Nanoindentation studies of full and empty viral capsids and the effects of capsid protein mutations on elasticity and strength

doi:10.1073/pnas.0601744103

Nanoindentation studies of full and empty viral capsids and the effects of capsid protein mutations on elasticity and strength

*Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095-1569;
^‡Faculty of Exact Sciences, Department of Physics and Astronomy, Vrije Universiteit, 1081 HV, Amsterdam, The Netherlands;
^§Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90095-1597; and
^‖III. Physikalisches Institut, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Communicated by Joan Selverstone Valentine, University of California, Los Angeles, CA, March 3, 2006
↵ ^†J.P.M. and I.L.I. contributed equally to this work. (received for review November 30, 2005)

Abstract

The elastic properties of capsids of the cowpea chlorotic mottle virus have been examined at pH 4.8 by nanoindentation measurements with an atomic force microscope. Studies have been carried out on WT capsids, both empty and containing the RNA genome, and on full capsids of a salt-stable mutant and empty capsids of the subE mutant. Full capsids resisted indentation more than empty capsids, but all of the capsids were highly elastic. There was an initial reversible linear regime that persisted up to indentations varying between 20% and 30% of the diameter and applied forces of 0.6–1.0 nN; it was followed by a steep drop in force that is associated with irreversible deformation. A single point mutation in the capsid protein increased the capsid stiffness. The experiments are compared with calculations by finite element analysis of the deformation of a homogeneous elastic thick shell. These calculations capture the features of the reversible indentation region and allow Young's moduli and relative strengths to be estimated for the empty capsids.

Footnotes

^¶To whom correspondence should be addressed. E-mail: knobler{at}chem.ucla.edu
Author contributions: W.S.K., C.M.K., G.J.L.W., and C.F.S. designed research; J.P.M., I.L.I., and M.M.G. performed research; J.P.M. and I.L.I. analyzed data; and J.P.M., C.M.K., G.J.L.W., I.L.I., and C.F.S. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations:

Abbreviations:

AFM,

atomic force microscopy;

CCMV,

cowpea chlorotic mottle virus;

ss,

salt stable;

FZ,

force–distance.