Interaction of Apoprotein from Porcine High-Density Lipoprotein with Dimyristoyl Lecithin. 2. Nature of Lipid-Protein Interaction

Abstract

The detailed molecular structure of the complex formed by the apoprotein from porcine high density lipoprotein and dimyristoyl phosphatidylcholine (lecithin) has been investigated by a range of physical techniques. The complex, an oblate ellipsoid with major axis 11.0 nm and minor axis 5.5 nm (see the accompanying paper), is comprised of a section of lecithin bilayer with apoprotein at the surface. The main site of interaction between protein and lipid is in the lipid glycerophosphorylcholine group region; as with native high density lipoprotein the surface of the particle consists of a mosaic of lecithin polar groups and protein. The formation of this mosaic reduces the cooperativity of the lecithin chain motions and changes the curvature of the lipid-water interface, as compared to a bilayer. Otherwise, there are no major changes in lecithin motions indicating that no strong binding of lipid to protein occurs. The interaction involves the intercalation of amphipathic, 60 :d a-helical, apoprotein molecules among the lecithin molecules so that the protein resides at the lipid-water interface. The apoprotein has a high affinity for the lipid-water interface but specific lipid-protein interactions are not involved.Although proteins which are associated with lipids in vivo tend to be more hydrophobic than highly water-soluble proteins [ 1,2], high hydrophobicity alone is insufficient to allow complex-formation between lipid and protein in solution [3]. It has been pointed out before that there are more specific requirements for the protein, and it is the purpose of this study [3 -6a] to elucidate these for a particular class of lipid-associated proteins, namely the apoproteins of serum high density lipoproteins.Earlier work has established the chemical structure [4,7] and morphology [8,9] of high density lipoprotein (density 1.07-1.21 g/ml); leading references to earlier work are cited in these papers and [6a]. The two major apoproteins isolated [lo, 111 from human serum high density lipoproteins have been designated [32] as apo A-I and apo A-11. The amino acid sequences of these two apolipoproteins from human serum are known [13,14]. The major peptides (apo A-I) from human and porcine apoproteins have been compared [4,15,16] and shown to have the same general properties. The ability of the total apoproteins to recombine with the total lipids of high density lipoprotein to reconstitute particles similar to the original ~_ _ _ Abhreviations. NMR, nuclear magnetic resonance; ESR, electron spin resonance; CD, circular dichroism. high density lipoprotein has been well documented [ll]. Also the interaction with lecithin alone to give lipid-protein complexes which can be isolated by density-gradient ultracentrifugation has been described [5,18] and shown to be particularly amenable to study by physico-chemical techniques [19 -241. The results presented here and in [6a] are part of a detailed study of the complex formed by the total apoprotein fraction derived froin porcine high density lipoprotein and dimyristoyl lecit...

The Orientation of Helix 4 in Apolipoprotein A-I-containing Reconstituted High Density Lipoproteins

“…The reasons for this apparent shortfall are not entirely clear although physical measurements on rHDL particles made in the past have suggested that protein to phospholipid interactions at the disc edge can account for a reduced bilayer thickness at the perimeter versus that in the center of the particle (35). It is also possible that the phospholipid head groups can bend slightly around the sides of the helices to interact with negative charges along the polar surface of the helix, shortening the effective distance that the protein needs to cover.…”

Crystal Structure of C-terminal Truncated Apolipoprotein A-I Reveals the Assembly of High Density Lipoprotein (HDL) by Dimerization

“…The first 43 residues are encoded by exon-3 and the 44 -243-region is encoded by exon-4 (6). Sequence analysis has suggested that the exon-3-encoded region forms a G* helix, and the exon-4-encoded region contains 10 tandem 11/22-residue repeats thought to form lipid-binding class A amphipathic helices that represent the fundamental lipid-binding motif (7)(8)(9). In prior studies, we derived consensus sequences for the two types of 11-residue repeats (A and B) that divide the exon-4-encoded region into a series of putative helical segments with different homologies (10).…”

Ionization Behaviour of Native Apolipoproteins and of Their Complexes with Lecithin

“…The best fit of the titration curve of the native apoA-I protein to a Scatchard model [12] was achieved by assuming 2 types of residues whose pK differ by 1.5 pH unit ( Table 1). As 5 of the tyrosines (residues 18, 29, 104, 169, 239) are in a non-helical segment, whereas two residues belong to a helix (residues 118, 195) [3,21], this might account for the pK difference.…”

“…[3] and Andrews ct a/. [21], the most probable locations of the helical regions in the apoA-I protein have been calculated. These regions would incfudc 16 lysines and 38 carboxylic acids together with 13 arginine residues.…”