Molecular characterization of a subgroup specificity associated with the rotavirus inner capsid protein VP2

Group A rotaviruses are classified into serotypes, based on the reactivity pattern of neutralizing antibodies to VP4 and VP7, as well as into subgroups (SGs), based on non-neutralizing antibodies directed against VP6. The inner capsid protein (VP2) has also been described as a SG antigen; however, little is known regarding the molecular determinants of VP2 SG specificity. In this study, we characterize VP2 SGs by correlating genetic markers with the immunoreactivity of the SG-specific monoclonal antibody (YO-60). Our results show that VP2 proteins similar in sequence to that of the prototypic human strain Wa are recognized by YO-60, classifying them as VP2 SG-II. In contrast, proteins not bound by YO-60 are similar to those of human strains DS-1 or AU-1 and represent VP2 SG-I. Using a mutagenesis approach, we identified residues that determine recognition by either YO-60 or the group A-specific VP2 monoclonal antibody (6E8). We found that YO-60 binds to a conformationally dependent epitope that includes Wa VP2 residue M328. The epitope for 6E8 is also contingent upon VP2 conformation and resides within a single region of the protein (Wa VP2 residues A440 to T530). Using a high-resolution structure of bovine rotavirus double-layered particles, we predicted these epitopes to be spatially distinct from each other and located on opposite surfaces of VP2. This study reveals the extent of genetic variation among group A rotavirus VP2 proteins and illuminates the molecular basis for a previously described SG specificity associated with the rotavirus inner capsid protein.