The lipid-free structure of apolipoprotein A-I: effects of amino-terminal deletions.

Deletion mutants of human apolipoprotein A-I (apo hA-I) have been produced from a bacterial expression system to explore the function of the specific domains comprising residues 1−43, 1−65, 88−98, and 187−243, respectively, in the lipid-free conformation and in the lipid-binding mechanism of apo hA-I. Initial studies on apo Δ(1−43)A-I and apo Δ(187−243)A-I have already been reported. To aid purification of these mutants, a histidine-containing N-terminal extension was incorporated (+his); in cases where comparison with the (−his) construct was possible, little effect on the physical properties due to the (+his) extension was found. All mutants have folded structures in their lipid-free state, however these structures differ widely in their relative thermodynamic stability and extent of secondary structure. The mutant with the fewest residues deleted, apo Δ(88−98)A-I(+his), has the least secondary structure (only 34% helix) and is also the least stable (ΔG = 2.9 kcal/mol). Determined from sedimentation vel...