SARS-CoV-2 Recombinant Spike Protein, aa16-685 is expressed in HEK293 cells and is one of four structural proteins encoded by the SARS-CoV-2 genome. The Spike Protein plays a key role in attachment to host cells, allowing invasion through clathrin-mediated endocytosis. The Spike Protein can be cleaved by host cell proteases after aa685 to yield the N-terminal S1 subunit and C-terminal S2 region. The S1 subunit is responsible for interacting with the host cell receptor (angiotensin-converting enzyme II) through a receptor-binding domain that is highly conserved with SARS-CoV. The S1 subunit has two conformations: a ‘down’ conformation in which the receptor is inaccessible, and an ‘up’ conformation in which the receptor is accessible. These conformational changes are key for monoclonal antibody drugs and vaccine development. SARS-CoV-2 Recombinant Spike Protein contains a polyhistidine tag at the amino terminus; it also contains a FLAG tag at the carboxy terminus.