Hypothetical model of the receptor-bound, membrane-inserted PA pore. The model is based on the pre-pore PA63 crystal structure6, channel conductance studies8, and the crystal structure of -haemolysin19. The barrel is formed by rearrangement in each monomer of the segment shown in red in Fig. 3. Each PA63 monomer is shown in a different colour. Residues 303–324 form the membrane-spanning region of the barrel. Seven copies of the CMG2 I domain bound to the heptamer are in blue. The 40 Å gap between the CMG2 I domain and the membrane may be occupied by a 100-residue domain of CMG2, C-terminal to the I domain, which precedes its membrane-spanning sequence.
在一项应有助于抗炭疽热毒素和抗癌药物开发的研究工作中，Santelli等人确定了连接在一个细胞表面受体上的炭疽热毒素的晶体结构。三个毒素成分中的一个与该受体的结合方式，非常像粘合素（一种粘合型分子）与细胞外基质结合的方式。除了为阻断该毒素提供潜在目标外，这项工作还可帮助科学家设计定向炭疽热毒素，它们能选择性地结合和破坏癌细胞。
Crystal structure of a complex between anthrax toxin and its host cell receptor
Anthrax toxin consists of the proteins protective antigen (PA), lethal factor (LF) and oedema factor (EF). The first step of toxin entry into host cells is the recognition by PA of a receptor on the surface of the target cell. Subsequent cleavage of receptor-bound PA enables EF and LF to bind and form a heptameric PA63 pre-pore, which triggers endocytosis. Upon acidification of the endosome, PA63 forms a pore that inserts into the membrane and translocates EF and LF into the cytosol. Two closely related host cell receptors, TEM8 and CMG2, have been identified. Both bind to PA with high affinity and are capable of mediating toxicity. Here, we report the crystal structure of the PA–CMG2 complex at 2.5 Å resolution. The structure reveals an extensive receptor–pathogen interaction surface mimicking the non-pathogenic recognition of the extracellular matrix by integrins. The binding surface is closely conserved in the two receptors and across species, but is quite different in the integrin domains, explaining the specificity of the interaction. CMG2 engages two domains of PA, and modelling of the receptor-bound PA63 heptamer suggests that the receptor acts as a pH-sensitive brace to ensure accurate and timely membrane insertion. The structure provides new leads for the discovery of anthrax anti-toxins, and should aid the design of cancer therapeutics.
Figure 1 Structure of the PA–CMG2 complex. Two orthogonal views are shown in ribbon representation. PA is coloured by domain (I–IV). CMG2 is blue; the metal ion is shown as a magenta ball. PA domain I is cleaved after receptor binding, leading to the loss of domain Ia (yellow) and the formation of PA63. All molecular graphics images were generated using the UCSF Chimera package29 (http://www.cgl.ucsf.edu/chimera).
Figure 2 The MIDAS motifs of the PA–CMG2 complex (a) and the collagen–integrin 21 complex5 (b). Coordinating side chains and two water molecules () are shown in ball-and-stick representation. The metal is shown in blue. D683 from PA, and a collagen glutamic acid, are in gold. Bond distances to the metal are 2.1 0.2 Å in both cases. The three MIDAS loops (L1–L3) are labelled in a.
Figure 3 Intermolecular contacts between PA domains II and IV and CMG2. Contacting regions are coloured blue and green for CMG2 and PA domain IV, respectively. The 2–3 loop and flanking regions of PA domain II, which are implicated in pore formation, are highlighted in red. The 2–3 loop is disordered in monomeric PA and is shown schematically as a dashed line. The histidine residues within PA domains II and IV and within the CMG2 I domain are shown coloured cyan and are in ball-and-stick representation. Mutation sites that reduce binding by >100-fold (D683, S337, G342, W346, I656, N657, I665, Y681, N682, P686, L687) are highlighted in gold.
Figure 4 Key elements of the PA–CMG2 interaction a, Solvent-accessible surface of the PA domain IV groove, with key side chains from three CMG2 loops (1–1, blue; 2–3, red; 2–3, green) shown in ball-and-stick representation. The 2–3 loop forms the ridge. The MIDAS metal is labelled (M). b, Comparison with integrin I domains in the 'open' conformation (CMG2, red; M, cyan; 2, green; L, blue) overlaid on the MIDAS motif. c, Surface of the CMG2 pocket into which the PA 3–4 loop (red ribbon) inserts, formed by three CMG2 side chains (shown in ball-and-stick representation) and the 4–4 loop (cyan).
