How Lipids Regulate Membrane Protein Function

Abstract

To what extent can our understanding of how water molecules interact with a water-soluble protein help us to understand how lipid molecules interact with a membrane protein? A first shell of water molecules is found covering the surface of a water-soluble protein, and water molecules are also found buried within the structure. Interactions of these water molecules with the protein help define its structure, and thus its function. Similarly, the surface of the transmembrane region of a membrane protein is covered with a first-shell of perturbed lipid molecules, referred to as the lipid annulus. Binding constants of lipids to these annular sites can be determined using a fluorescence quenching method, studying the quenching of the fluorescence of Trp residues in the protein by lipids with bromine-containing chains. Such studies show that the lipid annulus is heterogeneous - the mechanosensitive channel MscL, for example, contains a ‘hot-spot’ where anionic lipids bind with high affinity. Binding of anionic lipids to this hot-spot has a large effect on the flux through the MscL channel. Lipid molecules can also be found buried within the structure of a membrane protein, for example, at protein-protein interfaces in multimeric proteins. An example is provided by the homotetrameric potassium channel KcsA. The crystal structure of KcsA by MacKinnon shows an anionic lipid molecule bound at each monomer-monomer interface. Occupation of these sites by anionic lipid molecules is not required for tetramer formation, but is important for function. The open probability of the channel increases markedly with increasing anionic lipid content in the membrane, three of the four inter-subunit binding sites having to be occupied by anionic lipid for the channel to be open.