Mutational analysis of the ligand binding domain of the low density lipoprotein receptor

The ligand binding domain of the low density lipoprotein (LDL) receptor contains seven imperfect repeats of a 40-amino acid cysteine-rich sequence. Each repeat contains clustered negative charges that have been postulated as ligand-binding sites. The adjacent region of the protein, the growth factor homology region, contains three cysteine-rich repeats (A-C) whose sequence differs from those in the ligand binding domain. To dissect the contribution of these different cysteine-rich repeats to ligand binding, we used oligonucleotide-directed mutagenesis to alter expressible cDNAs for the human LDL receptor which were then introduced into monkey COS cells by transfection. We measured the ability of the mutant receptors to bind LDL, which contains a single protein ligand for the receptor (apoB-100), and beta-migrating very low density lipoprotein (beta-VLDL), which contains apoB-100 plus multiple copies of another ligand (apoE). The results show that repeat 1 is not required for binding of either ligand. Repeats 2 plus 3 and repeats 6 plus 7 are required for maximal binding of LDL, but not beta-VLDL. Repeat 5 is required for binding of both ligands. Repeat A in the growth factor homology region is required for binding of LDL, but not beta-VLDL. Repeat B is not required for ligand binding. These results support a model for the LDL receptor in which various repeats play additive roles in ligand binding, each repeat making a separate contribution to the binding event.

Transport-deficient mutations in the low density lipoprotein receptor. Alterations in the cysteine-rich and cysteine-poor regions of the protein block intracellular transport

Certain individuals with familial hypercholesterolemia (FH) produce mutant forms of the low density lipoprotein (LDL) receptor that fail to move from the endoplasmic reticulum to the Golgi complex. Here, we describe the cloning and expression of one such mutant allele, FH 429. The mutation causes a substitution of a Val for a Gly at residue 544. When recreated in an expressible cDNA, this substitution gives rise to an LDL receptor that is not transported to the cell surface and is rapidly degraded. Three previously mapped transport-deficient alleles of the LDL receptor were traced to the cysteine-rich repeats of the protein, suggesting that the generation of non-disulfide-bonded (free) cysteines might cause the block in transport. The FH 429 mutation is not located in a cysteine-rich region, however. We have attempted to test the role of cysteine by expressing mutant cDNAs that encode proteins blocked in transport and predicted to contain free cysteines. The results suggest that free cysteines are not obligatory for the blocked intracellular movement of mutant LDL receptors.

Isolation of angolide from Pithomyces atro-olivaceus and P. graminicola

Two additional Pithomyces species, P. atro-olivaceus and P. graminicola, produced the cyclotetradepsipeptide angolide, when grown in surface culture on a defined medium.

Human low density lipoprotein receptor expressed in Xenopus oocytes. Conserved signals for O-linked glycosylation and receptor-mediated endocytosis

The human low density lipoprotein (LDL) receptor is shown to carry out efficient receptor-mediated endocytosis in Xenopus laevis oocytes. Microinjection of mRNAs encoding the human receptor led to synthesis of a 120-kDa precursor possessing high mannose N-linked sugars and core O-linked sugars. During its transport to the cell surface, the protein increased in apparent size to 160 kDa, which is similar to the change that occurs in human cells. This increase was not seen when the receptor lacked the serine/threonine-rich region that undergoes O-linked glycosylation. The surface receptors bound 125I-LDL at 0 degrees C and internalized it with a half-time of 2 min when the cells were warmed to 19 degrees C. The rate of internalization was slowed by 7-fold when a single residue in the cytoplasmic domain (Tyr807) was changed to a cysteine, an alteration that slows incorporation into coated pits in mammalian cells. Deletion of the cytoplasmic domain abolished rapid internalization. We conclude that the signals for O-linked glycosylation and receptor-mediated endocytosis of the LDL receptor have been conserved throughout vertebrate evolution.

