THE NMR STRUCTURE OF THE PULMONARY SURFACTANT-ASSOCIATED POLYPEPTIDE SP-C IN AN APOLAR SOLVENT CONTAINS A VALYL-RICH ALPHA-HELIX

The nuclear magnetic resonance (NMR) structure of the pulmonary surfactant-associated lipopolypeptide C (SP-C) was determined in a mixed solvent of (CH3Cl)-H-2/(CH3OH)-H-2/0.1 M HCl 32:64:5 (v/v). Sequence-specific H-1 NMR assignments and the collection of conformational constraints were achieved with two-dimensional H-1 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 Angstrom for the polypeptide backbone atoms N, C-alpha, and C', and for all heavy atoms, respectively. The 35-residue polypeptide chain of SP-C forms an Lu-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 alpha-helix is about 37 Angstrom, and the helical segment of residues 13-28, which contains exclusively aliphatic residues with branched side chains, is 23-Angstrom long and about 10 Angstrom in diameter. The alpha-helix is outstandingly regular, with virtually identical chi(1) angles for all valyl residues. The observation of a helical structure of SP-C was rather unexpected, considering that Val is generally underrepresented in alpha-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.