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Journal of Virology, October 2002, p. 9972-9980, Vol. 76, No. 19
0022-538X/02/$04.00+0     DOI: 10.1128/JVI.76.19.9972-9980.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Large-Scale, pH-Dependent, Quaternary Structure Changes in an RNA Virus Capsid Are Reversible in the Absence of Subunit Autoproteolysis

Derek J. Taylor,^1,2 Neel K. Krishna,¹ Mary A. Canady,^1, Anette Schneemann,¹ and John E. Johnson^1,2*

Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037,¹ Department of Chemistry and Biochemistry, University of California---San Diego, La Jolla, California 92093²

Received 4 March 2002/ Accepted 24 June 2002

The assembly and maturation of the coat protein of a T=4, nonenveloped, single-stranded RNA virus, Nudaurelia capensis virus (NV), was examined by using a recombinant baculovirus expression system. At pH 7.6, the coat protein assembles into a stable particle called the procapsid, which is 450 Å in diameter and porous. Lowering the pH to 5.0 leads to a concerted reorganization of the subunits into a 410-Å-diameter particle called the capsid, which has no obvious pores. This conformational change is rapid but reversible until slow, autoproteolytic cleavage occurs in at least 15% of the subunits at the lower pH. In this report, we show that expression of subunits with replacement of Asn-570, which is at the cleavage site, with Thr results in assembly of particles with expected morphology but that are cleavage defective. The conformational change from procapsid to capsid is reversible in N570T mutant virus-like particles, in contrast to wild-type particles, which are locked into the capsid conformation after cleavage of the coat protein. The reexpanded procapsids display slightly different properties than the original procapsid, suggesting hysteretic effects. Because of the stability of the procapsid under near-neutral conditions and the reversible properties of the cleavage-defective mutant, NV provides an excellent model for the study of pH-induced conformational changes in macromolecular assemblies. Here, we identify the relationship between cleavage and the conformational change and propose a pH-dependent helix-coil transition that may be responsible for the structural rearrangement in NV.

Present address: GeneFormatics Incorporated, San Diego, CA 92121-3754.

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