Fig. 2. (A) Secondary structure of the p40^phox PX domain. The first -helix is not part of the core PX domain but was included in the crystallised p40^phox PX domain construct. MIL, membrane interaction loop; PxxP, polyproline region. (B) Tertiary structure of the overall topology of the p40^phox PX domain bound to PtdIns(3)P. The membrane interaction loop of the PX domain and the acyl chains of the PtdIns(3)P would insert into a membrane located towards the top of the diagram. (C) Details of the lipid-binding pocket shown in (B). Key residues are illustrated to demonstrate how each one is positioned to fulfil specific roles; Lys92 hydrogen bonds with the 1-phosphate, Tyr59 stacks against the inositol ring, Arg105 interacts with the 4- and 5-hydroyl groups, Arg58 contacts the 3-phosphate and Arg57 faces away from the lipid binding pocket, fulfilling a structural role in the domain.