In this report, we show that the echinoderm microtubule (MT)-associated protein (EMAP) and related EMAP-like proteins (ELPs) share a similar domain organization with a highly conserved hydrophobic ELP (HELP) domain and a large tryptophan-aspartic acid (WD) repeat domain. To determine the function of mammalian ELPs, we generated antibodies against a 70-kDa human ELP and showed that ELP70 coassembled with MTs in HeLa cell extracts and colocalized with MTs in the mitotic apparatus. To determine whether ELP70 bound to MTs directly, human ELP70 was expressed and purified to homogeneity from baculovirus-infected Sf9 cells. Purified ELP70 bound to purified MTs with a stoichiometry of 0.40 ؎ 0.04 mol of ELP70/mol of tubulin dimer and with an intrinsic dissociation constant of 0.44 ؎ 0.13 M. Using a nucleated assembly assay and video-enhanced differential interference contrast microscopy, we demonstrated that ELP70 reduced seeded nucleation, reduced the growth rate, and promoted MT catastrophes in a concentration-dependent manner. As a result, ELP70-containing MTs were significantly shorter than MTs assembled from tubulin alone. These data indicate that ELP70 is a novel MT destabilizer. A lateral destabilization model is presented to describe ELP70's effects on microtubules.

Vaults are large (13 Mda) ribonucleoprotein particles that are especially abundant in multidrug resistant cancer cells and have been implicated in nucleocytoplasmic drug transport. To understand how these large barrel-shaped complexes are transported through the cytosol, we examined the association of vaults with microtubules both in vitro and in vivo. Within cells, a subpopulation of vaults clearly associates with microtubules, and these vaults remain associated with tubulin dimers/oligomers when microtubules are disassembled by nocodazole treatment. In vitro, a microtubule-pull down assay using highly purified rat vaults and reassembled microtubules reveals that vaults exhibit concentration-dependent binding to microtubules that does not require the carboxyl terminal end of tubulin. Remarkably, negative staining for electron microscopy reveals that vault binding to microtubules is mediated by the vault caps; more than 82% of bound vaults attach to the microtubule lattice with their long axes perpendicular to the long axis of the microtubule. Five to six vault particles were bound per micron of microtubule, with no crosslinking of microtubules observed, suggesting that only one end of the vault can bind microtubules. Taken together, the data support the model of vaults as barrel-shaped containers that transiently interact with microtubules.