Surface study of apoB1694–1880, a sequence that can anchor apoB to lipoproteins and make it nonexchangeable

Abstract

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. It remains bound to secreted lipoproteins during lipid metabolism in plasma. The b1 region (residues 827-1880) of apoB has a high amphipathic b strand (AbS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbS-rich region between apoB37 and apoB41 (residues 1694-1880) was cloned, expressed, and purified. The interfacial properties were studied at the triolein/water (TO/W) and air/water (A/W) interfaces. ApoB [37][38][39][40][41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB [37][38][39][40][41] Apolipoprotein B (apoB) is a large protein (4536 residues) that plays an essential role in the formation of triacylglycerol (TAG)-rich lipoproteins by the intestine, as chylomicrons, or by the liver, as VLDL (1). The N-terminal 48% of apoB (apoB48) in the intestine and the full-length apoB (apoB100) in the liver play fundamental roles in the assembly with lipids, including phosphatidylcholine, TAG, and cholesterol, into a nascent emulsion particle which, after further modification, is secreted and ultimately enters the blood. These particles carry dietary (chylomicrons) or liver (VLDL) TAG through the blood to other tissues where they are acted upon by lipoprotein lipase to serve as a source of energy or for cell membrane and lipid droplet synthesis. ApoB is unique among apolipoproteins and a rare member of the family of proteins that bind irreversibly to lipid droplets (1, 2). Only a few other peptides share this irreversible binding, including the oleosins of oil bodies in seeds (3) and perhaps some viral core proteins such as those in hepatitis C virus (4). Oleosins and virus core proteins have a large hydrophobic, proline-rich region that has been suggested to anchor these peptides permanently to their respective emulsion particles. ApoB is the only nonexchangeable apolipoprotein: it remains with the particle from the time it is formed in the liver or intestine until it is removed and catabolized through receptor-mediated cell uptake (1, 2). All remaining plasma apolipoproteins are exchangeable, moving Abbreviations: AaH, amphipathic a helix; AbS, amphipathic beta strand; ApoB, apolipoprotein B; apoB48, N-terminal 48% of apoB; apoB100, full-length apoB; A/W, air/water; CSP, a consensus sequence peptide of exchangeable apolipoproteins; DD/W, dodecane/water; DPC, dodecylphosphocholine; GIXD, grazing incidence X-ray diffraction; HC/W, hydrocarbon/water; IDL, intermediate density lipoprotein; MTP, microsomal triglyceride transfer protein; TAG, triacylglycerol; TO/W, triolein/water.

Lipoprotein Structure and Dynamics: Low Density Lipoprotein Viewed as a Highly Dynamic and Flexible Nanoparticle

“…This observation supports the notion that apo-B100 in VLDL is loosely packed at the interface covering a large surface area with low interfacial surface tension [59]. During particle conversion from VLDL to LDL, however, the relative number of surface molecules increases and a higher molecular packing density leads to a compression of the lipid anchored protein regions and an overall stiffening of the LDL particle [60]. To conclude, the intrinsic conformational flexibility and elasticity of apo-B100 containing lipoprotein particles is most likely critical for specific affinities of lipoproteins to receptors, antibodies or enzymes.…”

“…However, to evaluate lipid-protein interactions physical parameter like interfacial elasticity or molecular dynamics have to be considered. In this context, it was suggested that the hydrophobic -sheet domains of apo-B100 act as elastic lipid anchors, whereas the amphipathic -helical domains respond rapidly to changes in surface pressure [59,60]. In any case, it can be assumed that alterations in the adsorption and penetration depth of apo-B100 in the phospholipid monolayer and in the lipid core are accompanied by structural rearrangements of the domains and changes in the orientation of the domains relative to each other.…”

Surface behavior of apolipoprotein A-I and its deletion mutants at model lipoprotein interfaces

“…Some relief from high pressure will occur as albumin removes fatty acids and perhaps 2-monoacylglycerol from the interface. Note that apoB is irreversibly bound to CM and VLDL by its amphipathic ␤ strand domains ( 60 ), whereas its other loosely bound domains can desorb, thus decreasing the area covered by apoB ( 62 ). These detached domains probably have an altered conformation and perhaps change their ligand binding properties.…”

“…We have been using a novel surface chemistry technique, oil-drop tensiometry, to study the pressure-mediated adsorption, desorption, and fl exibility behavior of apolipoproteins, their segments, and consensus peptides at physiologically relevant model lipoprotein surfaces ( 38,(51)(52)(53)(54)(55)(56)(57)(58)(59)(60)(61)(62), such as triolein/water (TO/W) and phosphatidylcholine/triolein/water (POPC/TO/W) interfaces. Our novel technique provides a unique point of view to look at the interaction between apolipoproteins and lipids.…”

Ubiquitination regulates the assembly of VLDL in HepG2 cells and is the committing step of the apoB-100 ERAD pathway

“…During biogenesis, a portion of the nascent apoB-100 may exist in an intermediate, uncommitted state: not fully secretion-competent, yet not terminally malfolded. ApoB-100 contains hydrophobic b domains that bind strongly to lipids ( 27 ). Until the quality control surveillance machinery is satisfi ed with the status of these domains, it appears that apoB-100 must remain accountable to the ERAD pathway.…”