Source: DESC1 and MSPL Activate Influenza A Viruses and Emerging Coronaviruses for Host Cell Entry
The viral hemagglutinin (HA) and spike (S) surface proteins are responsible for host cell entry of FLUAV (6) and CoVs (7, 8), respectively. Both proteins use their N-terminal surface units, termed HA1 (FLUAV) and S1 (CoVs), to engage cellular receptors, while the C-terminal transmembrane units, termed HA2 (FLUAV) and S2 (CoVs), facilitate fusion of the viral membrane with a host cell membrane—processes that are essential for infectious entry (6,–8). Notably, the HA and S proteins are synthesized as inactive precursors in infected cells and acquire the ability to drive membrane fusion only upon activation by host cell proteases (9, 10).
Take out “host cell” in the last sentence. Almost certainly it doesn’t have to occur in tbe infected host cell.