title: Structure of apolipoprotein B-100 in low density lipoproteins.
abstract: There is general consensus that amphipathic alpha-helices and beta sheets represent the major lipid-associating motifs of apolipoprotein (apo)B-100. In this review, we examine the existing experimental and computational evidence for the pentapartite domain structure of apoB. In the pentapartite nomenclature presented in this review (NH(2)-betaalpha(1)-beta(1)-alpha(2)-beta(2)-alpha(3)-COOH), the original alpha(1) globular domain (Segrest, J. P. et al. 1994. Arterioscler. Thromb. 14: 1674;-1685) is expanded to include residues 1;-1,000 and renamed the betaalpha(1) domain. This change reflects the likelihood that the betaalpha(1) domain, like lamprey lipovitellin, is a globular composite of alpha-helical and beta-sheet secondary structures that participates in lipid accumulation in the co-translationally assembled prenascent triglyceride-rich lipoprotein particles. Evidence is presented that the hydrophobic faces of the amphipathic beta sheets of the beta(1) and beta(2) domains of apoB-100 are in direct contact with the neutral lipid core of apoB-containing lipoproteins and play a role in core lipid organization. Evidence is also presented that these beta sheets largely determine LDL particle diameter. Analysis of published data shows that with a reduction in particle size, there is an increase in the number of amphipathic helices of the alpha(2) and alpha(3) domains associated with the surface lipids of the LDL particle; these increases modulate the surface pressure decreases caused by a reduction in radius of curvature. The properties of the LDL receptor-binding region within the overall domain structure of apoB-100 are also discussed. Finally, recent three-dimensional models of LDL obtained by cryoelectron microscopy and X-ray crystallography are discussed. These models show three common features: a semidiscoidal shape, a surface knob with the dimensions of the betaC globular domain of lipovitellin, and planar multilayers in the lipid core that are approximately 35 A apart; the multilayers are thought to represent cholesteryl ester in the smectic phase. These models present a conundrum: are LDL particles circulating at 37 degrees C spheroidal in shape, as generally assumed, or are they semidiscoidal in shape, as suggested by the models? The limited evidence available supports a spheroidal shape.
authors.affiliation: Department of Medicine, 630 Boshell Bldg., #3, UAB Medical Center, Birmingham, AL 35294-0012, USA. segrest@uab.edu
authors.forename: J P
authors.initials: JP
authors.lastname: Segrest
authors.forename: M K
authors.initials: MK
authors.lastname: Jones
authors.forename: H
authors.initials: H
authors.lastname: De Loof
authors.forename: N
authors.initials: N
authors.lastname: Dashti
mesh_terms: D000818:Animals; D053299:Apolipoprotein B-100; D001055:Apolipoproteins B / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D003198:Computer Simulation; D003902:Detergents; D006801:Humans; D008077:Lipoproteins, LDL / Q000737:chemistry; D008958:Models, Molecular; D011487:Protein Conformation; D017433:Protein Structure, Secondary; D011973:Receptors, LDL / Q000378:metabolism; D012995:Solubility; D013329:Structure-Activity Relationship
chemical_list: D053299:Apolipoprotein B-100; D001055:Apolipoproteins B; D003902:Detergents; D008077:Lipoproteins, LDL; D011973:Receptors, LDL