The capsid surrounds an approximately 7500-nucleotide, single-stranded RNA genome. proteins into the density maps. Binding of C3 to either 160S or 135S particles caused residues of the BC loop, located on the tip of a prominent peak known as the mesa, to move by an estimated 5 ?. C3 antibodies are neutralizing and can bind bivalently. The orientation of the bound Fabs in our reconstructions suggests that C3 neutralizes poliovirus by binding two adjacent BC loops on the YLF-466D same mesa and inhibiting conformational changes in the viral capsid. Keywords: antibodies, antibody neutralization, antigen-binding fragment (Fab), 135S cell-entry intermediate particle, 80S RNA-released particle, antibody-antigen interactions, antibody-protein interactions, C3 antibody, cryo-electron microscopy, electron cryo-microscopy, fragment antibody-binding (Fab), picornavirus, viral, virus-antibody interactions Introduction Antibody, antigen, or both may change conformation during antibody-antigen binding, e.g. (1). This example of an induced-fit is typically seen by comparing atomic-resolution structures. Here, by cryogenic electron microscopy (cryo-EM), we show evidence of a changed antigen conformation occurring in poliovirus. Poliovirus capsid is composed of 60 copies each of four capsid proteins (VP1, VP2, VP3, and VP4), arranged with icosahedral symmetry (Fig. 1). The capsid surrounds an approximately 7500-nucleotide, single-stranded RNA genome. VP4 and the amino-terminal segments of VP1, VP2, and VP3 lie on the inner surface of the capsid with extended conformations. The external surface of the capsid is constituted of VP1, VP2, and VP3. Open in a separate window FIGURE 1 Prominent structural features of poliovirusProminent structural features on the exterior of a surface-rendered 160S poliovirus particle (17) are labeled. The expanded representation shows the core -jellyroll subunit and BC loop of VP1 (2). The BC loop (cyan) is between -strands B and C (orange) and is located at the tip of the star-shaped mesa. In the surface rendering, purple signifies features closest to the center of the particle, and white represents features farthest from the center. A loop connecting the B and C -strands of the VP1 -jellyroll (the BC loop) is located at each tip of the star-shaped mesas that surround the particle fivefold axes (Fig. 1). As shown by several studies, this loop (residues 95C105 (2)) is a major epitope for neutralizing antibodies (3-10). One such antibody is monoclonal C3 (4). During cell-entry, poliovirus (sedimentation coefficient, 160S) binds to its receptor, CD155, which initiates conformational rearrangements and conversion to the cell-entry intermediate (135S) particle. After RNA is released, the empty capsid sediments at 80S. C3, which was raised by immunizing mice with the 80S particle (3), binds all three particles160S, 135S, and 80S (11). A crystal structure of the C3 Fab fragment, with a bound peptide corresponding to residues 93C103 of VP1, was determined previously (12). In this complex, the structure of the peptide differed significantly from its structure in the BC loop of the 160S particle, suggesting that the loop changes its structure upon antibody binding. This observation raised the question of whether the same phenomenon occurs when the antibody binds to an assembled particle. To address this question, we mixed the C3 Fab with 160S and GNG7 135S particles and solved the structures of the respective complexes by cryo-EM. In both particle states, our results show that the conformation of the BC loop is affected by binding this antibody; fitting of the C3 Fab (12) and capsid protein YLF-466D (2, 13) coordinates into the cryo-EM density maps indicated that the Fab-bound epitopes had altered YLF-466D positions. The resulting structures also suggest that bivalent antibody binding occurs as the two Fab arms bind adjoining epitopes on a single pentameric mesa and that C3-Ab-induced neutralization occurs because a conformational change that is required for infection is inhibited. Materials and Methods Preparation of viral and Fab complex Poliovirus virions (160S particles) and C3 Fab were prepared as described previously (12, 14, 15). 135S particles were prepared by heat treatment (50 C for 3 min.) of 160S particles in low-salt buffer containing 2 mM CaCl2 (13, 15). Equal volumes of 160S-particle (3.9 mg/ml) and C3-Fab (1.7 mg/ml) solutions were mixed, giving a Fab-to-virus ratio of 74:1. A similar ratio was.