The spike protein protrudes from the virus hull (bottom, beige) and mediates entry into the host cell.
It consists of three identical copies of a protein (a homo-trimer, in green) and binds to the cellular receptor called angiotensin converting enzyme 2 (not shown).
This triggers a cascade of events leading to the fusion of the cell and virus membrane. After the prefusion trimer is destabilized, a part of the spike protein is shed, leading to transition to a stable post-fusion conformation.
To engage a host cell receptor, the receptor-binding domain of the spike protein undergoes a hinge-like conformational rearrangement. The exact mechanism of this entry and fusion of SARS-CoV-2 with the host cell is still not fully understood.
The spike protein is heavily glycosylated (grey), meaning it carries sugar chains on the protein surface which form a slimy surface making it more difficult for the immune system to recognise the spike protein. The type of glycosylation depends on the host cell in which the protein was produced.