(A) Averaged delay times as a function of microtubule growth speed as shown in Figure 6A (tubulin concentration variation dataset) are fitted using either an 'end density' or 'total number' threshold model, either assuming simple 1-step maturation of microtubule end regions, as throughout this study, or assuming 2-step maturation kinetics (Maurer et al., 2014), as indicated. Both end density models yield good fits, predicting thresholds of 0.20 and 0.25 for the 1-step and 2-step maturation model, respectively. Both 'total number' models yielded unsatisfactory fits. (B) Averaged delay times as shown in Figure 6C (Mal3 dataset and its control) are fitted using an 'end density' threshold model either assuming 1-step or 2-step microtubule end maturation, as indicated. Predicted thresholds: Mal3 data – 0.28 and 0.29 for the 1-step and 2-step model, respectively; control without Mal3 – 0.14 and 0.17 for the 1-step and 2-step model, respectively. For the fits of the data without Mal3, k1 = 5km; for the fits of the data with Mal3, k1 = 20km (Maurer et al., 2014). The other fit parameters were as in Figure 6A and 6C.