The 1–3 subunit interface of AVR2, AVR4 and avidin. Stereo images of the 1–3 subunit interface of (A) AVR2-b, (B) AVR4 [PDB:1Y52] , and of (C) avidin expressed in E. coli [PDB:1VYO] (Airenne, Hytönen et al. unpublished) are shown. Lys-94 of AVR2 exists in two alternate conformations, which can form a hydrogen bond with the side-chain oxygen atom of Asn-115 or with the main-chain oxygen atom of Val-113, both from an adjacent subunit. Putative hydrogen bonds are shown by dashed lines. The carbon atoms of subunits 1 and 3 are coloured white and cyan, respectively.