Analytical Ultracentrifugation Studies of the Influenza M2 Homotetramerization Equilibrium in Detergent Solutions

Abstract

Though much progress has been made in the field of membrane protein folding, there is still much to learn about the association of transmembrane helices. Equilibrium analytical ultracentrifugation (EAUC) has been an important method of determining free energies of association for these types of systems. The M2 protein from the Influenza A virus, its transmembrane region and variants of that region represent over half of the thermodynamic data currently available for membrane proteins. Here, we consider the technical details of the EAUC methods used to study the stability of M2 in detergent solutions. Density-matching of detergent-buffer solutions yields precise values of the monomer-tetramer dissociation constant, and expressing these values in terms of peptide/detergent mole fraction units gives constant values over a modest range of experimentally relevant detergent concentrations. Furthermore, we have extended the EAUC method to calculate the number of detergent molecules bound to both monomer and tetramer species by employing a range of ²H₂O buffer compositions. Determination of peptide-bound detergent not only reinforces the importance of density matching, but it opens the door to future analytical ultracentrifugation experiments involving membrane proteins in alternative detergents and lipid bicelles.