Fig. 2.

Bundling normally protects nearly aligned MTs from crossover severing. (A) Without bundling, severing at crossovers is detrimental to MT alignment. (B) The theoretically estimated effect of protecting angles less than ${\theta }_p$ from severing: Spontaneous alignment may occur in the gray area ($0>G_c>G^{\ast }$). The thick red curve assumes that severing at intersections is equally effective as direct induced catastrophes, an assumption that necessarily overestimates their effectiveness and thus provides an upper bound. The thin red curve shows predictions for 50% effectiveness, illustrating the nonlinear approach toward this theoretical maximum. The cartoons at the bottom show the interaction functions used for C corresponding to the colored arrows: dark gray represents induced catastrophes, light gray represents crossovers that may be severed later, and white represents crossovers that are protected from severing. (C) The negative effect of crossing-only severing on alignment decreases if shallow angles (below ${\theta }_p$ = 20°, 40°, and 60°) are protected from severing and turns into a positive effect for ${\theta }_p$ = 40° and 60°. With less frequent induced catastrophes ($P_{cat}=0.09$), the potential impact of crossing-only severing is even larger (SI Appendix, Fig. S5). Error bars represent SEM, with n = 100 independent simulations per data point.