The structures proteins adopt in solution are being unravelled by researchers in Italy.

Claudia Leggio and co-workers at Sapienza University of Rome have developed a method for determining the folded structures of proteins in solution, information they say is crucial for understanding protein function. They derive 3D models from small-angle x-ray scattering (SAXS) data and validate them with light scattering measurements. 

The researchers' first goal was to reconstruct the solution form of human serum albumin (HSA). 'HSA's shape and dimensions at neutral pH has been a matter of controversy,' says Leggio, explaining the team's motivation. This is despite its being the most abundant protein in human plasma and involved in many physiological functions, including blood pH and pressure maintenance, she adds. 

Leggio's results imply that HSA adopts a heart-shaped structure (left) in solution rather than the cigar shaped ellipsoid (right) implied in earlier work

Early work indicated that HSA resembles a cigar-shaped ellipsoid, but recent studies have pointed to a heart-shaped structure in solution, similar to that obtained from x-ray crystallography. 'Our results show that in solution the defatted protein adopts a slightly more open heart-shaped structure than in the crystal,' says Leggio.

William Heller, an expert in structural molecular biology at Oak Ridge National Laboratory in Tennessee, says that sophisticated methods such as Leggio's are a prerequisite for studying proteins in solution. 'Such systems are challenging to study structurally since high-resolution crystallography is often not possible,' he explains, 'because the systems do not adopt a single conformation that will produce high quality crystals.' SAXS, on the other hand, can be used for systems that adopt several conformations, making it suitable for proteins in solution. But, Leggio points out, what SAXS measures is an average over an ensemble of conformations. 'The challenge is to combine the technique with others to take the averaging into account,' she adds.

Leggio says that her methods could also be extended to study protein folding processes and mechanisms. Heller agrees, describing the work as 'pointing a way forward for other researchers in the field toward understanding the structures of proteins and the conformational changes that take place during the performance of their biological function.'

Colin Batchelor