DNA-binding proteins analysed by SAXS

Abstract

Small Angle X-ray Scattering (SAXS) of macromolecules in solution is a technique that allows lowresolution structural analysis of proteins and their complexes, including protein-DNA complexes. The number of SAXS studies on macromolecular particles has increased significantly in the last years, due to improvement of algorithms and beamtime availability in synchrotron facilities. One advantage of this technique is that is no limited by factors like particle size or flexibility; on the contrary, it is possible to assess these or other features, like the coexistence of particles of different sizes in the same preparation. The experimental SAXS curve allows to fit against it a theoretical curve calculated using a crystallographic atomic model or an electron microscopy shape, which might be modified to optimise the curve fitting. Fitting optimisation can be attempted by exploring the conformational space of the particle on the basis of the available model, using techniques like simple manual displacement of domains or more sophisticated ones like normal mode analysis, and select those models that altogether yield a curve that fits best; such a methodology can be useful in determining the degree of flexibility of domains or interdomain segments. Macromolecular complexes are reconstructed using static threedimensional models of the complexes or by docking the individual components; by playing with the proportion of non-interacting vs interacting partners, is possible to establish the dynamics of the complex. Furthermore, by introducing the conformational-space analysis algorithms within macromolecular complexes is possible to explore the conformational changes induced by ligands, substrates or interacting partners. We are going to present a SAXS analysis of three phylogenetically-related proteins whose structure is based on homology models, and in one case involving a protein/DNA complex. The strategies applied will be analysed and the results discussedPeer Reviewe