The unprecedented public health and economic impact of the COVID-19 pandemic caused by infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have been met with an equally unprecedented scientific response. Much of this response has focused, appropriately, on the mechanisms of SARS-CoV-2 entry into host cells, and in particular the binding of the spike (S) protein to its receptor, angiotensin-converting enzyme 2 (ACE2), and subsequent membrane fusion.
This Review provides the structural and cellular foundations for understanding the multistep SARS-CoV-2 entry process, including S protein synthesis, S protein structure, conformational transitions necessary for association of the S protein with ACE2, engagement of the receptor-binding domain of the S protein with ACE2, proteolytic activation of the S protein, endocytosis and membrane fusion.
We define the roles of furin-like proteases, transmembrane protease, serine 2 (TMPRSS2), and cathepsin L in these processes, and delineate the features of ACE2 orthologues in reservoir animal species and S protein adaptations that facilitate efficient human transmission. We also examine the utility of vaccines, antibodies, and other potential therapeutics targeting SARS-CoV-2 entry mechanisms. Finally, we present key outstanding questions associated with this critical process.
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of COVID-19, which escalated into a global pandemic in 2020. SARS-CoV-2 is an enveloped, positive-sense, single-stranded RNA virus of the genus Betacoronavirus.
In addition to SARS-CoV, which was responsible for the 2002–2004 SARS epidemic and which shares 79% nucleotide sequence identity with SARS-CoV-2 the genus also includes human coronavirus (HCoV)-OC43, HCoV-HKU1, and Middle East respiratory syndrome coronavirus (MERS-CoV).
SARS-CoV-2 relies on its obligate receptor, angiotensin-converting enzyme 2 (ACE2), to enter cells. ACE2 was originally identified in 2003 as the receptor for SARS-CoV ACE2 is also the receptor for alphacoronavirus HCoV-NL63, which, together with another alphacoronavirus, HCoV-229E, and beta coronaviruses HCoV-OC43 and HCoV-HKU1, is a known causative agent of mild upper respiratory tract infections.
The coronavirus virion is made up of the nucleocapsid (N), membrane (M), envelope (E), and spike (S) proteins, which are structural proteins. The entry steps of the viral particles — encompassing attachment to the host cell membrane and fusion — are mediated by the S glycoprotein. S protein is assembled as a homotrimer and is inserted in multiple copies into the membrane of the virion giving it its crown-like appearance.
Entry glycoproteins of many viruses, including HIV-1, Ebola virus, and avian influenza viruses, are cleaved into two subunits — extracellular and transmembrane — in the infected cells (that is, the cleavage occurs before releasing of the virus from the cell that produces it).
Similarly, the S protein of some coronaviruses is cleaved into S1 and S2 subunits during their biosynthesis in the infected cells, while the S protein of other coronaviruses is cleaved only when they reach the next target cell.
SARS-CoV-2, like MERS-CoV, belongs to the first category: its S protein is cleaved by proprotein convertases such as furin in the virus-producer cells.
Therefore, the S protein on the mature virion consists of two non-covalently associated subunits: the S1 subunit binds ACE2 and the S2 subunit anchors the S protein to the membrane. The S2 subunit also includes a fusion peptide and other machinery necessary to mediate membrane fusion upon infection of a new cell