The structure of N1. (a) The dimer of N1 is viewed looking along the molecular twofold axis. BH1, BH2, BH3 and BH4 motifs are coloured green, magenta, yellow and blue, respectively and N and C termini are labelled. The right hand view is the same as in (b) and looks onto the surface groove and the mutated C40S residue is shown in red. For clarity, the helices of one monomer have been labelled. In the crystal structure of N1, the asymmetric unit has three dimers, each held together by anti-parallel interactions between helix α1 and α6 of each subunit, with a loss of 950 Å2 of solvent accessible surface. The interface does not involve the surface groove, which consequently is exposed and available to bind BH3 motifs. (b and c) Comparison of the structure of VACV N1 with Bcl-xL. Cartoons of N1 (b) and Bcl-xL (c), with structurally equivalent BH1, BH2, BH3 and BH4 motifs coloured as in panel (a). The position adopted by helix 3 of Bcl-xL when complexed with the Bim BH3 helix is drawn as a red semi-transparent loop and red arrows show movement of this helix upon peptide binding. (d) Sequence alignment of N1 and Bcl-xL based on structural alignment from SHP (Stuart et al., 1979). The core of the molecules, used in the structure based alignment, are highlighted in pink. Strictly conserved residues are in red blocks and similar residues in blue boxes. Secondary structural elements are coloured by BH motif definition, as above. To avoid breaking up the VACV N1 sequence, residues for Bcl-xL that are not matched by N1 are omitted, and the position and number of residues removed are indicated under the Bcl-xL sequence. The position of a disordered loop in Bcl-xL is marked by the green triangle.