Solid‐state NMR studies of the membrane‐bound closed state of the colicin E1 channel domain in lipid bilayers

The colicin El channel polypeptide was shown to be organized anisotropically in membranes by solid‐state NMR analysis of samples of uniformly 15N‐labeled protein in oriented planar phospholipid bilayers. The 190 residue C‐terminal colicin E1 channel domain is the largest polypeptide to have been characterized by 15N solid‐state NMR spectroscopy in oriented membrane bilayers. The 15N‐NMR spectra of the colicin E1 show that: (1) the structure and dynamics are independent of anionic lipid content in both oriented and unoriented samples; (2) assuming the secondary structure of the polypeptide is helical, there are both trans‐membrane and in‐plane helical segments; (3) trans‐membrane helices account for approximately 20‐25% of the channel polypeptide, which is equivalent to 38‐48 residues of the 190‐residue polypeptide. The results of the two‐dimensional PISEMA spectrum are interpreted in terms of a single trans‐membrane helical hairpin inserted into the bilayer from each channel molecule. These data are also consistent with this helical hairpin being derived from the 38‐residue hydrophobic segment near the C‐terminus of the colicin E1 channel polypeptide.