Probing Proton Translocation in Influenza A/M2 Proteoliposomes - A systematic Approach to Membrane-Protein Reconstitutions

Abstract: An improved method for reconstituting membrane proteins into artificial liposomes for quantitative functional analysis is presented. A number of key parameters for reconstitution by detergent removal are assessed in this thesis: The lipid-to-protein ratio, the detergent-to-lipid ratio and the lipid and cholesterol composition. New porphyrin-based pH-probes are evaluated. Based on this systematic, comprehensive approach to protein reconstitution, we present a robust system for quantitative proton-flux analysis, as demonstrated by influenza virus A M2 reconstitution into large unilamellar vesicles. The M2 protein is a small, single-span transmembrane protein, which plays an important role in the life cycle of influenza A virus and is the target of the adamantane series of anti-influenza drugs. This virus enters cells via the endosomes; as the endosomes acidify M2 facilitates proton transport into the viral interior, thereby disrupting matrix protein/RNA interactions required for infectivity. A mystery has been how protons can accumulate in the viral interior without developing a large electrical potential that impedes further inward proton translocation, which is required to effect a significant change in the internal effective pH. Here, we show that M2 has essential antiporter-like activity. This should lead to future investigations of the biophysical mechanism of transport, which will have implications for the design of new generations of M2-targeting drugs as well as furthering our understanding of cotransport.