The structure of protein polymers, revealed by HPLC-SAX S experiments

Serpinopathies are genetic diseases related to the deficiency of a serpin (SERin Protease Inhibitor) and/or its accumulation as polymer chain. The formation of protein polymers can also be triggered in solutions of wild-type serpins by mild thermal stress. The first model for the serpin polymer formation, involving the insertion of the solvent exposed reactive loop of molecule A into the central beta-sheet of molecule B , has been recently challenged by two different crystallographic structures showing a varying degree of domain-swapping. In a series of experiments we measured the SAXS patterns of samples of alpha1-antitrypsin (AAT) purified from plasma, incubated for short times at 55°C. The X-rays scattering was measured just after a chromatographic column that perform a partial separation of the polymers of different lengths. Furthermore, data obtained from SAXS experiments are known to be difficult to interpret when the systems are not homogeneous. To extract valuable information on the structure of AAT polymers we devised a model based on a collection of low resolution rigid monomer particles. Polydispersity was easily taken into account, thus allowing the fitting of the entire set of partially separated polymer populations at once.