The crystal structure of the N-terminal region of BUB1 provides insight into the mechanism of BUB1 recruitment to kinetochores

Abstract

The interaction of the central mitotic checkpoint component BUB1 with the mitotic kinetochore protein Blinkin is required for the kinetochore localization and function of BUB1 in the mitotic spindle assembly checkpoint, the regulatory mechanism of the cell cycle that ensures the even distribution of chromosomes during the transition from metaphase to anaphase. Here, we report the 1.74 angstroms resolution crystal structure of the N-terminal region of BUB1. The structure is organized as a tandem arrangement of three divergent units of the tetratricopeptide motif. Functional assays in vivo of native and site-specific mutants identify the residues of human BUB1 important for the interaction with Blinkin and define one region of potential therapeutic interest. The structure provides insight into the molecular basis of Blinkin-specific recognition by BUB1 and, on a broader perspective, of the mechanism that mediates kinetochore localization of BUB1 in checkpoint-activated cells.

Figure 1

Model Showing the Functions of BUB1 and BUBR1 in the Mitotic Spindle Checkpoint during Mitotic Progression (A) In prometaphase, the nuclear envelope breaks down and microtubules emanating from opposite poles attach to the kinetochores of individual sister chromatids. Core checkpoint components BUB1, BUBR1, BUB3, MAD1, and MAD2 are recruited to unattached kinetochores in SAC-activated cells. Kinetochore localization of BUB1 and BUBR1 is mediated by the mitotic kinetochore protein Blinkin. The latter also establishes physical interaction with the MIS12 and NDC80 complexes. Cytosolic BUBR1, BUB3, MAD2, and CDC20 associate to form mitotic checkpoint complexes, which interact with the anaphase promoter complex/cyclosome (APC/C) to render it inactive. (B) In metaphase, the bipolar attachment and alignment of all chromosomes at the center of the cell is reached and APC/C-CDC20 inhibition released by silencing of the SAC. This is followed by the onset of anaphase, in which sister chromatids separate and are pulled toward opposite poles of the cell. When the checkpoint is silenced, securing can be ubiquitinated by APC/C and degraded. This results in the release and activation of separase, which leads to the cleavage of mitotic cohesions at centromeres and chromosome arms to cause chromosome separation and mitotic progression from M-phase to interphase.

Figure 2

Sequence Conservation of the N-Terminal Domain of BUB1 and BUBR1 (A) The ten α helices seen in the structure of Sc-BUB1_(29-230) are indicated above the alignment by solid ribbons. Fully conserved residues in BUB1 and BUBR1 that are located in loop regions are highlighted in dark gray. Sc, Saccharomyces cerevisiae; Hs, Homo sapiens; Sp, Schizosaccharomyces pombe; Mm, Mus musculus; Dm, Drosophila melanogaster; Xl, Xenopus laevis; Gg, Gallus gallus. Six representative BUB1 sequences and five of BUBR1 were aligned using ClustalW (Thompson et al., 1994). The inverted triangles above the alignment (▾) show the trypsin cleavage sites identified after limited proteolysis and proteomics analysis. (B) Nano-ES mass spectrum of 45 μM sample (+15 to +12), in the m/z range 3000 and 4500. A single charge state distribution was observed centered on charge +14 at m/z 3510. These values correspond to a molecular weight of 49,313 Da, consistent with a dimer (Mw of monomeric and dimeric Sc-BUB1_[29-230] calculated from the amino acid sequence is 24,226 Da and 48,452 Da, respectively). (C) Comparison of the distance distribution [p(r)] function of Sc-BUB1_(29-230) (blue) against that of Sc-BUB1_(1-230) (red) provides evidence for conformational heterogeneity due to structural flexibility of the region encompassing residues 1–28. (D) The SAXS scattering profile simulation (continuous lines) based on the dimer observed in the crystal structure demonstrates that the overall conformation of Sc-BUB1_(1-230) (red) and Sc-BUB1_(29-230) (blue) is essentially conserved in solution.

