Conformation and lipid binding of the N-terminal (1-44) domain of human apolipoprotein A-I.
Because of its role in reverse cholesterol transport, human apolipoprotein A-I is the most widely studied exchangeable apolipoprotein. Residues 1-43 of human apoA-I, encoded by exon 3 of the gene, are highly conserved and less well understood than residues 44-243, encoded by exon 4. In contrast to residues 44-243, residues 1-43 do not contain the 22 amino acid tandem repeats thought to form lipid binding amphipathic helices. To understand the structural and functional roles of the N-terminal region, we studied a synthetic peptide representing the first 44 residues of human apoA-I ([1-44]apoA-I). Far-ultraviolet circular dichroism spectra showed that [1-44]apoA-I is unfolded in aqueous solution. However, in the presence of n-octyl beta-d-glucopyranoside, a nonionic lipid mimicking detergent, above its critical micelle concentration ( approximately 0.7% at 25 degrees C), sodium dodecyl sulfate, an ionic detergent, above its CMC ( approximately 0.2%), trimethylamine N-oxide, a folding inducing organic osmolyte, or trifluoroethanol, an alpha-helix inducer, alpha-helical structure was formed in [1-44]apoA-I up to approximately 45%. Characterization by density gradient ultracentrifugation and visualization by negative staining electron microscopy demonstrated that [1-44]apoA-I interacts with dimyristoylphosphatidylcholine (DMPC) over a wide range of lipid:peptide ratios from 1:1 to 12:1 (w/w). At 1:1 DMPC:[1-44]apoA-I (w/w) ratio, discoidal complexes with composition approximately 4:1 (w/w) and approximately 100 A diameter were formed in equilibrium with free peptide. At higher ratios, discoidal complexes were shown to exist together with a heterogeneous population of lipid vesicles with peptide bound also in equilibrium with free peptide. When bound to DMPC, [1-44]apoA-I has approximately 60% helical structure, independent of whether it forms discoidal or vesicular complexes. This helical content is consistent with that of the predicted G helix (residues 8-33). Our data provide the first strong and direct evidence that the N-terminal region of apoA-I binds lipid and can form discoidal structures and a heterogeneous population of vesicles. In doing so, approximately 60% of this region folds into alpha-helix from random coil. The composition of the 100 A discoidal complex is approximately 5 [1-44]apoA-I and approximately 150 DMPC molecules per disk. The helix length of 5 [1-44]apoA-I molecules in lipid-bound form is just long enough to wrap around the DMPC bilayer disk once.
Department of Physiology and Biophysics, Boston University School of Medicine, 715 Albany Street, Boston, Massachusetts 02118, USA.
Hongli L
HL
Zhu
David
D
Atkinson
D016632:Apolipoprotein A-I / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D002499:Centrifugation, Density Gradient; D002942:Circular Dichroism; D003902:Detergents; D005960:Glucosides / Q000737:chemistry; D006801:Humans; D008055:Lipids / Q000737:chemistry*; D008744:Methylamines / Q000737:chemistry; D010446:Peptide Fragments / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D012967:Sodium Dodecyl Sulfate / Q000737:chemistry; D012996:Solutions; D013816:Thermodynamics; D014270:Trifluoroethanol / Q000737:chemistry; D014867:Water / Q000737:chemistry
D016632:Apolipoprotein A-I; D003902:Detergents; D005960:Glucosides; D008055:Lipids; D008744:Methylamines; D010446:Peptide Fragments; D012996:Solutions; D014867:Water; C018619:octyl-beta-D-glucoside; D012967:Sodium Dodecyl Sulfate; D014270:Trifluoroethanol; C005855:trimethyloxamine