Crystallization of truncated human apolipoprotein A-I in a novel conformation

• Published in: Acta crystallographica. Section D, Biological crystallography. Biological crystallography., 1999-09, Vol.55 (9), p.1578-1583
• Main Authors: Borhani, David W., Engler, Jeffrey A., Brouillette, Christie G.
• Format: Article
• Language: English
• Subjects: apolipoprotein A-I; Apolipoprotein A-I - chemistry; conformation; Crystallization; Crystallography, X-Ray; Detergents - chemistry; Humans; Osmolar Concentration; Peptide Fragments - chemistry; Protein Conformation; Protein Structure, Secondary; Recombinant Proteins - chemistry
• ISSN: 1399-0047; 0907-4449; 1399-0047
• DOI: 10.1107/S0907444999008914

The crystallization of recombinant human apolipoprotein A‐I (apo A‐I), the major protein component of high‐density lipoprotein, in a new crystal form is described. The fragment crystallized, residues 44–243 of native apo A‐I [apo Δ(1–­43)A‐I], is very similar to intact native apo A‐I in its ability to bind lipid, to be incorporated into high‐density lipoproteins and to activate lecithin–cholesterol acyl transferase. Apo Δ(1–43)A‐I crystallizes, in the presence of β‐d‐octylglucopyranoside, in space group I222 or I212121, with unit‐cell parameters a = 37.11, b = 123.62, c = 164.65 Å and a diffraction limit of 3.2 Å. These form II crystals grow under conditions of significantly lower ionic strength than the original form I crystals (space group P212121, a = 97.47, b = 113.87, c = 196.19 Å, diffraction limit 3.0 Å). Packing arguments show that the unusual open conformation of apo Δ(1–43)A‐I found in the form I crystals cannot be packed into the smaller oddly proportioned form II unit cell. Monomeric apo Δ(1–43)A‐I, as either a four‐helix bundle (∼75 × 30 × 30 Å) or an extended helical rod (∼150 × 20 × 20 Å), can be packed into the form II unit cell. It is concluded, therefore, that apo Δ(1–43)A‐I may have crystallized in one of these distinct conformations in the form II crystals.