Microtubules are filamentous polymers essential for cell viability. Microtubule plus-end
tracking proteins (+TIPs) associate with growing microtubule plus ends and control
microtubule dynamics and interactions with different cellular structures during cell
division, migration, and morphogenesis. EB1 and its homologs are highly conserved
proteins that play an important role in the targeting of +TIPs to microtubule ends,
but the underlying molecular mechanism remains elusive. By using live cell experiments
and in vitro reconstitution assays, we demonstrate that a short polypeptide motif,
Ser-x-Ile-Pro (SxIP), is used by numerous +TIPs, including the tumor suppressor APC,
the transmembrane protein STIM1, and the kinesin MCAK, for localization to microtubule
tips in an EB1-dependent manner. Structural and biochemical data reveal the molecular
basis of the EB1-SxIP interaction and explain its negative regulation by phosphorylation.
Our findings establish a general "microtubule tip localization signal" (MtLS) and
delineate a unifying mechanism for this subcellular protein targeting process.