Physiological and molecular triggers for SARS-CoV membrane fusion and entry into host cells

Abstract

During viral entry, enveloped viruses require the fusion of their lipid envelope with host cell membranes. For coronaviruses, this critical step is governed by the virally-encoded spike (S) protein, a class I viral fusion protein that has several unique features. Coronavirus entry is unusual in that it is often biphasic in nature, and can occur at or near the cell surface or in late endosomes. Recent advances in structural, biochemical and molecular biology of the coronavirus S protein has shed light on the intricacies of coronavirus entry, in particular the molecular triggers of coronavirus S-mediated membrane fusion. Furthermore, characterization of the coronavirus fusion peptide (FP), the segment of the fusion protein that inserts to a target lipid bilayer during membrane fusion, has revealed its particular attributes which imparts some of the unusual properties of the S protein, such as Ca²⁺-dependency. These unusual characteristics can explain at least in part the biphasic nature of coronavirus entry. In this review, using severe acute respiratory syndrome coronavirus (SARS-CoV) as model virus, we give an overview of advances in research on the coronavirus fusion peptide with an emphasis on its role and properties within the biological context of host cell entry.

Keywords: Calcium; Coronavirus; Endosomes; Fusion peptide; SARS; Spike protein; Virus entry.

Fig. 1

Structural model of the SARS-CoV spike in pre-fusion uncleaved and monomeric form, based on pdb 3JCL (MHV spike). SARS-CoV S is colored with the S1 domain in blue and the S2 domain in orange, and is shown in in three different representations: space-filling, surface mesh and cartoon. The fusion peptide/predicted neutralizing epitope is shown green. The two proteolytic cleavage sites are shown in magenta, with S1/S2 exposed and the fusion peptide-proximal S2′ site protected. An enlarged cartoon depiction of the fusion peptide region is shown with key negatively charged and hydrophobic residues indicated.

Fig. 2

Predicted model of the CoV fusion peptide region, along with its interaction with the lipid bilayer. Conserved negatively charged and hydrophobic residues and a proposed location of Ca²⁺ ions are shown.

Fig. 3

Cartoon model of SARS-CoV entry into cells. In this model, fusion can take place in two Ca²⁺-containing compartments corresponding to the observed “early” and “late” entry pathways, with cleavage occurring by distinct activating proteases following receptor engagement.