Overexpression of low density lipoprotein (LDL) receptor eliminates LDL from plasma in transgenic mice

A complementary DNA encoding the human low density lipoprotein (LDL) receptor under control of the mouse metallothionein-I promoter was injected into fertilized mouse eggs, and a strain of mice expressing high levels of LDL receptors was established. After administration of cadmium, these mice cleared intravenously injected 125I-labeled LDL from blood eight to ten times more rapidly than did normal mice. The plasma concentrations of apoproteins B-100 and E, the two ligands for the LDL receptor, declined by more than 90 percent after cadmium treatment, but the concentration of another apoprotein, A-I, was unaffected. Therefore, overexpression of an endocytotic receptor can dramatically lower the ambient concentration of its ligand in vivo.

Multiple crm- mutations in familial hypercholesterolemia. Evidence for 13 alleles, including four deletions

The low density lipoprotein (LDL) receptors in fibroblasts from 132 subjects with the clinical syndrome of homozygous familial hypercholesterolemia were analyzed by immunoprecipitation with an anti-LDL receptor monoclonal antibody. 16 of the 132 cell strains (12%) synthesized no immunodetectable LDL receptor protein, indicating the presence of two mutant genes that failed to produce cross-reacting material (crm- mutations). DNA and mRNA from 15 of the 16 crm- patients, representing 30 crm- genes, were available for further study. Haplotype analysis based on 10 restriction fragment length polymorphisms (RFLPs) suggested that the 30 crm- genes represent 13 mutant alleles. Four of the alleles produced no mRNA. Three of these four mRNA- alleles had large deletions ranging from 6 to 20 kb that eliminated the promoter region of the gene. The fourth mRNA- allele did not contain any deletion or alteration in the promoter sequence; the reason for the mRNA- phenotype was not apparent. Nine alleles were positive for mRNAs, of which three encoded mRNAs of abnormal size. One of the abnormal mRNAs was produced by a gene harboring a deletion, and another was produced by a gene with a complex rearrangement. The third abnormal-sized mRNA (3.1 kb larger than normal) was produced by an allele that had no detectable alterations as judged by Southern blotting. The other six mRNA+ alleles appeared normal by Southern blotting and produced normal-sized mRNA but no receptor protein. The current studies demonstrate that mRNA analysis coupled with haplotype determination by Southern blot analysis can be used to classify crm- mutations at a genetic locus where multiple alleles exist.

Hemimethylation prevents DNA replication in E. coli

The DNA adenine methylase of E. coli methylates adenines at GATC sequences. Strains deficient in this methylase are transformed poorly by methylated plasmids that depend on either the pBR322 or the chromosomal origins for replication. We show here that hemimethylated plasmids also transform dam- bacteria poorly but that unmethylated plasmids transform them at high frequencies. Hemimethylated daughter molecules accumulate after the transformation of dam- strains by fully methylated plasmids, suggesting that hemimethylation prevents DNA replication. We also show that plasmids purified from dam+ bacteria are hemimethylated at certain sites. These results can explain why newly formed daughter molecules are not substrates for an immediate reinitiation of DNA replication in wild-type E. coli.

Deletion in the gene for the low-density-lipoprotein receptor in a majority of French Canadians with familial hypercholesterolemia

We found a large deletion (more than 10 kilobases) in the gene for the low-density-lipoprotein receptor in 63 percent of French Canadians with heterozygous familial hypercholesterolemia. The deletion also occurred in homozygous form in four of seven French Canadian homozygotes. The deletion removes the promoter and first exon of the gene, and it abolishes the production of messenger RNA for the low-density-lipoprotein receptor. The high frequency of this mutation is presumably related to a founder effect among the 8000 ancestors of present-day French Canadians, who have had relatively little cross-breeding with groups of other national origins. This deletion has not been observed in any other ethnic group. It can be detected by analysis of genomic DNA from blood leukocytes, thus allowing direct diagnosis of familial hypercholesterolemia in a majority of affected French Canadians.

mRNA for low density lipoprotein receptor in brain and spinal cord of immature and mature rabbits

