Secondary structure of human apolipoprotein A-I(1–186) in lipid-mimetic solution

The solution structure of an apoA-I deletion mutant, apoA-I(1–186) was determined by the chemical shift index (CSI) method and the torsion angle likelihood obtained from shift and sequence similarity (TALOS) method, using heteronuclear multidimensional NMR spectra of [u- 13C, u- 15N, u-50% 2H]apoA-I(1–186) in the presence of sodium dodecyl sulfate (SDS). The backbone resonances were assigned from a combination of triple-resonance data (HNCO, HNCA, HN(CO)CA, HN(CA)CO and HN(COCA)HA), and intraresidue and sequential NOEs (three-dimensional (3D) and four-dimensional (4D) 13C- and 15N-edited NOESY). Analysis of the NOEs, H α, C α and C′ chemical shifts shows that apoA-I(1–186) in lipid-mimetic solution is composed of α-helices (which include the residues 8–32, 45–64, 67–77, 83–87, 90–97, 100–140, 146–162, and 166–181), interrupted by short irregular segments. There is one relatively long, irregular and mostly flexible region (residues 33–44), that separates the N-terminal domain (residues 1–32) from the main body of protein. In addition, we report, for the first time, the structure of the N-terminal domain of apoA-I in a lipid-mimetic environment. Its structure (α-helix 8–32 and flexible linker 33–44) would suggest that this domain is structurally, and possibly functionally, separated from the other part of the molecule.