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 beta1 region (residues 827-1880) of apoB has a high amphipathic beta strand (AbetaS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbetaS-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[37-41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB[37-41] was removed from the aqueous phase. Adsorbed apoB[37-41] did not desorb and could not be forced off by increasing the surface pressure up to 23 mN/m. ApoB[37-41] adsorbed on the TO/W interface was completely elastic when compressed and expanded by +/-13% of its area. On an A/W interface, the apoB[37-41] monolayer became solid when compressed to 4 mN/m pressure indicating extended beta-sheet formation. It could be reversibly compressed and expanded between low pressure and its collapse pressure (35 mN/m). Our studies confirm that the AbetaS structure of apoB[37-41] is a lipid-binding motif that can irreversibly anchor apoB to lipoproteins.
Highlights
1348 Journal of Lipid Research Volume 50, 2009 and great conformational flexibility. The former allows Apolipoprotein B (apoB) to effectively recruit lipids to form stable nascent TAG-rich lipoprotein particles and to be anchored during lipid metabolism (1, 2) from nascent VLDL to LDL. The latter allows the conformational changes of anchored apoB on lipoprotein particles of different sizes and compositions, which promote multiple functions including binding to antibodies, enzymes, receptors, and oxidation products (28–31)
apolipoprotein B (ApoB) is rich in amphipathic b strand (AbS) and amphipathic a helix (AaH) and can be divided into five distinct super domains: NH2-ba1-b 1-a2-b2-a3-COOH (7, 32, 33)
The N-terminal 20% of apoB contains many subdomains with phospholipid and/or TAG binding regions which are folded during translocation in the endoplasmic reticulum in ways which allow it to be secreted with only a few molecules of lipid or as a more lipidated particle (38), especially if microsomal triglyceride transfer protein (MTP) is highly expressed (13)
Summary
Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. 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 enters the blood. Previous studies have shown that a 12 amino acid consensus amphipathic b strand (AbS) or a two-strand amphipathic b sheet modeled from B21 to B41 sequence (the first b sheet region of apoB) binds irreversibly to air/ water (A/W), hydrocarbon/water (HC/W), and TO/W interfaces, lowering the interfacial free energy (6). ApoB48 has only the b1 putative anchoring domain
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