Assembly of spikes into coronavirus particles is mediated by the carboxy-terminal domain of the spike protein

The type I glycoprotein S of corona virus, trimers of which constitute the typical viral spikes, is assembled into virions through noncovalent interac tions with the hi protein, sere we demonstrate that incorporation is mediat ed by the short carboxy-terminal segment comprising the transmembrane and e ndodomain. To this aim, He used the virus-like particle (VLP) system that,v e developed earlier for the mouse hepatitis virus strain A59 (MHV-A59) and, which we describe now also for the unrelated coronavirus feline infectious peritonitis virus (FIPV; strain 79-1146). Two chimeric MHV-FIPV S proteins were constructed, consisting of the ectodomain of the one virus and the tra nsmembrane and endodomain of the other. These proteins were tested for thei r incorporation into VLPs of either species. They were found to assemble on ly into viral particles of the species from which their carboxy-terminal do main originated. Thus, the 64-terminal-residue sequence suffices to draw th e 1308 (MHV)- or 1433 (FIPV)-amino-acid-long mature S protein into VLPs. Bo th chimeric S proteins appeared to cause cell fusion when expressed individ ually, suggesting that they were biologically fully active. This was indeed confirmed by incorporating one of the proteins into virions which thereby acquired a new host cell tropism? as will be reported elsewhere.