EBs Recognize a Nucleotide-Dependent Structural Cap at Growing Microtubule Ends

Summary

Growing microtubule ends serve as transient binding platforms for essential proteins that regulate microtubule dynamics and their interactions with cellular substructures. End-binding proteins (EBs) autonomously recognize an extended region at growing microtubule ends with unknown structural characteristics and then recruit other factors to the dynamic end structure. Using cryo-electron microscopy, subnanometer single-particle reconstruction, and fluorescence imaging, we present a pseudoatomic model of how the calponin homology (CH) domain of the fission yeast EB Mal3 binds to the end regions of growing microtubules. The Mal3 CH domain bridges protofilaments except at the microtubule seam. By binding close to the exchangeable GTP-binding site, the CH domain is ideally positioned to sense the microtubule's nucleotide state. The same microtubule-end region is also a stabilizing structural cap protecting the microtubule from depolymerization. This insight supports a common structural link between two important biological phenomena, microtubule dynamic instability and end tracking.

Abstract

Graphical Abstract

Highlights

► End-binding protein (EB) Mal3 bridges microtubule protofilaments except at the seam ► Mal3 binds next to the tubulin GTPase site and is positioned to sense GTP hydrolysis ► EBs recognize a stabilizing structural cap at growing microtubule ends ► EB end tracking and dynamic instability are mechanistically linked

Abstract

Subnanometer reconstruction of a microtubule end-binding protein (EB) in complex with microtubules reveals interactions with the GTP cap that protects the growing end from depolymerization. The results suggest a mechanism for how EBs track dynamic changes in filament length and provide a moving platform for recruiting regulatory proteins.