Structural data are critical for an understanding of how membrane transport systems function; unfortunately, high-resolution crystal structures for these systems are rare, primarily because of difficulties in crystallization. One alternative approach to the structure problem is to dissect the system into individual components and domains; that is, the integral membrane components of transporters are often multisubunit complexes that interact with soluble, peripheral subunits, or they may contain domains that can be expressed as soluble proteins. Structural information for individual domains and soluble components of the system will help to illuminate the mechanism, and this information can be integrated into models for the structure of the complete system. Small-angle X-ray scattering (SAXS) is a method that can be used to investigate the low-resolution structure and conformational changes of soluble proteins, without the requirement for crystals (for examples,seerefs.1 –9 ).
