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Journal of Virology, June 2000, p. 4967-4978, Vol. 74, No. 11
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Assembly of the Coronavirus Envelope: Homotypic Interactions between the M Proteins

Cornelis A. M. de Haan, Harry Vennema, and Peter J. M. Rottier*

Institute of Virology, Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, and Institute of Biomembranes, Utrecht University, 3584 CL Utrecht, The Netherlands

Received 22 December 1999/Accepted 1 March 2000

The viral membrane proteins M and E are the minimal requirements for the budding of coronavirus particles. Since the E protein occurs in particles only in trace amounts, the lateral interactions between the M proteins apparently generate the major driving force for envelope formation. By using coimmunoprecipitation and envelope incorporation assays, we provide extensive evidence for the existence of such M-M interactions. In addition, we determined which domains of the M protein are involved in this homotypic association, using a mutagenetic approach. Mutant M proteins which were not able to assemble into viruslike particles (VLPs) by themselves (C. A. M. de Haan, L. Kuo, P. S. Masters, H. Vennema, and P. J. M. Rottier, J. Virol. 72:6838-6850, 1998) were tested for the ability to associate with other M proteins and to be rescued into VLPs formed by assembly-competent M proteins. We found that M proteins lacking parts of the transmembrane cluster, of the amphipathic domain, or of the hydrophilic carboxy-terminal tail, or M proteins that had their luminal domain replaced by heterologous ectodomains, were still able to associate with assembly-competent M proteins, resulting in their coincorporation into VLPs. Only a mutant M protein in which all three transmembrane domains had been replaced lost this ability. The results indicate that M protein molecules interact with each other through multiple contact sites, particularly at the transmembrane level. Finally, we tested the stringency with which membrane proteins are selected for incorporation into the coronavirus envelope by probing the coassembly of some foreign proteins. The observed efficient exclusion from budding of the vesicular stomatitis virus G protein and the equine arteritis virus M protein indicates that envelope assembly is indeed a highly selective sorting process. The low but detectable incorporation of CD8 molecules, however, demonstrated that this process is not perfect.

* Corresponding author. Mailing address: Institute of Virology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, P.O. Box 80.165, 3508 TD Utrecht, The Netherlands. Phone: 31-30-2532462. Fax: 31-30-2536723. E-mail: P.Rottier{at}vet.uu.nl.

Journal of Virology, June 2000, p. 4967-4978, Vol. 74, No. 11
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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