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Nihon Rinsho. Japanese Journal of... Dec 1994The gene encoding human apo B has been mapped to the short arm of chromosome 2 in the p23-p24 region. The apo B gene extends over 43 kb and is composed of 29 exons and... (Review)
Review
The gene encoding human apo B has been mapped to the short arm of chromosome 2 in the p23-p24 region. The apo B gene extends over 43 kb and is composed of 29 exons and 28 introns. Both apo B 100 and apo B 48 were encoded by the same gene. All intestinal cDNA clones contained a single C to T base substitution in the codon CAA encoding Gln2153 in apo B 100 cDNA, resulting in a translational stop. In human intestinal cells, apo B mRNA is recognized by a specific enzyme that modifies cytosine 6666 to a uracil, introducing a stop codon. Recently, a human apo B mRNA editing protein was cloned. The cDNA sequence predicts a translation product of 236-aa residues. The human apo B mRNA editing protein is a cytidine deaminase and exists as a homodimer. Familial hypobetalipoproteinemia can be caused by mutations in the apoB gene that interfere with the translation of a full-length apo B molecule. Frequently, a truncated apo B molecule can be detected in the plasma lipoproteins of familial hypobetalipoproteinemia. Abetalipoproteinemia is caused by defect of the gene encoding the large subunit of a microsomal triglyceride transfer protein.
Topics: Apolipoproteins B; Carrier Proteins; Cholesterol Ester Transfer Proteins; Female; Glycoproteins; Humans; Hypobetalipoproteinemias; Male; Mutation; RNA, Messenger
PubMed: 7853698
DOI: No ID Found -
Annual Review of Nutrition 2000Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small... (Review)
Review
Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.
Topics: Animals; Apolipoproteins B; Base Sequence; Chylomicrons; Gene Expression Regulation; Humans; Intestine, Small; Lipid Metabolism; Lipids; Liver; RNA Editing; RNA Processing, Post-Transcriptional; RNA, Messenger
PubMed: 10940331
DOI: 10.1146/annurev.nutr.20.1.169 -
Journal of Lipid Research Sep 2001There 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... (Review)
Review
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.
Topics: Animals; Apolipoprotein B-100; Apolipoproteins B; Binding Sites; Computer Simulation; Detergents; Humans; Lipoproteins, LDL; Models, Molecular; Protein Conformation; Protein Structure, Secondary; Receptors, LDL; Solubility; Structure-Activity Relationship
PubMed: 11518754
DOI: No ID Found -
The Journal of Nutrition Feb 1999Over the past five years, several laboratories have used transgenic and gene-targeted mice to study apolipoprotein (apo) B biology. Genetically modified mice have proven... (Review)
Review
Over the past five years, several laboratories have used transgenic and gene-targeted mice to study apolipoprotein (apo) B biology. Genetically modified mice have proven useful for investigating the genetic and environmental factors affecting atherogenesis, for defining apoB structure/function relationships, for understanding the regulation of the apoB gene expression in the intestine, for defining the "physiologic rationale" for the existence of the two different forms of apoB (apoB48 and apoB100) in mammalian metabolism and for providing mechanistic insights into the human apoB deficiency syndrome, familial hypobetalipoproteinemia. This review will provide several examples of how genetically modified mice have contributed to our understanding of apoB biology, including our new discovery that human heart myocytes secrete nascent apoB-containing lipoproteins.
