Thermostability Assays: a Generic and Versatile Tool for Studying the Functional and Structural Properties of Membrane Proteins in Detergents.

There are very few generic methods to assess the stability and functional properties of membrane proteins solubilized in detergent. For this purpose, a thiol-reactive fluorochrome N-[4-(7-diethylamino-4-methyl-3-coumarinyl)phenyl]maleimide (CPM) can be used. An unfolding profile is obtained when the fluorochrome becomes fluorescent on reaction with cysteine residues that have been exposed during thermal denaturation of the protein population. The method was initially developed to optimize the stability of membrane proteins for crystallization studies, but in the course of our work we found many other applications. First, the assay can be used to study the binding of inhibitors, substrates, lipids, and other effectors to membrane proteins. Second, the assay can be used to understand the dynamics of proteins, allowing states to be defined by changes in accessibility of cysteine residues or by changes in specific amino acid interactions. Finally, the assay can be used to study state-dependent domain interactions, for example, as part of regulatory mechanisms. The CPM thermostability assay represents a broadly applicable and versatile tool for a wide range of applications in the functional and structural analysis of membrane proteins.