Apolipoprotein amphipathic helices

Dimyristoyl-PC liposomes can bind apolipoproteins reversibly, as described in the preceding section however, at the transition temperature of the lipid (24°C) and at sufficiently high proportions of apolipoprotein to dimyristoyl-PC (1/3 or greater, wt/wt), the apolipoproteins can solubilize the liposomes to give rise to small discs analogous to nascent HDL. The rate of the liposome disruption and solubilization depends on the temperature of the reaction. It is highest at the onset of the main phase transition of dimyristoyl-PC (when lattice defects in the PC bilayer, into which apolipoprotein amphipathic helices can penetrate, are maximal) and decreases a thousand-fold on either... 

Lam CW, Yuen YP, Cheng WF, Chan YW, Tong SF (2006) Missense mutation Leu72Pro located on the carboxyl terminal amphipathic helix of apolipoprotein C-II causes familial chy-lomicronemia syndrome. Clin Chim Acta 364 256-259... 

The major lipoproteins of insect hemolymph, the lipophorins, transport diacylglycerols. The apolipo-phorins have molecular masses of -250, 80, and sometimes 18 kDa.34-37a The three-dimensional structure of a small 166-residue lipophorin (apolipophorin-III) is that of a four-helix bundle. It has been suggested that it may partially unfold into an extended form, whose amphipathic helices may bind to a phospholipid surface of the lipid micelle of the lipophorin 35 A similar behavior may be involved in binding of mammalian apolipoproteins. Four-helix lipid-binding proteins have also been isolated from plants.38 See also Box 21-A. Specialized lipoproteins known as lipovitellins... 

Fig. 2. A model for lipoprotein structure based on the interactions between apolipopro-teins and lipid constituents. The surface monolayer is composed of phospholipids and apolipoproteins. The apoproteins contain helical regions which are amphipathic. The hydrophobic surface of the amphipathic helix interacts with the fatty acyl chains of phospholipids, and the hydrophilic surface is exposed to the aqueous environment of the polar head groups and the plasma. Adapted from Pownall et al.. (1981).

The amphipathic helix, in which residues are spaced so that the helical periodicity places hydrophobic side chains on one side of the helix and hydrophilic side chains on the other, is a common structural motif used by the peripheral apolipoproteins to bind lipid (Segrest et al., 1992) it is also a structural element present in globular proteins (Perutz et al., 1965). 

Class L (lytic) amphipathic helices include venoms such as bombolitins and mastoparan from Hymenoptera that are hemolytic (Argiolas and Pisano, 1983) antibiotics such as magainins, isolated from Xenopus laevis skin (Zasloff et al., 1988) and seminal plasmin from semen (Sitaram and Nagaraj, 1989). As the name implies, these peptides disrupt artificial phospholipid bilayers, although magainin and seminal plasmin are not hemolytic. Unlike the apolipoproteins and peptide hormones (class H), each peptide of this class consists entirely of an amphipathic helix. 

Fig. 5. Structural relevance of 1 l-mer/22-mer tandem repeats in the exchangeable apolipoproteins. WHEEL/SNORKEL analysis of a 22-mer formed from a tandem repeat of the 11-mer class A amphipathic helix sequence, ELLEALKAKLA. The thin arrows indicate the angle (20°) between tandem repeated residues within the 22-mer (e.g., residue 1 to residue 12) the thick arrows indicate the angle (40°) between tandem repeated residues between two successive 22-mers (e.g., residue I to residue 23).

As previously noted, apoA-I contains Pro-punctuated tandem repetitive amphipathic helical domains. In apoE, however, the major lipidbinding domain maps to a class A amphipathic helix motif (residues 202-266 see Fig. 7) with no Pro punctuations (Fig. 12A). Thus, this is by far the longest unbroken amphipathic helix among the exchangeable apolipoproteins (65 residues) and one in which the polar-nonpolar interface is in register throughout its length due to a four-amino-acid deletion compared with apoA-I. 

The key structural features predicted for the amphipathic helix by the original model (Segrest et al., 1974) enabled three laboratories to study independently how amino acid variability determined the properties of the amphipathic helix (Kanellis et al., 1980 Fukushima et al., 1980 Sparrow et al., 1981). The strategy adapted by these investigators was based, not on the primary sequence of naturally occurring apolipoproteins, but on incorporating the periodicity of the secondary structural features of the amphipathic helix motif into the sequences of the peptide analogs. 

One role of the class Y amphipathic helices found in apoA-IV and apoA-I appears to be to serve as low-affinity lipid-associating domains. The snorkel hypothesis predicts that this class of amphipathic helix would not penetrate as deeply into phospholipid surfaces as would class A and thus would have lower lipid affinity. This prediction is supported by experimental evidence (based on Trp fluorescence blue shifts, ease of Trp fluorescence quenching, and liposomal leakage) that apoA-IV sits higher in a phospholipid monolayer than do the other exchangeable apolipoproteins that contain class A amphipathic helices (Weinberg and Jordan, 1990). 

Apolipoprotein A-I is the primary protein component of HDL.23 2513 Most of the 243 residues consist of a nearly continuous amphipathic a helix with kinks at regularly spaced proline residues.26 28 Two disulfide-linked ApoA-I molecules may form a belt that encircles the discoid lipoprotein.2513 ApoA-II is the second major HDL protein, but no dearly specialized function has been identified.29 30 ApoA-I, II, and IV, apoC-I, II, and III, and apoE all have multiple repeats of 22 amino acids with sequences that suggest amphipathic helices. Tire 391-residue ApoA-IV has 13 tandem 22-residue repeats. Proline and glycine are present in intervening hinge regions.23 This may enable these proteins to spread over and penetrate the surfaces of the lipoprotein micelles. Most of these proteins are encoded by a related multigene family.7 303... 

THE AMPHIPATHIC a HELIX A MULTIFUNCTIONAL STRUCTURAL MOTIF IN PLASMA APOLIPOPROTEINS... 

The 1 l-mer/22-mer evolutionary pathway for apolipoproteins can be explained as a result of the 3.6 amino acid residues per turn periodicity of an 0 helix 11 residues form three complete turns of an a helix. Consequently, tandem duplication of an 11-residue amphipathic a helix produces a 22-residue amphipathic a helix (Fig. 5). There is little twist (20° or less) between the polar and nonpolar faces of the two identical 11-mer halves (Segrest et al., 1990 Anantharamaiah et al., 1990a). This 11-mer/ 22-mer motif also means that continuous amphipathic helices significantly longer than 22 residues can exist e.g., there will be a twist of 40° or less between the polar and nonpolar faces of two tandem identical 22-mer amphipathic helices (Fig. 5). 

Various computer algorithms have predicted the existence of amphipathic a-helix segments, coinciding quite well with the 22-mer repeats of the exchangeable apolipoproteins. 

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