Noroviruses (NVs) are the most important pathogen of epidemic nonbacterial gastroenteritis. The recent finding that NVs recognize human histo-blood group antigens (HBGAs) as receptors provided a new approach to study the pathogenesis of NVs. Using computational and site-directed mutagenesis approaches, our investigators previously identified a plausible binding pocket in the P domain of the NV capsids. In this study, we further characterize the role of the P domain in the interaction with human HBGA receptors using three NV strains representing three binding patterns. Our results show that the isolated P domain, although it did not form virus-like particles (VLPs), formed dimers, and the dimers bound HBGAs with the same patterns as those of the intact viral capsids. In contrast, the S domain, which formed small, thin-layer VLPs, did not bind A, B, or H HBGAs. A chimera containing the S domain of VA387 and the P domain of MOH revealed a binding pattern of the P donor strain (MOH). Deletion experiments revealed that an intact P domain is necessary for receptor binding. The P domain dimers are stable over a broad range of pH (2 to 11) or under strong denaturing conditions. Taken together, our results suggest that the P domain of NV contains essential elements for strain-specific binding to receptors. Further study of the P domain will provide useful information about the virus-receptor interaction. The high yield and easy production of the recombinant P protein in the Escherichia coli expression system will provide a simple approach to this goal.Noroviruses (NVs), formally called Norwalk-like viruses, belong to one of two genera of human caliciviruses, the Norovirus and Sapovirus genera, within the Caliciviridae. NVs contain a single-stranded, positive-sense RNA genome of about 7.7 kb (12, 15). The viral genome encodes one major structural protein of ϳ60 kDa that is responsible for the building of the viral capsid (15). In addition, a minor structural protein of ϳ20 kDa was identified in the recombinant Norwalk virus capsid (7), which is encoded by the third open reading frame of the genome (15). The function and location of the protein in the capsid remain unclear, although a recent study suggested that this basic protein stabilizes the capsid protein and protects the virus-like particles (VLPs) from disassembly (1). NVs are difficult to study due to the lack of a cell culture and an animal model. The successful expression of NV capsid proteins (8,13,14,16,17) and the fact that the capsid proteins spontaneously form empty VLPs provided a valuable way for development of diagnostic assays for studying the immunology, epidemiology, and pathogenesis of NVs.The X-ray crystal structure of the prototype Norwalk virus VLPs showed that the Norwalk virus capsid is composed of 180 capsid protein monomers that form a Tϭ3 icosahedral capsid (20). Each of the capsid proteins has two major domains, the S and P domains, linked by a hinge of 8 amino acids (aa). The S domain (residues 1 to 217) is responsible for the formation of t...