| Id |
Subject |
Object |
Predicate |
Lexical cue |
| T240 |
0-225 |
Sentence |
denotes |
To date, the cell entry mechanism of SARS-CoV and SARS-CoV-2 has been understood in its general details and it is based on a concerted action of receptor binding and proteolysis of the S protein (Figure 5; Tang et al., 2020). |
| T241 |
226-417 |
Sentence |
denotes |
Ultrastructural studies showed a metastable “prefusion” V-shaped trimer composed by three S1 heads sitting on top of a trimeric S2 stalk anchored into the virus membrane (Walls et al., 2016). |
| T242 |
418-635 |
Sentence |
denotes |
The RBD constantly switches between a standing-up (“open”) position for receptor binding and a lying-down (“closed”) configuration, the latter allowing immune evasion (Figure 6; Song et al., 2018; Wrapp et al., 2020). |
| T243 |
636-754 |
Sentence |
denotes |
Yet only one of the three RBD in trimeric S can flip up at a time and interact with the receptor (Wrapp et al., 2020). |
| T244 |
755-917 |
Sentence |
denotes |
The second key feature of the fusion mechanism is “priming” by host proteases, which recognize and cleave a short peptide motif at the S1/S2 boundary (Figure 4B). |
| T245 |
918-1281 |
Sentence |
denotes |
This cleavage does not disassemble S1 from S2 in pre-fusion conditions (Belouzard et al., 2009), but the binding interaction of RBD with its receptor, accompanied by a further cleavage in a second site in S2 (S2’site, upstream of FP, Figure 4B), triggers the possible dissociation of S1 and the irreversible refolding of S2 into a “post-fusion” state (Figure 4B). |
| T246 |
1282-1517 |
Sentence |
denotes |
In detail, HR1 undergoes a dramatic “jack-knife” conformational change, converting four helical stretches that run in an antiparallel fashion into a single long (∼130 aa) α-helix (Heald-Sargent and Gallagher, 2012; Walls et al., 2017). |
| T247 |
1518-1632 |
Sentence |
denotes |
At first, three of these helices assemble into a homotrimeric bundle and stick the FP into the host cell membrane. |
| T248 |
1633-1787 |
Sentence |
denotes |
Then, HR2 (one for each S2 chain) fold backward and bind to HR1, yielding the “six-helix bundle fusion core” (6-HB) of post-fusion S2 (Song et al., 2018). |
| T249 |
1788-2020 |
Sentence |
denotes |
This conformational foldback brings the FP (at N-terminus of HR1) and the TM (at the C terminus of HR2) close to each other, so that the viral and host cell membranes approach until their outer leaflets merge (hemifusion, Figure 5). |
| T250 |
2021-2231 |
Sentence |
denotes |
Eventually the inner leaflets merge (pore formation), enabling a connection between the interior of the virus and the host cell cytoplasm, that allows the delivery of viral genome (Figure 5; Tang et al., 2020). |