Topics: Animals; Apolipoproteins B; Disease Models, Animal; Gene Targeting; Humans; Hypobetalipoproteinemias; Lipoproteins; Mice; Mice, Transgenic; Mutation; Myocardium
PubMed: 10064308
DOI: 10.1093/jn/129.2.451S -
Advances in Experimental Medicine and... 1991Apolipoprotein (apo-) B100 is the exclusive apolipoprotein of low density lipoproteins (LDL0, which transport most of the plasma cholesterol in humans. Mutations in... (Review)
Review
Apolipoprotein (apo-) B100 is the exclusive apolipoprotein of low density lipoproteins (LDL0, which transport most of the plasma cholesterol in humans. Mutations in apo-B100 can cause either hypocholesterolemia or hypercholesterolemia. Familial hypobetalipoproteinemia, which leads to hypocholesterolemia, has been shown to be caused by defects in the apo-B gene that terminate translation prematurely and result in the production of truncated proteins. The mutations responsible for the hypocholesterolemia have been either single nucleotide substitutions or deletions. Familial defective apo-B100, which leads to hypercholesterolemia, is caused by a point mutation in the receptor-binding domain of apo-B100. The mutation disrupts the binding of LDL to the LDL receptor, thereby disrupting LDL receptor-mediated catabolism and resulting in hypercholesterolemia. A variant form of apo-B, apo-B48, is also critical for lipoprotein metabolism. Apolipoprotein B48 is obligatory for the secretion of chylomicrons. It is formed from an RNA-edited apo-B mRNA in which codon 2153 has been converted from a CAA (glutamine) codon to a premature UAA (stop) codon. The first cytosine in this codon is deaminated to form uracil. The minimum nucleotide recognition sequence for the editing mechanism has been reported to be between 26 and more than 63 nucleotides surrounding codon 2153. The apo-B mRNA editing mechanism, which appears to be a cytosine deaminase, and its regulation are being actively investigated.
Topics: Apolipoprotein B-48; Apolipoproteins B; Cholesterol; Humans; Mutation
PubMed: 1858554
DOI: 10.1007/978-1-4684-5904-3_2 -
Advances in Lipid Research 1985
Review
Topics: Animals; Apolipoproteins B; Chylomicrons; Genetic Variation; Humans; Lipoproteins; Lipoproteins, HDL; Lipoproteins, LDL; Lipoproteins, VLDL; Molecular Weight; Protein Conformation; Rats; Receptors, LDL
PubMed: 3895843
DOI: 10.1016/b978-0-12-024921-3.50007-1 -
Atherosclerosis Nov 1987The complete amino acid sequence of the liver-synthesized apolipoprotein B (apoB) species, apoB 100, has been derived from cloned cDNA. The protein consists of 4536... (Review)
Review
The complete amino acid sequence of the liver-synthesized apolipoprotein B (apoB) species, apoB 100, has been derived from cloned cDNA. The protein consists of 4536 amino acids (+ a 27 amino acid signal sequence). Cysteine is clustered in the N-terminal 1/10 of the protein, suggesting the presence of a stabilized tertiary structure in this part of the molecule. Three types of structure are suggested to be of importance for the binding of the protein to lipids; (i) hydrophobic sequences with a high probability for beta-sheet structure, (ii) strict amphipathic beta-sheets, and (iii) amphipathic alfa-helices. An apoB 100 molecule is completed within 10-14 min and secreted after approximately 30 min, 1/3 of which is due to the transfer through the endoplasmic reticulum (ER), while 2/3 is spent in the Golgi apparatus. ApoB 100 is co-translationally N-glycosylated and 25% of the oligosaccharide chains is processed in the Golgi compartment. Other posttranslational modifications that have been discussed include covalent acylation and phosphorylation. It has also been suggested that the lipid moiety of the apoB 100 lipoproteins are modified during the passage through the Golgi apparatus. The site of lipoprotein assembly is suggested to be separated from the site of apoB 100 synthesis, and apoB 100 appears to be co-translationally bound to the ER membrane and from this transferred to the ER lumen. Based on these observations a model for the assembly of apoB 100 lipoproteins is discussed in this paper. The intestinal derived apoB species, apoB 48, has a molecular mass of 210 kDa and appears to correspond to the N-terminal 48% of apoB 100. The mechanism by which apoB 48 is formed is still not known. Available data indicate that the protein is formed within the intestinal cells, these data also argue against the possibility that apoB 48 is formed by posttranslational proteolysis of apoB 100. The formation of a separate apoB 48 mRNA by alternative splicing has been suggested, based on the observation of a 7 kb mRNA which corresponds to the 5' portion of the apoB 100 mRNA. However, the most abundant apoB mRNA species found in the intestine have a size that corresponds to that of the apoB 100 mRNA, furthermore the observation that apoB 48 appears to terminate in a 7.5 kb exon that appears to lack alternative splice sites, does not favour the possibility of alternative splicing.
