The Small Envelope Protein E Is Not Essential for Murine Coronavirus Replication

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Abstract
The importance of the small envelope (E) protein in the assembly of coronaviruses has been demonstrated in several studies. While its precise function is not clearly defined, E is a pivotal player in the morphogenesis of the virion envelope. Expression of the E protein alone results in its incorporation into vesicles that are released from cells, and the coexpression of the E protein with the membrane protein M leads to the assembly of coronavirus-like particles. We have previously generated E gene mutants of mouse hepatitis virus (MHV) that had marked defects in viral growth and produced virions that were aberrantly assembled in comparison to wild-type virions. We have now been able to obtain a viable MHV mutant in which the entire E gene, as well as the nonessential upstream genes 4 and 5a, has been deleted. This mutant (ΔE) was obtained by a targeted RNA recombination method that makes use of a powerful host range-based selection system. The ΔE mutant produces tiny plaques with an unusual morphology compared to plaques formed by wild-type MHV. Despite its low growth rate and low infectious titer, the ΔE mutant is genetically stable, showing no detectable phenotypic changes after several passages. The properties of this mutant provide further support for the importance of E protein in MHV replication, but surprisingly, they also show that E protein is not essential.