Hybridization studies with [32P]cDNA probes revealed detectable amounts of mRNA for the low density lipoprotein (LDL) receptor in the central nervous system (CNS) of rabbits. mRNA levels were highest in the medulla/pons and spinal cord, which were the most heavily myelinated regions that were studied. Lower, but detectable levels were present in cerebral cortex, hypothalamus, thalamus, midbrain, and cerebellum. In the medulla/pons and spinal cord, the levels of receptor mRNA were in a range comparable to that detected in the liver. The levels of receptor mRNA in whole brain were constant from 3 days of age to adulthood and, thus, did not vary in proportion to the rate of myelin synthesis. LDL receptor mRNA in the CNS was produced by the same gene that produced the liver and adrenal mRNA as revealed by the demonstration of a deletion in the neural mRNA of Watanabe-heritable hyperlipidemic (WHHL) rabbits identical to the deletion in the LDL receptor gene of these mutant animals. Using antibodies directed against the bovine LDL receptor, we showed that LDL receptor protein is present in the medulla/pons of adult cows. The cell types that express LDL receptors in the CNS and the functions of these receptors are unknown.

Three direct repeats and a TATA-like sequence are required for regulated expression of the human low density lipoprotein receptor gene

The low density lipoprotein receptor is encoded by a "housekeeping" gene that is transcribed in most mammalian cells and is subject to negative feedback regulation by sterols. To determine the basis for this regulated expression, we performed a transfection analysis with hybrid genes containing up to 6500 base pairs of 5' flanking DNA from the low density lipoprotein receptor gene fused to the coding region of the bacterial chloramphenicol acetyltransferase gene. These studies identified a 177-base pair fragment of 5' flanking DNA that is sufficient for expression as well as negative regulation by sterols. The positive elements within this region were further defined by analysis of a series of 15 mutations in which overlapping 10-base pair segments were scrambled by site-specific mutagenesis. These studies identified the positive elements as three imperfect direct repeats of 16 base pairs and a TATA-like sequence. The three repeats contain a sequence that is homologous to the consensus DNA sequence recognized by transcription factor Sp1.

Cloning and expression of recombinant, functional ricin B chain

The cDNA encoding the B chain of the plant toxin ricin has been cloned and expressed in monkey kidney COS-M6 cells. The recombinant B chain was detected by labeling the transfected cells with [35S]methionine and [35S]-cysteine and demonstrating the secretion of a protein with a Mr of 30,000-32,000 that was not present in the medium of mock-transfected COS-M6 cells. This protein was specifically immunoprecipitated by an anti-ricin or anti-B-chain antibody and the amount of recombinant B chain secreted by the COS-M6 cells was determined by a radioimmunoassay. Virtually all of the recombinant B chain formed active ricin when mixed with native A chain; it could also bind to the galactose-containing glycoprotein asialofetuin as effectively as native B chain. These results indicate that the vast majority of recombinant B chains secreted into the medium of the COS-M6 cells retain biological function.

Acid-dependent ligand dissociation and recycling of LDL receptor mediated by growth factor homology region

A domain in the low-density lipoprotein receptor contains three cysteine-rich 'growth factor' repeats like those that occur in many proteins. When this domain is deleted, the receptor no longer releases its ligand at acid pH, it is no longer recycled efficiently and it is rapidly degraded after ligand binding.

42 bp element from LDL receptor gene confers end-product repression by sterols when inserted into viral TK promoter

The LDL receptor, which mediates the cellular uptake of cholesterol, is subject to classic end-product repression when cholesterol accumulates in the cell. We here show that the sensitivity to end-product repression depends upon a 42 bp element in the 5'-flanking region of the human LDL receptor gene. This sequence, designated sterol regulatory element 42 (SRE 42), contains two 16 bp direct repeats that exhibit positive and negative transcriptional activities. Cells transfected with a fusion gene containing SRE 42 inserted into the promoter of the herpes simplex viral TK gene produced abundant mRNA when grown without sterols. When sterols were present, the mRNA was reduced by 57%-95%, depending on the number of copies of SRE in the fusion gene. These transfection data plus DNAase I footprinting experiments suggest a model of end-product repression in which the end product (sterols) opposes the action of a positive transcription factor that binds to a discrete promoter element.

