Freezing immunoglobulins to see them move

doi:10.1073/pnas.0400119101

Freezing immunoglobulins to see them move

^*Centro Interdipartimentale per lo Studio delle Dinamiche Complesse, Università di Firenze, Via G. Sansone 1, 50019 Florence, Italy; ^‡Cell and Molecular Biology Department, Karolinska Institutet, 171 77 Stockholm, Sweden; and ^§Institute de Physique Théorique, École Polytechnique Fédérale de Lausanne, BP CH-1015, Lausanne, Switzerland

Edited by Roger D. Kornberg, Stanford University School of Medicine, Stanford, CA (received for review January 7, 2004)

Abstract

The issue of protein dynamics and its implications in the biological function of proteins are arousing greater and greater interest in different fields of molecular biology. In cryo-electron tomography experiments one may take several snapshots of a given biological macromolecule. In principle, a large enough collection of snapshots of the molecule may then be used to calculate its equilibrium configuration in terms of the experimentally accessible degrees of freedom and, hence, to estimate its potential energy. This information would be crucial in order to analyze the biological functions of biomolecules by directly accessing the relevant dynamical indicators. In this article, we analyze the results of cryo-electron tomography experiments performed on monoclonal murine IgG2a antibodies. We measure the equilibrium distribution of the molecule in terms of the relevant angular coordinates and build a mechanical model of the antibody dynamics. This approach enables us to derive an explicit expression of the IgG potential energy. Furthermore, we discuss the configuration space at equilibrium in relation to results from other techniques, and we set our discussion in the context of the current debate regarding conformation and flexibility of antibodies.

Footnotes

↵ ¶ To whom correspondence should be addressed. E-mail: ulf.skoglund{at}cmb.ki.se.
↵ † L.B., D.F., and F.P. contributed equally to this work.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: cryo-ET, cryo-electron tomography; Fab arms, fragment antigen-binding arms.
↵ ∥ An argument to test a posteriori the validity of our theoretical framework (including the hypothesis of factorization) is provided in Appendix 2, which is published as supporting information on the PNAS web site.