In the view of the above results, we decided to study a TBCC truncation mutant containing the N-terminal domain FTY720 overexpressed in HeLa cells. In contrast to the cytoplasmic pattern observed for the full-length polypeptide, the TBCC N-terminal domain produced a dot-like pattern, distributed at the perinuclearcentrosomal region . As observed for the full-length construct, TBCC N-terminal domain overexpression was also associated with a number of metaphase aberrations . These results confirm a role for TBCC at the centrosome and support the hypothesis that the TBCC N-terminal domain is masked within this organelle. These data led us to study in more detail the TBCC N-terminal domain. Fig. 5A shows the superposition of the 20 conformers of the TBCC N-terminal domain determined by NMR. The structure is a left-handed 3-stranded a-helix bundle composed of 3 antiparallel and almost coaxial a-helices: a2, N56-R77; a3, V81-S101; a4, A107-L131 connected by short linkers: loop 2, A78-S80; loop 3, V102-A106. The N-terminal portion of this domain has not a defined orientation relative to the protein core and shows regions with partial helix formation . In particular, residues E33-K44 and N49-E55 adopt helical conformations with populations of ,60 and ,38%, respectively as estimated on the basis of their conformational shifts . No NOEs connect these nascent helices to the rest of the protein. The entire N-terminal region is structurally disordered relative to the domain and samples all the available conformational space. The structured part of the protein , is well-defined with low pairwise RMSD values . Average interhelical angles of 170u BU 4061T between helix a2 and a3, 6u between helix a2 and a4, and 173u between helix a3 and a4 are obtained for the ensemble. The compact helix bundle confers the molecule a rodlike shape with a volume of 11000 A �� 3 and a global accessible surface area of 6400 A �� 2 . Helical wheel projections show that the sequences of the three helices conforming the TBCC��s bundle fulfil the characteristic heptad pattern of lefthanded coiled coils . The side chains of a significant number of hydrophobic residues are deeply buried in the protein core, pointing to the interior of the helix bundle .