Function
The homotrimeric spike glycoprotein on the virus envelope mediates the entry into cell. Every monomer consists of the two subunits S1 and S2. SARS-CoV-2 spike S1 subunit binds the cellular receptor called angiotensin converting enzyme 2 (ACE2). Binding triggers a cascade of events leading to the fusion of cell and virus membrane. After the prefusion trimer is destabilized, the S1 subunit is shedded leading to transition of the S2 subunit to a stable postfusion conformation. To engage a host cell receptor, the receptor-binding domain (RBD) of S1 undergoes hinge-like conformational rearrangement that transiently hide or expose the residues necessary for receptor binding. [1]
Structure Description
Spike subunits S1 and S2 can be divided into several subdomains. The S1 subunit comprises a signal sequence (SS) on the N-terminal end followed by a N-terminal domain (NTD) and the receptor binding domain (RBD). After two small subdomains (SD1/2), we find two protease cleavage sites (S1/S2 and S2’).The S2 subunit is composed of a fusion peptide (FP), two heptad repeats (HR1 and 2), a central helix (CH), a connector domain (CD), a transmembrane domain (TM) and the cytoplasmic tail (CT). [1]
The structure of the receptor binding domain (RBD), in complex with the human ACE2 receptor, shows that interaction happens via the spike protein RBD and the ACE2 N-terminal peptidase domain. The RBD consists of a twisted five stranded antiparallel β-sheet (β1, β2, β3, β4 und β7) forming the core together with short connecting helices and loops. The spike receptor binding motif (RBM), containing most of the ACE2 contacting residues, is located as an extended insertion between the β4 and β7 strands consisting of short β-sheets (β5 and β6), α-helices (α4 and α5) and loops. The ACE2 N-terminal peptidase domain has two lobes that form the substrate binding site. The contact between RBM and ACE2 is made at the bottom side of the ACE2 small lobe, with a concave outer surface in the RBM accommodating the N-terminal helix of the ACE2 and thus generating an interface of 1687 Å^2. This interface contains a network of different interactions, including hydrophilic interactions with 13 hydrogen bonds and 2 salt bridges. Key residues for for receptor binding include the amino acids Leu544, Phe486, Gln493, and Asn 501. Leu 544 interacts with ACE2 residues Asp30, Lys31 and His34. Phe486, interacts with ACE2 GLN24, Leu79, Met82 (by van der Waals forces) and Tyr 83. Gln 493 forms a hydrogen bond with ACE2 Glu35 and interacts with Lys31 and His34. Another Hydrogen bond is formed between ACE2 Tyr 41 and Asn501 of one α-helix of the RBM. Further, Asn501 also interacts with the amino acid residues Lys353, Gly354 and Asp355. Outside the RBM, there is another unique ACE2-interacting residue Lys417, forming a salt bride with ACE2 Asp30. [2] [3]
Fusion and Entry Mechanism
The task of the spike protein is to initiate the fusion and entry with/ into the host cell. A key role in mediating these processes are the domains S-HR1 and S-HR2. The exact mechanism of entry and fusion of SARS-CoV-2 with/ into the host cell is still not fully known but it could be possible that the 2019-nCoV may have similar membrane fusion mechanism as that of SARS-CoV. The putative antiviral mechanism is, that after binding of RBD S1 subunit of 2019-nCoV spike protein to the receptor ACE2 on the host cell, S2 subunit changes conformation by inserting FP into the cell membranes, triggering the association between the HR1 and HR2 domains to form a six-helix-bundle, which brings the viral and cellular membranes in close proximity for fusion.[4]
Glycosilation of the Spike Protein
Coronavirus spike proteins are densely decorated by heterogenous N-linked glycans protruding from the trimer surface. SARS-CoV-2 S comprises 22 N-linked glycosylation sequons per protomer. N-linked glycans play a key role in proper protein folding and in priming by host proteases [5] Since glycans can shield the amino acid residues and other epitopes from cells and antibody recognition, glycosylation can enable the coronavirus to evade both the innate and adaptive immune responses. [2] [6]