The low density lipoprotein receptor. Identification of amino acids in cytoplasmic domain required for rapid endocytosis

The 50-residue cytoplasmic domain of the low density lipoprotein receptor (amino acids 790-839) directs the receptor to coated pits, thereby facilitating rapid endocytosis of bound low density lipoprotein. To determine the structural features required for this targeting, we produced 24 mutations in the cytoplasmic domain through use of oligonucleotide-directed mutagenesis. The first 22 amino acids of the cytoplasmic domain (residues 790-811) are sufficient for rapid internalization. The amino acid at position 807 is especially critical. Aromatic residues (tyrosine, phenylalanine, or tryptophan) at this position allow rapid internalization. Charged or uncharged aliphatic residues do not substitute. Although the requirements at the neighboring positions (806 and 808) are less stringent, the insertion of proline at position 806 is detrimental. These specificities suggest that the juxtamembranous region of the cytoplasmic domain participates in protein:protein interactions that allow the low density lipoprotein receptor to cluster in coated pits.

Duplication of seven exons in LDL receptor gene caused by Alu-Alu recombination in a subject with familial hypercholesterolemia

A defective LDL receptor gene in a child with familial hypercholesterolemia produces a receptor precursor that is 50,000 daltons larger than normal (apparent Mr 170,000 vs. 120,000). The elongated protein resulted from a 14 kilobase duplication that encompasses exons 2 through 8. The duplication arose from an unequal crossing-over between homologous repetitive elements (Alu sequences) in intron 1 and intron 8. The mutant receptor has 18 contiguous cysteine-rich repeat sequences instead of the normal nine. Seven of these duplicated repeats are derived from the ligand-binding domain, and two repeats are part of the epidermal growth factor precursor homology region. The elongated receptor undergoes normal carbohydrate processing, its apparent molecular weight increases to 210,000, and the receptor reaches the cell surface where it binds reduced amounts of LDL but undergoes efficient internalization and recycling. The current findings support an evolutionary model in which homologous recombination between repetitive elements in introns leads to exon duplication during evolution of proteins.

Alu-Alu recombination deletes splice acceptor sites and produces secreted low density lipoprotein receptor in a subject with familial hypercholesterolemia

A Japanese subject with homozygous familial hypercholesterolemia was found to have a 7.8-kilobase deletion in the gene for the low density lipoprotein receptor. The deletion joins intron 15 to the middle of exon 18, which encodes the 3' untranslated region, thereby removing all 3' splice acceptor sites distal to intron 15. By S1 nuclease mapping, we demonstrated that the 5' splice donor site of intron 15 is no longer used. Instead a continuous transcript is produced in which exon 15 is followed by the remaining segments of intron 15 and exon 18. The translational reading frame of exon 15 continues for 165 nucleotides into intron 15 before a termination codon is reached. This mRNA should produce a truncated receptor that lacks the normal membrane-spanning region and cytoplasmic domain and that has 55 novel amino acids at its COOH terminus. A cDNA expression vector containing this sequence produced a receptor that behaved similarly to the truncated protein produced by the Japanese patient, i.e. greater than 90% of the receptor was secreted from the cell, and the receptors remaining on the surface showed defective internalization. The deletion in this subject resulted from a recombination between two repetitive sequences of the Alu family, one in intron 15 and the other in exon 18. To date, Alu sequences have been observed at the deletion joints of all four gross deletions in the low density lipoprotein receptor gene that have been characterized. Within these Alu sequences, six out of the seven breakpoints have occurred in the left arm. These data suggest that recombination between Alu sequences may be a frequent cause of deletions in the human genome.

