ACE2 and infection: The S1 subunit binds to the human angiotensin-converting enzyme-2 (ACE-2) receptors, which in turn mediate the fusion of the virus, the key to the infection process, whereas the S2 subunit contains fusion peptide (FP), heptad repeat 1 (HR1), central helix, connector domain, heptad repeat 2 (HR2), transmembrane domain (TM), and cytoplasmic domain (CD).