sufficient to allow the Wee1A and Cdc25C phosphorylation to approach steady state. (B, C) Time courses. Extracts were incubated with 100 nM Δ65-cyclin B1 for the lengths of time indicated. Panel B shows one experiment; panel C shows average results from two independent experiments.

Figure 3. A Non-Linear Response Can Be Reconstituted in Vitro in the Absence of Phosphatase

Purified full length recombinant Cdc25C (150 nM) was phosphorylated for 1 hour by various concentrations of recombinant p13 Suc1-Δ65-cyclin B1-Cdk1AF complexes in the presence of MgATP (500 µM). Data from nine experiments were scaled, pooled, and fitted to a Hill equation The Hill coefficient n_H was 2.3 with a 95% confidence interval of 1.9 to 2.7.

Figure 4. The Reconstituted Ultrasensitivity is Decreased by Reducing the Number of Available Phosphosites

(A) Phosphorylation of a recombinant N-terminal fragment of Cdc25C (N200 Cdc25C) parallels that of endogenous Cdc25C. Interphase extracts were incubated with different concentrations of Δ65-cyclin B1 and CDK1AF plus purified recombinant N200 Cdc25C. The hyperphosphorylation of endogenous Cdc25C and N200 Cdc25C were assessed by immunoblotting with Cdc25C antibody. (B) Eliminating three conserved TP sites decreases ultrasensitivity. N200 Cdc25C or the same N-terminal fragment with three sites changed to glutamates (N200-3E, with T48E, T67E and T138E) were incubated with varying concentrations of recombinant p13 Suc1-Δ65-cyclin B1-Cdk1AF (complexes in the presence of MgATP (500 µM)) plus [γ-³²P]ATP. Phosphorylation of the N-terminal fragments was quantified by autoradiography. Scaled data were pooled and fitted to a three-parameter Hill equation. Hill coefficients were 4.5 (N200) with a 95% confidence interval of 2.7 to 6.3, and 0.9 (N200-3E) with a 95% confidence interval of 0.5 to 1.2.

Figure 5. Ultrasensitivity in Extracts Does Not Decrease as Intermolecular Competition Is Reduced

Hyperphosphorylation of Cdc25C was carried out in normal extracts (blue circles) and extracts diluted 10-fold (red squares). Cdc25C was kept at endogenous levels (150 nM) in the diluted extract by addition of recombinant protein. Hyperphosphorylation was measured by immunoblotting and quantitation. Results from individual experiments were scaled to the fitted maximum levels of phosphorylation (b) and EC50 values (K), and combined. The line shown is a fit to all of the pooled scaled data. The Hill coefficient was estimated to be 8.

Figure 6. Schematic Views of Three Ways Multisite Phosphorylation Can Generate Ultrasensitivity

Figure 7. A Model Based on the Experimentally-Observed Response Functions for Wee1 and Cdc25C Accounts for the Bistability of the Mitotic Trigger

(A) Experimental data for the hysteretic response of Cdk1 to recombinant Δ65-cyclin B1 in Xenopus egg extracts. Data are taken from a previous publication (Pomerening et al., 2003). (B) Schematic view of the regulation of cyclin B1-Cdk1 activity by cyclin B1, Wee1A, and Cdc25C. (C) Theoretical steady-state responses based on Equation 6, for five assumed values of k_Wee1A/k_Cdc25C (left to right: 0.125, 0.25, 0.5 (green), 1, 2). (D) Rate-balance plots. The calculated rates of Cdk1 activation (red curves) and inactivation (blue curve) as a function of the concentration of active cyclin B1-Cdk1. The five red curves correspond to five assumed total cyclin concentrations. The ratio k_Wee1A/k_Cdc25C is assumed to be 0.5. (E) Robustness of the bistable response. For the full model and various modified models, we calculated response curves for 100 randomly-generated parameter sets. The ranges of the parameters were:

1. bkgd_Cdc25C = 0 to 0.4 (experimental estimate = 0.2)

   bkgd_Wee1A = 0 to 0.4 (experimental estimate = 0.2)

   EC50_Cdc25C = 20 to 80 nM (experimental estimate = 30 nM)

   EC50_Wee1A = 20 to 80 nM (experimental estimate = 35 nM)

   n_Cdc25C = 5 to 15 (experimental estimate = 11)

   n_Wee1A = 1.5 to 8 (experimental estimate = 3.5)

   k_Wee1A/k_Cdc25C = 0 to 1.5

Random parameter sets were generated and curves were plotted using Mathematica 6.0.3.