While the target of many antimicrobial peptides is the microbial lipid bilayer itself, membrane-binding peptide hormones and bacterial toxins likely act on proteins located in the membrane. Besides membrane-bound peptides, which can be structurally characterized easily by solution NMR spectroscopy, there are an enormous number of membrane-binding proteins.
Although the 3D structure determination of soluble proteins by X-ray crystallography and NMR spectroscopy is well-established, far fewer structures of membrane-bound proteins have been solved to date. Structural studies of membrane proteins are considerably complicated by their amphipathic or hydrophobic nature and resulting solubility problems in aqueous buffers. Less than 2% of all structures in the Protein Data Bank (PDB) are membrane-bound proteins although they account for about 30% of all expressed proteins.įrequently, their limited stability in the absence of lipids constitutes an additional problem. Noteworthy, about 45% of all pharmaceuticals target G-protein coupled receptors, which are also a group of membrane-bound proteins. The limited structural information available for these proteins stems either from difficulties in crystallization or their large size, resulting in broad signals and limited spectral dispersion in NMR spectra.
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