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Previously we have identified the Drosophila orbit gene whose hypomorphic mutations cause abnormal chromosome segregation (Inoue et al., 2000). The orbit encodes Orbit/Mast, a 165-kDa microtubule-associated protein (MAP) with GTP-binding motifs. Two human homologues of the Orbit/Mast, CLASP1 (hOrbit1) and CLASP2 (hOrbit2) have been identified. Using an antibody to CLASP1/hOrbit1 polypeptide, we confirmed that the polypeptide of about 150 kDa associates with microtubule purified from the porcine brain. Thus, we conjectured that CLASP1 may be a human orthologue of the Drosophila Orbit/Mast, and therefore we named it h (human) Orbit1. We constructed the plasmid for expression of a fusion protein of the putative microtubule-binding domain (1-662 out of 1289 residues) of hOrbit1 and the green fluorescent protein (GFP), and then, transfected the plasmid into Tet off cells derived from HeLa cell. Confocal laser scanning microscopic observation revealed that the GFP-fluorescence associated with short and thin filaments in the perinuclear region during the short period after plasmid transfection, and colocalized with only part of the microtubules. GFP fluorescence was later detected on the abnormally longer and thick bundles of microtubule filaments. Finally the bundles formed networks in the perinuclear region. The results suggest that the GFP-hOrbit1 N-terminal fragment (GFP-hOrbit1 NF) binds to the newly formed microtubules rather than the pre-formed ones, and that displacement of the endogenous hOrbit by the fragment might cause abnormal bundling of microtubules. Interestingly, the expression of the GFP-hOrbit1 NF results in cell death associated with nuclear fragmentation.

Microtubule Bundle Formation and Cell Death Induced by the Human CLASP/Orbit N-Terminal Fragment