Structural Analysis of Determinants of Histo-Blood Group Antigen Binding Specificity in Genogroup I Noroviruses

Abstract

Human noroviruses (NoVs) cause acute epidemic gastroenteritis. Susceptibility to the majority of NoV infections is determined by genetically controlled secretor-dependent expression of histo-blood group antigens (HBGAs), which are also critical for NoV attachment to host cells. Human NoVs are classified into two major genogroups (genogroup I [GI] and GII), with each genogroup further divided into several genotypes. GII NoVs are more prevalent and exhibit periodic emergence of new variants, suggested to be driven by altered HBGA binding specificities and antigenic drift. Recent epidemiological studies show increased activity among GI NoVs, with some members showing the ability to bind nonsecretor HBGAs. NoVs bind HBGAs through the protruding (P) domain of the major capsid protein VP1. GI NoVs, similar to GII, exhibit significant sequence variations in the P domain; it is unclear how these variations affect HBGA binding specificities. To understand the determinants of possible strainspecific HBGA binding among GI NoVs, we determined the structure of the P domain of a GI.7 clinical isolate and compared it to the previously determined P domain structures of GI.1 and GI.2 strains. Our crystallographic studies revealed significant structural differences, particularly in the loop regions of the GI.7 P domain, altering its surface topography and electrostatic landscape and potentially indicating antigenic variation. The GI.7 strain bound to H-and A-type, Lewis secretor, and Lewis nonsecretor families of HBGAs, allowing us to further elucidate the structural determinants of nonsecretor HBGA binding among GI NoVs and to infer several contrasting and generalizable features of HBGA binding in the GI NoVs. IMPORTANCEHuman noroviruses (NoVs) cause acute epidemic gastroenteritis. Recent epidemiological studies have shown increased prevalence of genogroup I (GI) NoVs. Although secretor-positive status is strongly correlated with NoV infection, cases of NoV infection associated with secretor-negative individuals are reported. Biochemical studies have shown that GI NoVs exhibit genotypedependent binding to nonsecretor histo-blood group antigens (HBGAs). From our crystallographic studies of a GI.7 NoV, in comparison with previous studies on GI.1 and GI.2 NoVs, we show that genotypic differences translate to extensive structural changes in the loop regions that significantly alter the surface topography and electrostatic landscape of the P domain; these features may be indicative of antigenic variations contributing to serotypic differentiation in GI NoVs and also differential modulation of the HBGA binding characteristics. A significant finding is that the threshold length and the structure of one of the loops are critical determinants in the binding of GI NoVs to nonsecretor HBGAs.

Structural basis for norovirus neutralization by an HBGA blocking human IgA antibody

“…3A). Some of these loop regions have been observed to have sequence and structural changes in other genogroups and within genotypes contributing to variations of HBGA binding specificities (13,15,21). The paratope of Fab 5I2 comprises three of the six CDRs, including CDRL1 (residues [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40] and CDRL3 (residues 96-103) in the light chain and CDRH3 (residues 97-113) in the heavy chain (Fig.…”

“…The H381 residue in the T loop of NV P domain is critical for IgA 5I2 binding, as it is involved in multiple stabilizing interactions with the antibody and is not conserved in other GI genotypes. Although this residue in GI.1 is far removed from the HBGA binding site, because of the conformational changes, the structurally corresponding residue S391 in GI.7 becomes a part of the primary HBGA binding site (13), clearly illustrating a coordinated interplay between antigenic variation and HBGA binding in the evolution of NoVs.…”

“…The P domain is further divided into P1 and P2 subdomains, with the latter being an insertion in the P1 subdomain. Evolutionarily, the P2 subdomain is the least conserved and is implicated in strain diversity, differential HBGA binding, and antigenicity (12,13).HuNoVs are suggested to evolve through a coordinated interplay between differential HBGA binding specificities and antigenic variations that allow emerging strains to escape host immunity. Differential HBGA binding has been previously well characterized in GI and GII HuNoVs (14,15).…”

“…The P domain is further divided into P1 and P2 subdomains, with the latter being an insertion in the P1 subdomain. Evolutionarily, the P2 subdomain is the least conserved and is implicated in strain diversity, differential HBGA binding, and antigenicity (12,13).…”

Role of histo-blood group antigens in primate enteric calicivirus infections

“…In addition, retrospective HBGA analysis of recently reported Bangladeshi patients, infected with rhesus enteric calicivirus (TV strain), should also be informative [16] . Latest structural analysis of GI human NoVs revealed critical extension of the P domain loop region that appears to be responsible for binding of the GI.7 NoV with non-secretor HBGAs [34] . The "extended P domain loop" is not present on other GI NoVs that are known to bind with secretor HBGAs.…”