Severe acute respiratory syndrome coronavirus 2
First, the virus enters the cell with the help of the S protein or spike protein expressed on the surface of the virus beyond its envelope.  The spike protein attaches to the human cell receptor hACE2 or  human angiotensin-converting enzyme 2 . ACE2 is found on the cells of the epithelial cell linings of the lungs, kidneys, and intestines. After the attachment of the S- protein to the virus, the S protein splits into its two components- S1 head and S2 stalk . The S1 head is now able to bind to the TMPRSS2 receptor which is a serine protease enzyme containing a transmembrane domain. The interaction of TMPRSS2 and S1 trigger the cell membrane of the human cell to merge with that of the virus leading to viral genome entry into cells via endosomes. The viral genome in the endosomes which convert to lysosomes is then processed by cathepsin, which is then replicated in the human cells and released after viral packaging and maturation in the cells to infect others. In October 2020, scientists said they had seen SARS-CoV-2 use another receptor, Neuropilin-1, to enter cells with the help of the furin cleavage site in its genome which was not found in any of its previous ancestors.