Antibody-mediated cell entry of SARS-CoV-2 (preprint)

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) enters host cells by first engaging its cellular receptor angiotensin converting enzyme 2 (ACE2) to induce conformational changes in the virus-encoded spike protein and fusion between the viral and target cell membranes. We report here that certain monoclonal neutralizing antibodies against distinct epitopic regions of the receptor-binding domain of the spike can replace ACE2 to serve as a receptor and efficiently support membrane fusion and viral infectivity. These receptor-like antibodies can function in the form of a complex of their soluble immunoglobulin G with Fc-gamma receptor I, a chimera of their antigen-binding fragment with the transmembrane domain of ACE2 or a membrane-bound B cell receptor, indicating that ACE2 and its specific interactions with the spike protein are dispensable for SARS-CoV-2 entry. These results suggest that antibody responses against SARS-CoV-2 may expand the viral tropism to otherwise nonpermissive cell types; they have important implications for viral transmission and pathogenesis.

Cryo-EM structure of SARS-CoV-2 postfusion spike in membrane (preprint)

Entry of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) into host cells depends on refolding of the virus-encoded spike protein from a prefusion conformation, metastable after cleavage, to a lower energy, stable postfusion conformation. This transition overcomes kinetic barriers for fusion of viral and target cell membranes. We report here a cryo-EM structure of the intact postfusion spike in a lipid bilayer that represents single-membrane product of the fusion reaction. The structure provides structural definition of the functionally critical membrane-interacting segments, including the fusion peptide and transmembrane anchor. The internal fusion peptide forms a hairpin-like wedge that spans almost the entire lipid bilayer and the transmembrane segment wraps around the fusion peptide at the last stage of membrane fusion. These results advance our understanding of the spike protein in a membrane environment and may guide development of intervention strategies.

Distinct conformational states of SARS-CoV-2 spike protein (preprint)

The ongoing SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) pandemic has created urgent needs for intervention strategies to control the crisis. The spike (S) protein of the virus forms a trimer and catalyzes fusion between viral and target cell membranes - the first key step of viral infection. Here we report two cryo-EM structures, both derived from a single preparation of the full-length S protein, representing the prefusion (3.1[A] resolution) and postfusion (3.3[A] resolution) conformations, respectively. The spontaneous structural transition to the postfusion state under mild conditions is independent of target cells. The prefusion trimer forms a tightly packed structure with three receptor-binding domains clamped down by a segment adjacent to the fusion peptide, significantly different from recently published structures of a stabilized S ectodomain trimer. The postfusion conformation is a rigid tower-like trimer, but decorated by N-linked glycans along its long axis with almost even spacing, suggesting possible involvement in a mechanism protecting the virus from host immune responses and harsh external conditions. These findings advance our understanding of how SARS-CoV-2 enters a host cell and may guide development of vaccines and therapeutics.