Topics: Amino Acid Sequence; Animals; Apolipoproteins B; Endoplasmic Reticulum; Molecular Sequence Data; Molecular Weight
PubMed: 3318851
DOI: 10.1016/0021-9150(87)90088-8 -
Advances in Protein Chemistry 1994ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine,... (Review)
Review
ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.
Topics: Amino Acid Sequence; Animals; Apolipoproteins B; Humans; Lipoproteins, LDL; Molecular Sequence Data; Triglycerides
PubMed: 8154370
DOI: 10.1016/s0065-3233(08)60641-5 -
Journal of Lipid Research Feb 1997Apolipoprotein-B-48 is a structural protein exclusively associated with post-prandial lipoproteins (chylomicrons). Apolipoprotein B-48 would be a useful marker to...
Apolipoprotein-B-48 is a structural protein exclusively associated with post-prandial lipoproteins (chylomicrons). Apolipoprotein B-48 would be a useful marker to monitor the kinetics of chylomicrons in vivo, however, its quantitation is limited because of a low concentration in plasma and lack of specific antibodies. Dynamic sieving capillary electrophoresis (DSCE) has recently become widely available for the separation of nanomolar quantities of proteins by size and electrophoretic mobility. Here we describe the potential of DSCE to accurately quantitate apolipoprotein mass in one ml of plasma. Separation of human serum apolipoproteins was achieved through an uncoated fused silica glass capillary column with quantitation based on area response at 220 nm. The retention times of human apolipoprotein B-48, apolipoprotein B-100 and albumin were 8.96 min +/- 0.57%, 10.21 min +/- 0.72%, and 6.56 min +/- 0.4%, respectively (phase-standardized to internal reference). A significant correlation (r = 0.99) was observed between apolipoprotein concentrations and peak area response for mass ranges of 30-40 micrograms/ml. DSCE provides an alternative method for quantifying apolipoprotein B-48 and therefore, may be useful for studying postprandial lipoprotein metabolism.
Topics: Animals; Apolipoprotein B-100; Apolipoprotein B-48; Apolipoproteins B; Cattle; Electrophoresis, Capillary; Electrophoresis, Polyacrylamide Gel; Humans; Sensitivity and Specificity; Serum Albumin
PubMed: 9162759
DOI: No ID Found -
Journal of Internal Medicine Nov 2005Apolipoprotein (apo) B exists in two forms apoB100 and apoB48. ApoB100 is present on very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and... (Review)
Review
Apolipoprotein (apo) B exists in two forms apoB100 and apoB48. ApoB100 is present on very low-density lipoproteins (VLDL), intermediate density lipoproteins (IDL) and LDL. ApoB100 assembles VLDL particles in the liver. This process starts by the formation of a pre-VLDL, which is retained in the cell unless converted to the triglyceride-poor VLDL2. VLDL2 is secreted or converted to VLDL1 by a bulk lipidation in the Golgi apparatus. ApoB100 has a central role in the development of atherosclerosis. Two proteoglycan-binding sequences in apoB100 have been identified, which are important for retaining the lipoprotein in the intima of the artery. Retention is essential for the development of the atherosclerotic lesion.
Topics: Apolipoprotein B-100; Apolipoprotein B-48; Apolipoproteins B; Arteriosclerosis; Glycosaminoglycans; Humans; Hyperlipidemias; Lipoproteins, VLDL; Models, Biological; Proteoglycans; Risk Factors; Triglycerides
PubMed: 16238675
DOI: 10.1111/j.1365-2796.2005.01556.x