C-terminal arginine cluster is essential for receptor binding of norovirus capsid protein

Noroviruses are the major viral pathogens of epidemic acute gastroenteritis affecting people worldwide. They have been found to recognize human histo-blood group antigens as receptors. The P domain of norovirus capsid protein was found to be responsible for binding to viral receptors, and the recombinant P protein forms P dimers and P particles in vitro. In this study, we demonstrate that a highly conserved arginine (R) cluster at the C terminus of the P domain is critical for receptor binding and P particle formation of the P proteins. Deletions of the R cluster abolished these functions. Replacement of the R cluster with histidines (another positively charged amino acid) resulted in low efficiency of receptor binding and P particle formation, while replacement with alanines led to loss of both functions completely. The R cluster also contains a highly conserved trypsin digestion site. A treatment of capsid protein or P domain mutants from both genogroup I (Norwalk virus) and genogroup II (VA387) noroviruses with trypsin resulted in a removal of the R cluster and the S domain, leaving a P polypeptide of 31.3 kDa (Norwalk virus) or 34.3 kDa (VA387), similar to the soluble P protein found in vivo. Our findings imply that the proteolytic process could be a necessary step for norovirus replication in the host.