The spike protein (S protein) is a SARS-CoV-2 virus (coronavirus) component that helps the virus enter healthy cells. It latches onto receptors called ACE2 (angiotensin-converting enzyme 2), penetrating host cells and causing infection. The spike protein forms a structure called a spike on the outside of an enveloped virus. It’s what gives coronaviruses their crown-like appearance. The word corona is Latin for “crown.”