An Interaction between the Nucleocapsid Protein and a Component of the Replicase-Transcriptase Complex Is Crucial for the Infectivity of Coronavirus Genomic RNA

Abstract

ABSTRACT The coronavirus nucleocapsid (N) protein plays an essential role in virion assembly via interactions with the large, positive-strand RNA viral genome and the carboxy-terminal endodomain of the membrane protein (M). To learn about the functions of N protein domains in the coronavirus mouse hepatitis virus (MHV), we replaced the MHV N gene with its counterpart from the closely related bovine coronavirus (BCoV). The resulting viral mutant was severely defective, even though individual domains of the N protein responsible for N-RNA, N-M, or N-N interactions were completely interchangeable between BCoV and MHV. The lesion in the BCoV N substitution mutant could be compensated for by reverting mutations in the central, serine- and arginine-rich (SR) domain of the N protein. Surprisingly, a second class of reverting mutations were mapped to the amino terminus of a replicase subunit, nonstructural protein 3 (nsp3). A similarly defective MHV N mutant bearing an insertion of the SR region from the severe acute respiratory syndrome coronavirus N protein was rescued by the same two classes of reverting mutations. Our genetic results were corroborated by the demonstration that the expressed amino-terminal segment of nsp3 bound selectively to N protein from infected cells, and this interaction was RNA independent. Moreover, we found a direct correlation between the N-nsp3 interaction and the ability of N protein to stimulate the infectivity of transfected MHV genomic RNA (gRNA). Our results suggest a role for this previously unknown N-nsp3 interaction in the localization of genomic RNA to the replicase complex at an early stage of infection.