Biological solution scattering: recent achievements and future challenges

In the post-genomic age it is apparent that as structures of larger macromolecules and their complexes are investigated, structure-function investigations are often confronted with the necessity to apply a portfolio of tools for biochemical and biophysical characterization. A survey of the published literature over the last decade reveals that publications in the area of structural biology employing neutron or X-ray scattering as one of their techniques tripled since 1995. Yet, taken as a whole, the contribution from small-angle scattering (SAS) to research papers dealing with structural analyses is still only of the order of 1% in 2005 (for comparison, less than 0.5% in 1995). Nevertheless, the last few years saw stimulating biological applications and analysis procedures which emphasize the growing potential of SAS applications for the structural studies of macromolecules in solution. The usage of SAS largely consists of low-resolution reconstructions of molecules with partial or without presumption of structural details, consistency analysis of high-resolution crystallographic structures and their corresponding low-resolution models determined in the solution state, and rigid-body refinement of multi-subunit assemblies including complexes and full-length multidomain proteins. Complementary structural information obtained from SAS in conjunction with data acquired by protein crystallography, NMR, molecular dynamics or computational docking provides a means to link low- and high-resolution models essential for the elucidation of biomolecular organization, interactions and function. The capabilities as well as limitations of determining low-resolution structures of multidomain proteins, macromolecular complexes and assemblies are highlighted in three examples. The first example is the characterization of the conformation of the PDZ region of SAP97, a multidomain protein involved in the regulation and localization of membrane receptor molecules. The second example is the characterization of the structural features of the TIM10 complex, an escort molecule for mitochondrial inner-membrane proteins, and the third example is the description of the shape and pH-induced conformational transition of a full-length bacterial potassium channel. The latter two in particular benefited from neutron scattering with contrast variation by using H-D labelling of the macromolecular complex or the solvent, respectively.