The structure of ABP. (a) Ribbon representation of the ABP structure. The component β-strands of the β-barrel are labelled A–H and the loops surrounding the binding pocket are labelled A–B to G–H. The two helices occurring `downstream' of the β-barrel are labelled α1 and α2. The positions of the two disulfide bonds are shown as stick diagrams and the positions of the C- and N-termini are indicated. The right-hand figure was obtained by rotation of the left-hand figure around the axis shown. (b) Amino-acid sequence alignment of the ABP-coding sequence produced as recombinant protein with the related proteins NP2 and NP4. The N-terminal methionine is largely removed post-translationally, giving the mature polypeptide shown in the alignment. Amino-acid identities are marked in red. The boundaries of the important secondary-structure elements from Fig. 2(a) are marked below the alignment. The numbering at the right end of each sequence line indicates the amino-acid position for that sequence. The numbering above the sequences marks every ten positions of the alignment.