Membrane proteins are key components of living cells. They constitute one-third of the ORFs of virtually all genomes, perform crucial roles, and are targeted by a huge number of drugs.
When present in their native environment, transmembrane proteins are inserted via hydrophobic segments in a lipidic bilayer. Their in vitro characterization requires the extraction of proteins from the membrane and the maintenance of them in a soluble and native state. This is generally achieved using amphiphilic compounds, termed detergents, yielding so-called protein-detergent complexes (pdc). Several membrane proteins are naturally found as oligomers, and maintaining this precise assembly is a crucial issue for further studies.
In this article, we describe a method to determine the quaternary structure of membrane proteins and to follow its retention during protein handling. It is well known that classical size-exclusion chromatography (SEC) using column calibration does not apply in the case of pdc, whose volume and shape also depend on the detergent fraction. Light scattering coupled to chromatography is a promising technique, it does not suffer from the usual bias affecting batch measurements, and it allows the evaluation of sample homogeneity, oligomerization state, protein-protein interaction, and complex formation with other chemical species.
An illustration of this method is provided by the quaternary structure study of the Methanosarcina mazei CorA transporter in two detergents. Crystallographic studies of a homologous CorA revealed that the protein works as a pentamer to conduct ions across the membrane and constitutes a suitable system to characterize the effect of detergents on its oligomeric state.