Synctium Formation in Coronaviruses
It's easy to see why GoF experiments would focus on the furin cleavage site.
'Of all natural MHV (mouse hepatitis virus) isolates, MHV-2 is the only strain with an uncleaved spike protein. Isolated mutants of MHV-2, which, in contrast to the parental virus, were capable of inducing synctium formation, appeared to have acquired this ability through a single mutation restoring an apparently lost cleavage site.
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Interestingly, mutants of MHV-2 carrying a proper furin cleavage site appeared to have lost their sensitivity both to lysosomotropic agents (ex., chloroquine) and to cathepsin inhibitors.'
(Bosch and Rottier, Nidovirus Entry Into Cells, in Nidoviruses, ASM Press, 2008)
Virus entry into cells can be promoted by inhibition of such things as chloroquine and leupeptin. Thus, synctium formation does link to the absence and presence of furin cleavage sites, which target the S1-S2 junction.
If the reader understands the next paragraph, they will have gained much insight into GoF experiments, hydroxychloroquine and furin cleavage sites.
'As this cleavage induces cell-cell fusion, it is conceivable that cathepsin L also targets the Si-S2 junction. This is consistent with the already-mentioned rescue of SARS-CoV entry inhibition by leupeptin and lysosomotropic agents (ex., chloroquine) by trypsin treatment of cell-bound virions, as well as with the formation of synctia in SARS-CoV spike protein-expressing cells and in MHV-2-infected cells that is observed after introduction of a furin cleavage site at the S1-S2 junction of these spike proteins.
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One might argue that all spike proteins, including those of group 1 coronaviruses, originally contained a furin cleavage site. Spike protein cleavage during biogenesis might have been unfavorable for some coronaviruses, as it could lead to spike inactivation through S1-S2 dissociation.
Viruses with uncleaved spikes might thus have benefitted somehow from the loss of furin cleavage. This idea is supported by the presence of what has been suggested to be a vestigial furin cleavage site (DRTRG) approximately in the middle of the TGEV spike protein.'
(ibid.)
This last statement is noteworthy considering the RGD motif of Dr. Quay's Twitter page showing the Chinese military virus, ZC45. TGEV is the pig coronavirus that can subsist on stainless steel surfaces and remain viable for up to 28 days. For the RGD motif, see this thread, post #2,288.
(USMB search 'rgd motif')