Enhanced local symmetry interactions globally stabilize a mutant virus capsid that maintains infectivity and capsid dynamics.
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-A 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.
Speir
Jeffrey A
JA
Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Rd., La Jolla, CA 92037, USA.
Bothner
Brian
B
Qu
Chunxu
C
Willits
Deborah A
DA
Young
Mark J
MJ
Johnson
John E
JE
D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D017795:Bromovirus / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism / Q000648:ultrastructure; D002213:Capsid / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004591:Electrophoresis, Polyacrylamide Gel; D016679:Genome, Viral; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D008970:Molecular Weight; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D012367:RNA, Viral / Q000032:analysis; D012697:Serine Endopeptidases / Q000494:pharmacology; D012965:Sodium Chloride / Q000494:pharmacology; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014357:Trypsin / Q000494:pharmacology; D014867:Water / Q000737:chemistry
D012367:RNA, Viral; D014867:Water; D012965:Sodium Chloride; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase; D014357:Trypsin