Indroduction to Small Angle Scattering (SAS) – P. Timmins

Small Angle Scattering for Biological Macromolecular Complexes

In the last decade, the number of high resolution protein structures has increased drastically as seen from the Protein Data Bank which contains now more 30 000 structures. Tremendous progress in the automation of protein overproduction, crystallization, data collection and data treatment has facilitated the crystallographic studies of macromolecules. The challenge for the next decade is to integrate the molecular descriptions with their cellular environment. For this, structures of larger macromolecular complexes have to be deciphered. These multi-component complexes often participate in signal transduction pathways and have therefore dynamic structures that have to be unravelled by multiple approaches combining various resolutions and spatial scales. Crystallographic approaches are very powerful in solving structures of proteins or macromolecular complexes at atomic resolution when they adopt well-defined conformations and can therefore be crystallized. More labile conformations, characteristic of modular proteins or transient complexes have to be characterized by other approaches. The combination of high-resolution techniques with low or medium resolution approaches leads to quasi-atomic models. This is well illustrated by several examples of virus structures where electron microscopy on single viruses provides for the overall viral envelope and X-ray crystallography of individual proteins leads to atomic structures that can be fitted into these envelopes. Small angle scattering has been used since several decades, although its use was very often limited to very low resolution, the so-called Guinier region, analysis of which allows the determination of the radius of gyration and the molecular mass. In the last few years, experimental X-ray set-ups have improved and higher resolution scattering data are now exploitable. Further mathematical developments made it possible to fit a molecular envelope to the interatomic distance distribution, the P(r) function, deduced from the small angle scattering data 1. Small angle X-ray scattering is now a powerful tool for determining molecular envelopes that can be combined with high-resolution structures. In contrast to electron microscopy, the method can be used for smaller macromolecules and is therefore complementary. Neutron small angle scattering provides additional information for macromolecular complexes that are made of several types of molecules such as proteins, nucleic acids or lipids. Contrast variation experiments obtained by exchanging the solvent for deuterated or partially deuterated solvent enhances the signal from one component. SAXS and SANS are therefore tools of major importance in tackling systems biology.

Neutron small angle scattering instruments have been developed at ILL since several years and are available to the community. An X-ray small angle instrument is partially available for biological studies at ESRF, but does not respond to the increasing demand of the scientific community. This project aims to reinforce the development of small angle scattering for biological studies in general in Grenoble, by improving the instrumental facilities and helping in good quality data collection. It should also help in bridging the gap between the experimental data and the final molecular modeling in combination with X-ray or NMR partial structures.