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Journal of Virology, April 2006, p. 3582-3591, Vol. 80, No. 7
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.7.3582-3591.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Enhanced Local Symmetry Interactions Globally Stabilize a Mutant Virus Capsid That Maintains Infectivity and Capsid Dynamics

Jeffrey A. Speir,1,{dagger} Brian Bothner,1,{dagger},{ddagger} Chunxu Qu,1,§ Deborah A. Willits,2 Mark J. Young,2 and John E. Johnson1*

Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037,1 Department of Plant Sciences and Plant Pathology, Montana State University, Bozeman, Montana 597172

Received 6 July 2005/ Accepted 12 January 2006

Structural transitions in viral capsids play a critical role in the virus life cycle, including assembly, disassembly, and release of the packaged nucleic acid. Cowpea chlorotic mottle virus (CCMV) undergoes a well-studied reversible structural expansion in vitro in which the capsid expands by 10%. The swollen form of the particle can be completely disassembled by increasing the salt concentration to 1 M. Remarkably, a single-residue mutant of the CCMV N-terminal arm, K42R, is not susceptible to dissociation in high salt (salt-stable CCMV [SS-CCMV]) and retains 70% of wild-type infectivity. We present the combined structural and biophysical basis for the chemical stability and viability of the SS-CCMV particles. A 2.7-Å resolution crystal structure of the SS-CCMV capsid shows an addition of 660 new intersubunit interactions per particle at the center of the 20 hexameric capsomeres, which are a direct result of the K42R mutation. Protease-based mapping experiments of intact particles demonstrate that both the swollen and closed forms of the wild-type and SS-CCMV particles have highly dynamic N-terminal regions, yet the SS-CCMV particles are more resistant to degradation. Thus, the increase in SS-CCMV particle stability is a result of concentrated tethering of subunits at a local symmetry interface (i.e., quasi-sixfold axes) that does not interfere with the function of other key symmetry interfaces (i.e., fivefold, twofold, quasi-threefold axes). The result is a particle that is still dynamic but insensitive to high salt due to a new series of bonds that are resistant to high ionic strength and preserve the overall particle structure.

* Corresponding author. Mailing address: Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Rd., MB-31, La Jolla, CA 92037. Phone: (858) 784-2947. Fax: (858) 784-8660. E-mail: jackj{at}scripps.edu.

{dagger} J.A.S. and B.B. contributed equally to this work.

{ddagger} Present address: Department of Chemistry and Biochemistry, Montana State University, Bozeman, MT 59717.

§ Present address: Ludwig Institute for Cancer Research, University of California at San Diego, La Jolla, CA 92093.

Journal of Virology, April 2006, p. 3582-3591, Vol. 80, No. 7
0022-538X/06/$08.00+0     doi:10.1128/JVI.80.7.3582-3591.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.





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