Phosphoregulation of the budding yeast EB1 homologue Bim1p by Aurora/Ipl1p

EB1 (end binding 1) proteins have emerged as central regulators of microtubule (MT) plus ends in all eukaryotes, but molecular mechanisms controlling the activity of these proteins are poorly understood. In this study, we show that the budding yeast EB1 protein Bim1p is regulated by Aurora B/Ipl1p-mediated multisite phosphorylation. Bim1p forms a stable complex with Ipl1p and is phosphorylated on a cluster of six Ser residues in the flexible linker connecting the calponin homology (CH) and EB1 domains. Using reconstitution of plus end tracking in vitro and total internal reflection fluorescence microscopy, we show that dimerization of Bim1p and the presence of the linker domain are both required for efficient tip tracking and that linker phosphorylation removes Bim1p from static and dynamic MTs. Bim1 phosphorylation occurs during anaphase in vivo, and it is required for normal spindle elongation kinetics and an efficient disassembly of the spindle midzone. Our results define a mechanism for the use and regulation of CH domains in an EB1 protein.