Interaction between human apolipoprotein A-Ⅰ and phosphatidylcholine vesicles

Abstract

The interactions of human apolipoprotein A-I (apo A-I0 with dipalmitoylphosphatidylcholine (DPPC) vesicles in the vesicular complex at low protein concentration and in the micellar complex at high protein concentration are compared. The C-terminal segment of this protein with relative molecular weight (Mr) of about 11,000 is protected from the trypsin treatment in the apo A-I-vesicle complexes. A segment within the sequence from Leu-189 to Arg-215 of apo A-I penetrates into the hydrophobic interior of the membrane according to the hydrophobic labeling experiment using 3-(trifluoromethyl)-3-(m-[$^{125}I$]iodophenyl)diazirine([$^{125}I$]-TID). No appreciable stretch of apo A-I in the micellar complex is found to be protected from the tryptic digestion. This indicates that the apo A-I interactions with lipid bilayer in vesicular and micellar forms are different. The binding equilibrium of apo A-I with DPPC vesicles at low protein concentration, as revealed by hydrophobic labeling study of the bilayer-penetrating segment, is reached in about one hour, while the formation of micellar complexes at high protein concentration takes about 24 h at 42$^\circ$C. The time-dependent labeling studies with photoreactive phosphatidylcholine (PC) at high apo A-I concentration suggest an initial interaction with head group region of the bilayer followed by the interaction with the tail ends of the acyl chain of the lipid. A possible mode of micellization process is discussed. hydroxylamine treated apo A-I without the C-terminal region is also found to induce micellization of DPPC vesicles.