FIGURE 8:

Comparison of V_g and J (panels A and B) and an alternative method for measuring CC_Elongation (panels C and D). Left panels: simplified model; right panels: detailed model. In all panels, the gray dashed lines represent CC_Elongation as measured by Q3 (Figure 7, A and B) and CC_NetAssembly as measured by Q1 ≈ Q2 (Figure 3, A and B). (A, B) Overlay of V_g measured from the time-step analysis of growing individual MTs (square symbols; replotted from Figure 7, A and B) and J_Constant measured from the net rate of change in average MT length of the MT population (circle symbols; replotted from Figure 5, C and D), both from the constant [free tubulin] simulations. Interpretation of panels A, B: These data show that at high [free tubulin], J_Constant approaches the V_g of individual MTs. These two data sets converge because at sufficiently high [free tubulin] individual MTs are growing (nearly) all the time, as seen in the length histories (Figure 5, A and B). Thus, CC_Elongation, which was obtained from V_g in Figure 7, should also be obtainable by extrapolating from the J_Constant data at sufficiently high [free tubulin]. (C,D) Extrapolation to obtain Q7 from J_Constant. Interpretation of panels C, D: In each of the models, the value of Q7 is approximately equal to Q3 ≈ Q6 (Figure 7). Thus, Q7 provides another way of estimating CC_Elongation. Methods: Regression lines were fitted to J_Constant for [free tubulin] in ranges where the J_Constant data are approximately linear as a function of [free tubulin]: from 6 to 7 µM for the simplified model (panel C) and from 14 to 20 µM for the detailed model (panel D). Q7 is the x-intercept of the regression line. Note that the [free tubulin] ranges used for determination of Q7 are higher than those used for Q3 and Q6 because the Q7 extrapolation requires conditions where (almost) all MTs in the population are growing (few if any depolymerization phases).