The interaction of TOGp with microtubules and tubulin

Abstract

TOGp is the human homolog of XMAP215, a Xenopus microtubule-associated protein that promotes rapid microtubule assembly at plus ends. These proteins are thought to be critical for microtubule assembly and/or mitotic spindle formation. To understand how TOGp interacts with the microtubule lattice, we cloned full-length TOGp and various truncations for expression in a reticulocyte lysate system. Based on microtubule co-pelleting assays, the microtubule binding domain is contained within a basic 600-amino acid region near the N terminus, with critical domains flanking a region homologous to the microtubule binding domain found in the related proteins Stu2p (S. cerevisiae) and Dis1 (S. pombe). Both full-length TOGp and the N-terminal fragment show enhanced binding to microtubule ends. Full-length TOGp also binds altered polymer lattice structures including parallel protofilament sheets, anti-parallel protofilament sheets induced with zinc ions, and protofilament rings, suggesting that TOGp binds along the length of individual protofilaments. The C-terminal region of TOGp has a low affinity for microtubule polymer but binds tubulin dimer. We propose a model to explain the microtubule-stabilizing and/or assembly-promoting functions of the XMAP215/TOGp family of microtubule-associated proteins based on the binding properties we have identified.