Trade-off between high SCF occupancy and fast SR exchange rate.
A: Left axis shows SCF activity as measured by the steady state concentration of Cul1.SR1. Right axis shows the exchange rate as measured by the leading eigenvalue of the Jacobian matrix (ρl). As the total Cand1 concentration increases the SCF activity (solid lines) decreases while the exchange rate (dashed lines) concomitantly increases. As the relative binding affinity η (Eq 2) decreases both the SCF response curve as well as the curve characterizing the exchange rate develop a sharp threshold near Cand1T = Cul1T (marked by arrow head). The horizontal dotted line indicates the maximal exchange rate (cf. Eq 9). B: Exchange rate (|ρl|) vs. SCF occupancy ([Cul1.SR1]) drawn from the curves in panel A. Note that the curves are overlapping. We have also indicated the positions along the curve where, depending on the value of η, the concentration of Cand1 equals that observed in cells (cf. Table 1). C: Left panel shows the time scale for the exchange of substrate receptors (τs = 1/|ρl|) as a function of the total Cand1 concentration for the parameters listed in Table 1. Right panel shows the corresponding time course for the assembly of Cul1.SR1 after adding 100nM SR1 to a steady-state mixture containing 300nM Cul1, 560nM SR2 and Cand1 as indicated by the dashed lines in the left panel. The dashed lines in the right panel indicate the value of τs obtained from the intersection of the dashed lines with the black solid line in the left panel.
