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.
Zhu
Hongli L
HL
Department of Physiology and Biophysics, Boston University School of Medicine, 715 Albany Street, Boston, Massachusetts 02118, USA.
Atkinson
David
D
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