CLIP-170 tracks growing microtubule ends by dynamically recognizing composite EB1/tubulinbinding sites

Abstract

The microtubule cytoskeleton is crucial for the internal organization of eukaryotic cells. Several microtubuleassociated proteins link microtubules to subcellular structures. A subclass of these proteins, the plus end - binding proteins (+TIPs), selectively binds to the growing plus ends of microtubules. Here, we reconstitute a vertebrate plus end tracking system composed of the most prominent +TIPs, end-binding protein 1 (EB1) and CLIP-170, in vitro and dissect their end-tracking mechanism. We fi nd that EB1 autonomously recognizes specifi c binding sites present at growing microtubule ends. In contrast, CLIP-170 does not end-track by itself but requires EB1. CLIP-170 recognizes and turns over rapidly on composite binding sites constituted by end-accumulated EB1 and tyrosinated α-tubulin. In contrast to its fi ssion yeast orthologue Tip1, dynamic end tracking of CLIP-170 does not require the activity of a molecular motor. Our results demonstrate evolutionary diversity of the plus end recognition mechanism of CLIP-170 family members, whereas the autonomous end-tracking mechanism of EB family members is conserved. © 2008 Bieling et al.