Figure 3

Overall Structure of the N-Terminal Domain of BUB1 (A) The two molecules observed in the crystal asymmetric unit associate to form a dimer with noncrystallographic two-fold symmetry. (B) The structure viewed 90° rotated along the minor axis. (C) Electron density for α helices H5, H6, H7, and H9 and the connecting loop (after density modification) contoured at 1.4 σ. The final, refined model is shown using a ball-and-stick representation. The α helices, loops, and side chains are clearly visible in the initial map. Water molecules are shown as red spheres. (D) Ribbon diagram showing that this domain consists of ten α helices with a core arrangement of a triple repeat of the TPR motif (TPR1 orange; TPR2 magenta; TPR3 cyan). (E,F) Superposition of the three TPRs of Sc-BUB1_(29-230). Each molecule representation was generated with Pymol (DeLano, 2002).

Figure 4

Projection of Conserved Regions onto the Protein Surface (A) Residue conservation is shown according to the sequence-conservation score of ProSkin (Deprez et al., 2005). Blue and white denote conserved and variable regions, respectively. Conserved residues of the GN/DD and GIG motifs are shown in blue, two of the residues that define the hydrophobic pocket are shown in green, and the CHES molecule is shown using a ball-stick representation. (B) Electrostatic surface potential computed with GRASP (Nicholls et al., 1991). The electrostatic potential is contoured at the 10 kT/e level, with red denoting negative potential and blue denoting positive potential. Note the prominent acidic loop defined by the conserved residues E52 to D56 and that defined by the less conserved residues E167, E171, E188, E193, E198, D205, and E208. The structure is view rotated 90° along the vertical axis from that in (A). (C) The loops encompassing residues D53-D56 and G137-I-G139 (red) show conformations unique among TPRs of high local-structure similarity (Hs-PEX TPR is gray, Hs-HOP TPR2 is cyan, and Hs-PP5 TPR is yellow).

Figure 5

Analysis of the Hs-BUB1-Blinkin Interaction (A) Yeast two-hybrid analysis of diverse Hs-BUB1 mutants. The effect of the mutant P23G could not be established as it showed self-activation. (B) Projection of residues important for binding Blinkin onto the protein surface (salmon color). Residues whose mutation did not compromise the binding with Blinkin are shown in blue. (C) Mapping of Hs-BUB1 mutations associated with cancer. Residues absent in the deletion mutant Δ76–141 are shown in green and residues E36D, A130S, and H151D in brown.

Figure 6

Mechanistic Implications (A) The two Sc-BUB1_(29-230) protomers (show in yellow and magenta, respectively) form a groove that binds the C-terminal residues I215-S230 of a symmetry-related molecule (green). Several of the conserved residues that define an acidic cluster (orange color) are shown in the yellow protomer. Residues of the other protomer (magenta) that interact with residues I215-S230 of a symmetry-related molecule are shown in blue. (B) Superposition of TPR-containing domains of Sc-BUB1, Hs-PP5, Hs-HOP, and Hs-PEX5 shows that they form a similar concave face. Hs-PP5 and Hs-HOP bind the same pentapeptide sequence MEEVD (Hsp90, blue and red, respectively) in a different mode. Peroxisomal targeting signal-1 peptide (PTS-1, brown) binds Hs-PEX5 in a manner that resembles that of Hsp90-HOP. In Hs-BUB1, residues of the Blinkin-binding region map onto α helices equivalent to those of Hs-PP5, Hs-HOP, and Hs-PEX5 engage in peptide binding, suggesting a similar binding mode. In Sc-BUB1_(29-230) various residues of these helices (H2 and H4) are involved in the interaction with one CHES molecule (salmon) and others with the C-terminal region of a symmetry-related Sc-BUB1 molecule (green), suggesting that other residues participate in additional protein-protein interactions. (C) The TPR-containing domain of Hs-BUB1 and Hs-BUBR1 bind to the N-terminal region of Blinkin, whereas the physical interaction of BUB3 with BUB1 and BUBR1 involves the GLEBS motif. Putative MAD1 and MAD2 binding sites can be identified in the region preceding the kinase domain. The different affinity of Blinkin for Hs-BUB1 and Hs-BUBR1 might permit the simultaneous control of the recruitment of these proteins to the kinetochore. (D) Close-up view of the interaction between CHES and the hydrophobic pocket of Sc-BUB1_(29-230). One CHES molecule is located inside the small hydrophobic pocket constituted by residues L62, M65, I66, S69, L84, M88, I110, W113, and L117 (brown area).