Abstract

The microtubule cytoskeleton performs essential mechano-chemical tasks in eukaryotic cells. Growing microtubule ends serve as transient binding platforms for several proteins that regulate microtubule dynamics and interactions with cellular substructures. End Binding proteins (EBs) recruit most of these proteins. They autonomously recognize an extended region at growing microtubule ends with unknown structural characteristics. Using in vitro reconstitutions in combination with fluorescence microscopy we dissect the interplay between EBs and their recruited proteins. Using cryo-electron microscopy and subnanometer single particle reconstruction and fluorescence imaging, we construct a pseudo-atomic model of how EBs bind to the end region of growing microtubules. They bridge protofilaments except at the microtubule seam. By binding close to the exchangeable GTP binding site of tubulin, they are ideally positioned to sense the microtubule’s nucleotide state. Strikingly, the microtubule end region that is recognized by EBs is also a stabilizing structural cap protecting the microtubule from depolymerisation. This reveals a common structural link between two important biological phenomena, microtubule dynamic instability and end tracking.