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J. Virol. doi:10.1128/JVI.01577-06
Copyright (c) 2006, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Exceptional Flexibility in the Sequence Requirements for Coronavirus Small Envelope Protein (E) Function

Wadsworth Center, New York State Department of Health, and Department of Biomedical Sciences, State University of New York, Albany, New York 12201

^* To whom correspondence should be addressed. Email: masters{at}wadsworth.org.

	Abstract

The small envelope protein (E) plays a role of central importance in the assembly of coronaviruses. This was initially established by studies demonstrating that cellular expression of only E protein and the membrane protein (M) was necessary and sufficient for the generation and release of virus-like particles. To investigate the role of E protein in the whole virus, we previously generated E gene mutants of mouse hepatitis virus (MHV) that were defective in viral growth and produced aberrantly assembled virions. Surprisingly, however, we were also able to isolate a viable MHV mutant (E) in which the entire E gene, as well as the nonessential upstream genes 4 and 5a, were deleted. We have now constructed an E knockout mutant that confirms that the highly defective phenotype of the E mutant is due to loss of the E gene. Additionally, we have created substitution mutants in which the MHV E gene was replaced by heterologous E genes from viruses spanning all three groups of the coronavirus family. Group 2 and 3 E proteins were readily exchangeable for that of MHV. However, the E protein of a group 1 coronavirus, transmissible gastroenteritis virus, became functional in MHV only after acquisition of particular mutations. Our results show that proteins encompassing a remarkably diverse range of primary amino-acid sequences can provide E protein function in MHV. These findings suggest that E protein facilitates viral assembly in a manner that does not require E protein to make sequence-specific contacts with M protein.

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