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Journal of Virology, October 1999, p. 8152-8159, Vol. 73, No. 10
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Amino Acid Substitutions within the Leucine Zipper Domain of the Murine Coronavirus Spike Protein Cause Defects in Oligomerization and the Ability To Induce Cell-to-Cell Fusion

Zongli Luo, Avery M. Matthews, and Susan R. Weiss*

Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104-6076

Received 1 March 1999/Accepted 6 July 1999

The murine coronavirus spike (S) protein contains a leucine zipper domain which is highly conserved among coronaviruses. To assess the role of this leucine zipper domain in S-induced cell-to-cell fusion, the six heptadic leucine and isoleucine residues were replaced with alanine by site-directed mutagenesis. The mutant S proteins were analyzed for cell-to-cell membrane fusion activity as well as for progress through the glycoprotein maturation process, including intracellular glycosylation, oligomerization, and cell surface expression. Single-alanine-substitution mutations had minimal, if any, effects on S-induced cell-to-cell fusion. Significant reduction in fusion activity was observed, however, when two of the four middle heptadic leucine or isoleucine residues were replaced with alanine. Double alanine substitutions that involved either of the two end heptadic leucine residues did not significantly affect fusion. All double-substitution mutant S proteins displayed levels of endoglycosidase H resistance and cell surface expression similar to those of the wild-type S. However, fusion-defective double-alanine-substitution mutants exhibited defects in S oligomerization. These results indicate that the leucine zipper domain plays a role in S-induced cell-to-cell fusion and that the ability of S to induce fusion may be dependent on the oligomeric structure of S.

* Corresponding author. Mailing address: Department of Microbiology, University of Pennsylvania School of Medicine, Philadelphia, PA 19104-6076. Phone: (215) 898-8013. Fax: (215) 573-4858. E-mail: weisssr{at}mail.med.upenn.edu.

Journal of Virology, October 1999, p. 8152-8159, Vol. 73, No. 10
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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Copyright © 1999 by the American Society for Microbiology. All rights reserved.