The NMR Structure of the Pulmonary Surf actant-Associated Polypeptide SP-C in an Apolar Solvent Contains a Valyl-Rich α-Helix

The nuclear magnetic resonance (NMR) structure of the pulmonary surfactant-associated lipopolypeptide C (SP-C) was determined in a mixed solvent of C²H₃Cl/C²H₃OH/O.1 M HCl 32:64:5 (v/v). Sequence-specific ¹H NMR assignments and the collection of conformational constraints were achieved with two-dimensional ¹H NMR, and the structure was calculated with the distance geometry program DIANA. The root mean square deviations for the well-defined polypeptide segment of residues 9–34 calculated for the 20 best energy-minimized DIANA conformers relative to their mean are 0.5 and 1.3 Å for the polypeptide backbone atoms N, C^α, and C^′, and for all heavy atoms, respectively. The 35-residue polypeptide chain of SP-C forms an α-helix between positions 9 and 34, which includes two segments of seven and four consecutive valyls that are separated by a single leucyl residue. The N-terminal hexapeptide segment, which includes two palmitoylcysteinyls, is flexibly disordered. The length of the α-helix is about 37 Å, and the helical segment of residues 13–28, which contains exclusively aliphatic residues with branched side chains, is 23-Å long and about 10 Å in diameter. The α-helix is outstandingly regular, with virtually identical χ¹ angles for all valyl residues. The observation of a helical structure of SP-C was rather unexpected, considering that Val is generally underrepresented in α-helices, and it provides intriguing novel insights into the structural basis of SP-C functions as well as into general structural aspects of protein-lipid interactions in biological membranes.