Antibody inhibition of the transcriptase activity of the rotavirus DLP: A structural view

On entering the host cell the rotavirus virion loses its outer shell to bec ome a double-layered particle (DLP). The DLP then transcribes the 11 segmen ts of its dsRNA genome using its own transcriptase complex, and the mature mRNA emerges along the 5-fold axis. Ln order to better understand the trans cription mechanism and the role of VP6 in transcription we have studied thr ee monoclonal antibodies against VP6: RV-238 which inhibits the transcripta se activity of the DLP; and RV-133 and RV138 which have no effect on transc ription. The structures obtained by cryo-electron microscopy of the DLP/Fab complexes and by X-ray crystallography of the VP6 trimer and the VP6/Fab-2 38 complex have been combined to give pseudo-atomic structures. Steric hind rance between the Fabs results in limited Fab occupancy. In particular, the re are on average only three of a possible five Fabs-238 which point toward s the 5-fold axis. Thus, Fabs-238 are not in a position to block the exitin g mRNA, nor is there any visible conformational change in VP6 on antibody b inding at a resolution of 23 Angstrom. However, the epitope of the inhibiti ng antibody involves two VP6 monomers, whereas, those of the non-inhibiting antibodies have an epitope on only one VP6. Thus, the inhibition of transc ription may be a result of inhibition of a possible change in the VP6 confo rmation associated with the transcription of mRNA. (C) 2001 Academic Press.