Crystal structure of truncated human apolipoprotein A-I suggests a lipid-bound conformation

Abstract

The structure of truncated human apolipoprotein A-I (apo A-I), the major protein component of high density lipoprotein, has been determined at 4-A resolution. The crystals comprise residues 44-243 (exon 4) of apo A-I, a fragment that binds to lipid similarly to intact apo A-I and that retains the lipid-bound conformation even in the absence of lipid. The molecule consists almost entirely of a pseudo-continuous, amphipathic alpha-helix that is punctuated by kinks at regularly spaced proline residues; it adopts a shape similar to a horseshoe of dimensions 125 x 80 x 40 A. Four molecules in the asymmetric unit associate via their hydrophobic faces to form an antiparallel four-helix bundle with an elliptical ring shape. Based on this structure, we propose a model for the structure of apo A-I bound to high density lipoprotein.

Figure 1

Stereoview of the solvent-flattened, noncrystallographic symmetry-averaged experimental electron density map, contoured at 1.25σ at 4-Å resolution, with the current model depicted. Helices A4 (right) and B6 (left) of apo Δ(1–43)A-I are shown, as are two Pt sites (magenta octahedra). The Trp, Tyr, Phe, Met, and His residues of these helices are labeled.

Figure 2

Stereoviews of the apo Δ(1–43)A-I C^α trace. (a) The apo Δ(1–43)A-I monomer, molecule A. The N- and C-termini are labeled, as are each of the secondary structural elements described in the text. These elements are also colored differently. (b and c) The apo Δ(1–43)A-I A/B dimer, and the tetramer. The molecules are colored as in a to facilitate identification of the alignment of the extensive intermolecular interactions.

Figure 4

ribbons (34) representation illustrating the elliptical and curved shape of the apo Δ(1–43)A-I tetramer. (a) The tetramer is shown with the three noncrystallographic (pseudo)dyads: molecule A is gold, molecule B is purple, molecule C is pink, and molecule D is green. (b) View down the A/C dyad. (c) As in b, rotated 90° around the horizontal axis—i.e., viewed down the A/D pseudo-dyad. (d) As in c, rotated ≈70° around the vertical axis.

Figure 3

Amino acid sequence of apo A-I. The secondary structural elements are indicated below the sequence, as is the N-terminal region that is not present in apo Δ(1–43)A-I. Proline residues are shown in italics. Residues in helices 2–7 located at positions a and d of the modified heptad repeat are shown in bold.

Figure 5

Hypothetical model of apo A-I bound to spherical HDL. Two dimers of apo Δ(1–43)A-I are shown as CPK models, colored as in Fig. 4. The lipid head groups are represented by blue balls. At the top of the model are helices A10, D5, C5, and B10 (left to right). Separation of helices A10 and B10 of the A/B dimer allows the C/D dimer to pass through the gap, over the top of the sphere.