SARS-CoV-2 Recombinant Spike Protein, aa319-541 is expressed in Pichia pastoris 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. At the amino terminus of the polypeptide chain, SARS-CoV-2 Recombinant Spike Protein contains a polyhistidine tag and a SUMOstar site.