Cooperation of an RNA packaging signal and a viral envelope protein in coronavirus RNA packaging

Murine coronavirus mouse hepatitis virus (MHV) produces a genome-length mRNA, mRNA 1, and six or seven species of subgenomic mRNAs in infected cells. Among these mRNAs, only mRNA 1 is efficiently packaged into MHV particles. MHV N protein binds to all MHV mRNAs, whereas envelope M protein interacts only with mRNA 1. This M protein-mRNA 1 interaction most probably determines the selective packaging of mRNA 1 into MHV particles. A short cis-acting MHV RNA packaging signal is necessary and sufficient for packaging RNA into MHV particles. The present study tested the possibility that the selective M protein-mRNA 1 interaction is due to the packaging signal in mRNA 1. Regardless of the presence or absence of the packaging signal, N protein bound to MHV defective interfering RNAs and intracellularly expressed non-MHV RNA transcripts to form ribonucleoprotein complexes; M protein, however, interacted selectively with RNAs containing the packaging signal. Moreover, only the RNA that interacted selectively with M protein was efficiently packaged into MHV particles. Thus, it was the packaging signal that mediated the selective interaction between M protein and viral RNA to drive the specific packaging of RNA into virus particles. This is the first example for any RNA virus in which a viral envelope protein and a known viral RNA packaging signal have been shown to determine the specificity and selectivity of RNA packaging into virions.