Structural and dynamic interfacial properties of the lipoprotein initiating domain of apolipoprotein B

Abstract

To better understand the earliest steps in the assembly of triglyceride (TG)-rich lipoproteins, we compared the biophysical and interfacial properties of two closely related apolipoprotein B (apoB) truncation mutants, one of which contains the complete lipoprotein initiating domain (apoB20.1; residues 1-912), and one of which, by virtue of a 50 amino acid C-terminal truncation, is incapable of forming nascent lipoproteins (apoB19; residues 1-862). Spectroscopic studies detected no major differences in secondary structure, and only minor differences in conformation and thermodynamic stability, between the two truncation mutants. Monolayer studies revealed that both apoB19 and apoB20.1 bound to and penetrated egg phosphatidylcholine (EPC) monolayers; however, the interfacial exclusion pressure of apoB20.1 was higher than apoB19 (25.1 mN/m vs. 22.8 mN/m). Oil drop tensiometry revealed that both proteins bound rapidly to the hydrophobic triolein/water interface, reducing interfacial tension by ?20 mN/m. However, when triolein drops were first coated with phospholipids (PL), apoB20.1 bound with faster kinetics than apoB19 and also displayed greater interfacial elasticity (26.9 6 0.8 mN/m vs. 22.9 6 0.8 mN/m). These data establish that the transition of apoB to assembly competence is accompanied by increases in surface activity and elasticity, but not by significant changes in global structure. Apolipoprotein B (apoB) is a 4,536 amino acid residue secretory glycoprotein that serves as the major structural component of triglyceride (TG)-rich lipoproteins secreted by the liver (very low density lipoproteins) and intestine (chylomicrons) (1-5). The assembly of apoB-containing lipoproteins is believed to occur in two stages: in the first stage, a precursor apoB-containing emulsion particle is formed in the endoplasmic reticulum (ER) concurrently with apoB translation; in the second stage, these nascent lipoproteins fuse with TG-rich emulsion particles produced in the smooth ER (6-11). Both steps require the activity of microsomal TG transfer protein (MTP), a dedicated ER-localized cofactor that is essential for apoBcontaining lipoprotein assembly and secretion (12)(13)(14).The mechanism by which apoB is lipidated during the initial phase of TG-rich lipoprotein assembly is not well understood. It was first proposed that apoB intercalates into the inner leaflet of the ER membrane during its translation and then entrains membrane lipid as it desorbs from the ER membrane (15-18). The observation that apoB becomes associated with the ER membrane immediately after translation (16), and that N-terminal fragments of apoB bind to hydrophobic surfaces (19)(20)(21)(22)(23)(24), suggests that apoB possesses avid surface activity, which is an essential requisite of this model. Recently, however, have proposed a different model for the lipidation of apoB that is based upon sequence similarities between apoB and vitellogenin. This model postulates that the N terminal ?1,000 residues of apoB form two b-sheets that comprise the s...

Vitellogenin Recognizes Cell Damage through Membrane Binding and Shields Living Cells from Reactive Oxygen Species

“…The Vg domain (the N-sheet in combination with the ␣-helical subdomain) association with lipid surfaces has been described previously in human apoB (15,17). We narrow down this property to the ␣-helical part in the honey bee.…”

“…The Vg domain has lipid affinity, and it appears to be important for the assembly of the lipoprotein complex (15,16). The apoB Vg domain binds to egg phosphatidylcholine (PC) monolayers and dimyristoylphos-phatidylcholine multilamellar vesicles that have been used in the studies of the apoB-lipid packing (15,17), which takes place at the endoplasmic reticulum membrane. There is further evidence of direct binding of apoB to the epithelial cell membrane via the Vg domain (18), but the mechanisms and consequences of this cell binding are unclear.…”

Impact of murine intestinal apolipoprotein A-IV expression on regional lipid absorption, gene expression, and growth

“…Given the importance of interfacial phenomena in the initiation of TG-rich particle assembly ( 45,46 ) and the fi nding that apoA-IV displays the highest dynamic interfacial elasticity of any apolipoprotein ( 20 ), we have proposed that apoA-IV might increase the effi ciency of intestinal lipid absorption by facilitating particle expansion during the second step of TG-rich lipoprotein assembly ( 20 ). The observation that apoA-IV expression in IPEC-1 intestinal cells increases the size of secreted TG-rich lipoproteins supports this hypothesis ( 22 ).…”

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

“…ApoB17 was shown to bind to phospholipids (39), phospholipid-TAG emulsions (40), and TAG drops (41). ApoB 19 and 20.1 also bind to TAG drops (42). ApoB 5.9, corresponding to the Nterminal b barrel, does not bind to phospholipids; however, fragments of apoB such as apoB [6][7][8][9][10][11][12][13][14][15][16][17], [6][7][8][9][10][11][12][13], etc.…”