Interaction of human apolipoprotein A-I with dipalmitoylphosphatidylcholine in vesicular and micellar complexes.
The interactions of human apolipoprotein A-I (apo A-I) with dipalmitoylphosphatidylcholine (DPPC) in vesicular complexes at low protein concentrations and in micellar complexes at high protein concentrations are compared. The C-terminal segment of this protein, with a relative molecular weight (Mr) of about 11,000, is protected on trypsin treatment of apo A-I-vesicle complexes. A segment within the sequence from Leu-189 to Arg-215 of apo A-I penetrates the hydrophobic interior of the membrane, as found in a hydrophobic labeling experiment involving 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)-diazirine ([125I]TID). No appreciable stretch of apo A-I in micellar complexes was found to be protected from the tryptic digestion. This indicates that the interactions of apo A-I with lipids in the vesicular and micellar complexes are different. The binding equilibrium of apo A-I as to DPPC vesicles at low protein concentrations, as studied by hydrophobic labeling of the bilayer-penetrating segment, is reached within about 1 h, while the formation of micellar complexes at high protein concentrations takes about 24 h at 42 degrees C. Time-dependent labeling studies involving photoreactive phosphatidylcholine (PC) with high apo A-I concentrations suggested an initial interaction with the head group region of the bilayer followed by interaction with the tail ends of the acyl chains of the lipid. A possible mechanism for the micellization process is discussed.
Bae
Y S
YS
Department of Biological Science and Engineering, Korea Advanced Institute of Science, Seoul.
Kim
H
H
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000378:metabolism*; D000595:Amino Acid Sequence; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000378:metabolism*; D001389:Azirines; D006801:Humans; D007457:Iodine Radioisotopes; D008051:Lipid Bilayers / Q000378:metabolism*; D008566:Membranes; D008823:Micelles; D008969:Molecular Sequence Data; D010446:Peptide Fragments; D013997:Time Factors; D014357:Trypsin
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D001389:Azirines; D007457:Iodine Radioisotopes; D008051:Lipid Bilayers; D008823:Micelles; D010446:Peptide Fragments; D015060:1,2-Dipalmitoylphosphatidylcholine; C033174:3-(trifluoromethyl)-3-(3-iodophenyl)diazirine; D014357:Trypsin