The spike glycoprotein is the most important antigen in the pandemic strain of coronavirus, SARS-CoV-2, the causative agent of the respiratory disease COVID-19. With a complex transmembrane domain and exposed in a trimeric form on the surface of the novel virus, the spike protein can be divided into S1 and S2 subunits linked by a furin cleavage site.
The S1 subunit contains the receptor-binding domain (RBD) that binds to the human ACE2 receptor, enriched on the surface of upper respiratory tract epithelial cells. As a consequence, the formation of the RBD-ACE2 complex is known to drive proteolytic cleavage of the peak, exposing the S2 subunit. As a result, S2 undergoes significant molecular rearrangements that lead to membrane fusion and virion internalization.
To make the spike proteinase stable, specific single point mutations were made in the sequence of the furin cleavage linker. In this way, it was possible to express a stable version of the peak with a long lifespan, a crucial change that enables the development of research, diagnostic and therapeutic applications.
The D614G mutation of the peak was first detected in Europe in early March and corresponds to a single nucleotide change (GAA at position 23,403) that made the peak more flexible than the early variant and consequently improved its infectivity. Due to its improved properties, the G614 variant, also known as the “G” clade, became the dominant version of the spike protein in SARS-CoV-2 in late March 2020 on a global scale. Other studies revealed that the D614G mutation can also improve not only the stability of virions but also their fitness and replication.
This specific version of the spike is monomeric and is expressed in Chinese hamster ovary (CHO) cells with high quality and purity. Therefore, the peak can serve as a control for standard screening tests (eg ELISA, flow cytometry, WB, etc.). Furthermore, due to its vital role in the early stages of infection, this protein continues to guide the development of new diagnostic tools, biotherapeutics, and vaccines.