Fig. 2.

Structure of the lipocalin fold. a Characteristic features of the lipocalin fold. An unwound view of the lipocalin fold orthogonal to the axis of the barrel. The nine β-strands of the antiparallel β-sheet are shown as arrows and labelled A–I. The N-terminal 310-like helix and C-terminal α-helix (labelled A1) are also marked. The hydrogen-bonded connection of two strands is indicated by a pair of dotted lines between them. Connecting loops are shown as solid lines and labelled L1–L7. The two ends of the β-barrel are topologically distinct. One end has four β-hairpins (L1, L3, L5, and L7); the opening of the internal ligand-binding site is here and so is called the open end of the molecule. The other has three β-hairpin loops (L2, L4, and L6); the N-terminal polypeptide chain crosses this end of the barrel to enter strand A via a conserved 310 helix affecting closure of this end of the barrel: the closed end of the molecule. Those parts which form the three main structurally conserved regions (SCRs) of the fold, SCR1, SCR2, and SCR3, are marked as boxes. Three sequence motifs which correspond to these SCRs are shown (MOTIF 1, MOTIF 2, and MOTIF 3). The first three sequences are from kernel lipocalins and the second three from outlier lipocalins. Note that MOTIF 1 is well conserved in all sequences, whereas the other two, particularly MOTIF 2, are only well conserved in kernel lipocalin sequences. b The lipocalin β-barrel in cross-section β-strands is shown as triangles. Triangles pointing downwards indicate a strand direction into the plane of the paper and those pointing upwards indicate a strand direction out of the plane of the paper. The view shown, down the axis of the barrel, is orthogonal to that in a. Connecting loops are shown as continuous lines. Labelling and features shown are as in a. The closure of the sheet to form the lipocalin β-barrel breaks the symmetry of its elliptical cross-section, distinguishing between its two foci and suggesting a sidedness to the barrel also apparent in the location of the marked SCRs.

Reproduced with permission from Flower et al. (1996) [2]