Alternative conformations of a major antigenic site on RSV F

Jones, Harrison G.; Battles, Michael B.; Lin, Chun-Chi; Bianchi, Siro; Corti, Davide; McLellan, Jason S.

To further investigate the conformational variability of site Ø and to identify the epitope on prefusion RSV F recognized by RSD5-GL, we determined the crystal structure of prefusion RSV F in complex with the RSD5-GL Fab to 3.5 Å resolution ( Table 1 ). The crystal structure shows that three RSD5-GL Fabs bind to the membrane-distal apex of the prefusion RSV F trimer ( Fig 3A) . RSD5-GL binds slightly lower on the trimer than AM22, bridging antigenic site Ø and the recently defined site V [32, 44] . The interactions of RSD5-GL with prefusion RSV F are more diverse than that of AM22, with the CDR H2 and CDR H3 as well as all three CDRs of the light chain making contacts with the F protein ( Fig 3B) . In addition, the contacts on prefusion RSV F span multiple regions including the F2 loop as well as α3 and α4 of F1, burying a total surface area of 855 Å 2 . The RSD5-GL heavy chain buries 577 Å 2 (67%) on the surface of prefusion RSV F. The CDR H3 interacts with α4 and the F2 loop, forming hydrogen bonds with Asp200 and mainchain atoms of Lys65, respectively, whereas the CDR H2 contacts α3 and forms a salt bridge with Lys168. The light chain contributes to the interface through contacts with α4, including hydrogen bonds with Asp200 and main-chain atoms of Pro205 and Gln210. Similar to AM22, the prefusion specificity of RSD5-GL can be explained by the dramatic rearrangement of α3 and α4 to form the single elongated α5-helix upon conversion to the postfusion conformation, which dismantles the RSD5-GL epitope.