Diverse noroviruses infect humans and animals via the recognition of host-specific glycan ligands. Genogroup II (GII) noroviruses consist of human noroviruses (huNoVs) that generally bind histo-blood group antigens (HBGAs) as host factors and three porcine norovirus (porNoV) genotypes (GII.11/18/19) that form a genetic lineage lacking HBGA-binding ability. Thus, these GII porNoVs provide an excellent model to study norovirus evolution with host ligand specificity changes. Here we solved the crystal structures of a native GII.11 porNoV P protein and a closely-related GII.3 huNoV P protein complexed with an HBGA, focusing on the HBGA-binding sites (HBSs) compared with the previously known ones to understand the structural basis of the host ligand specificity change. We found that the GII.3 huNoV binds HBGAs via a conventional GII HBS that uses an arginine instead of the conserved aromatic residue for the required Van der Waals interaction, while the GII.11 porNoV HBS loses its HBGA-binding function because of two mutations (Q355/V451). A mutant that reversed the two mutated residues back to the conventional A355/Y451 restored the HBGA-binding function of the GII.11 porNoV P protein, which validated our observations. Similar mutations are also found in GII.19 porNoVs and a GII.19 P protein mutant with double reverse mutations restored the HBS function. This is the first reconstruction of a functional HBS based on one with new host specificity back to its parental one. These data shed light on the molecular basis of structural adaptation of the GII porNoVs to the pig hosts through mutations at their HBSs.

中文翻译：

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GII 猪诺如病毒与其密切相关的 GII 人诺如病毒的宿主配体特异性变化的结构基础。

多种诺如病毒通过识别宿主特异性聚糖配体来感染人类和动物。基因组 II (GII) 诺如病毒由通常结合组织血型抗原 (HBGA) 作为宿主因子的人诺如病毒 (huNoV) 和三种猪诺如病毒 (porNoV) 基因型 (GII.11/18/19) 组成，形成缺乏遗传谱系的HBGA 结合能力。因此，这些 GII porNoV 提供了一个极好的模型来研究诺如病毒随宿主配体特异性变化的进化。在这里，我们解析了天然 GII.11 porNoV P 蛋白和密切相关的 GII.3 huNoV P 蛋白与 HBGA 复合的晶体结构，重点关注 HBGA 结合位点 (HBS) 与之前已知的结合位点进行比较，以了解宿主配体特异性改变的结构基础。我们发现 GII.3 huNoV 通过传统的 GII HBS 结合 HBGA，该传统的 GII HBS 使用精氨酸而不是保守的芳香残基来实现所需的范德华相互作用，而 GII.11 porNoV HBS 由于两个突变而失去了 HBGA 结合功能（Q355/V451）。将两个突变残基逆转回常规 A355/Y451 的突变体恢复了 GII.11 porNoV P 蛋白的 HBGA 结合功能，这验证了我们的观察。在 GII.19 porNoV 中也发现了类似的突变，并且具有双反向突变的 GII.19 P 蛋白突变体恢复了 HBS 功能。这是功能性 HBS 的第一次重建，其基础是具有新宿主特异性的 HBS 回其亲代宿主。这些数据揭示了 GII porNoV 通过 HBS 突变对猪宿主进行结构适应的分子基础。