The interaction of TOGp with microtubules and tubulin

TOGp is the human homolog of XMAP215, a Xenopus microtubule-associated prot ein that promotes rapid microtubule assembly at plus ends. These proteins a re thought to be critical for microtubule assembly and/or mitotic spindle f ormation. To understand how TOGp interacts with the microtubule lattice, we cloned full-length TOGp and various truncations for expression in a reticu locyte assays, the microtubule binding domain is contained within a basic 6 00-amino acid region near the N terminus, with critical domains flanking a region homologous to the microtubule binding domain found in the related pr oteins Stu2p (S. cerevisiae) and Dis1 (S. pombe). Both full-length TOGp and the N-terminal fragment show enhanced binding to microtubule ends. Full-le ngth TOGp also binds altered polymer lattice structures including parallel protofilament sheets, antiparallel protofilament sheets induced with zinc i ons, and protofilament rings, suggesting that TOGp binds along the length o f individual protofilaments. The C-terminal region of TOGp has a low affini ty for microtubule polymer but binds tubulin dimer. We propose a model to e xplain the microtubule-stabilizing and/or assembly-promoting XMAP215/TOGp o f microtubule-associated proteins based on the binding properties we have i dentified.