Modelling the Effects of Microtubule Plus End Tapering on EB1 Binding

Abstract

EB1 is a protein that tracks growing microtubule plus-ends, manifesting as a comet-like structure slightly behind the microtubule tip (‘EB1 tip tracking’). EB1 localizes proteins to growing microtubule plus-ends that normally have little or no affinity for microtubules, and thus EB1 plays a central role in multiple cellular processes. The canonical model for EB1 tip tracking suggests that EB1 binds a complete pocket between four GTP-tubulin heterodimers, and then dissociates as GTP hydrolyzes to GDP. Recent work, however, suggests that EB1 may preferentially bind to incomplete binding pockets, which have less than four tubulin heterodimers comprising the binding pocket. Here, EB1 has a dramatically increased binding rate onto incomplete binding pockets (edge sites) as compared to complete binding pockets (lattice sites), due to steric hindrance to binding. This finding suggests that increased microtubule ‘tip tapering’, which is characterized by a larger difference between the shortest and longest microtubule protofilaments, could enhance the efficiency of EB1 tip tracking due to increased availability of edge versus lattice sites. Thus, to examine the degree to which microtubule tip tapering could predict EB1 tip tracking, we created a stochastic model that allowed us to quantitatively compare the effect of characteristics, such as the degree of microtubule tip tapering, EB1 on-rates, and EB1 off-rates, on the efficiency of EB1 tip tracking. Our model predicts that variations in microtubule tip tapering, as well as EB1 on and off rates, all alter the net amount of EB1 binding to microtubules. However, microtubule tip tapering uniquely controls the EB1 localization profile relative to the microtubule tip, thus providing a testable model prediction. Our model therefore suggests that the degree of microtubule tapering can uniquely alter the binding profile of EB1 along the microtubule plus end.