AvaII polymorphism in the human LDL receptor gene

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The Lebanese allele at the low density lipoprotein receptor locus. Nonsense mutation produces truncated receptor that is retained in endoplasmic reticulum

We here describe a mutant low density lipoprotein receptor gene that produces a shortened receptor protein lacking three domains: the region of clustered O-linked carbohydrates, the membrane-spanning region, and the cytoplasmic tail. The defect is attributable to a single nucleotide substitution that creates a premature termination codon at amino acid 660, eliminating 180 residues from the mature protein. The truncated protein retains only two domains: a complete ligand-binding region (residues 1-292) and a partial epidermal growth factor precursor homology region (residues 293-659). The termination codon occurs in the middle of a cysteine-rich sequence that is part of the epidermal growth factor precursor homology domain. The mutant protein is present in markedly reduced amounts and may be translated at a reduced rate. After synthesis, most of the receptor remains within the cell for several hours with its N-linked carbohydrate in an unprocessed endoglycosidase H-sensitive form. This finding suggests that the shortened receptor leaves the endoplasmic reticulum at an abnormally slow rate, which is likely attributable to abnormal folding of the truncated protein. The mutation creates a new restriction site for the enzyme HinfI, thus permitting diagnosis by Southern blotting of genomic DNA. Two copies of this mutant gene were present in each of four unrelated Arab patients with homozygous familial hypercholesterolemia (three from Lebanon and one from Syria). We believe that this mutation, hereafter referred to as the "Lebanese allele," is responsible for the extraordinarily high incidence of familial hypercholesterolemia in Lebanon.

Structure of the Saccharomyces cerevisiae HO gene and analysis of its upstream regulatory region

The HO gene product of Saccharomyces cerevisiae is a site-specific endonuclease that initiates mating type interconversion. We have determined the nucleotide sequence of a 3,129-base-pair (bp) segment containing HO. The segment contains a single long open reading frame encoding a polypeptide of 586 amino acids, which has unusual (unbiased) codon usage and is preceded by 762 bp of upstream region. The predicted HO protein is basic (16% lysine and arginine) and is calculated to have a secondary structure that is 30% helical. The corresponding transcript is initiated approximately 50 nucleotides prior to the presumed initiation codon. Insertion of an Escherichia coli lacZ gene fragment into the putative HO coding segment inactivated HO and formed a hybrid HO-lacZ gene whose beta-galactosidase activity was regulated by the mating type locus in the same manner as HO (repressed by a 1-alpha 2). Upstream regions of 1,360 and 762 bp conferred strong repression; 436 bp led to partial constitutivity and 301 bp to full constitutivity. Thus, DNA sequences that confer repression of HO by a1-alpha 2 are at least 250 nucleotides upstream of the transcription start point and are within 436 nucleotides of the HO initiation codon. The progressive loss of repression suggests that both the -762 to -436 and the -436 to -301 intervals contain sites for regulation by a1-alpha 2. The HO gene contains two distinct regions that promote autonomous replication of plasmids in S. cerevisiae. These regions contain sequences that are homologous to the two conserved sequences that are associated with ARS activity.

Deletion of exon encoding cysteine-rich repeat of low density lipoprotein receptor alters its binding specificity in a subject with familial hypercholesterolemia

The proposed ligand binding domain of the low density lipoprotein (LDL) receptor consists of a 40-amino acid cysteine-rich unit that is repeated with some variation seven times. We describe here a mutant allele at the LDL receptor locus in which one of the seven repeats has been deleted. This mutation was found in a patient with the clinical syndrome of homozygous familial hypercholesterolemia. By molecular cloning, we show that the deletion arose by homologous recombination between repetitive Alu sequences in intron 4 and intron 5 of the gene. The deletion removes exon 5, which normally encodes the sixth repeat of the ligand binding domain. In the resultant mRNA, exon 4 is spliced to exon 6, preserving the reading frame. This mRNA produces a shortened protein that reaches the cell surface and reacts with anti-receptor antibodies but does not bind LDL, which contains apoprotein B-100 as its major protein component. Surprisingly, the deleted protein retains the ability to bind and internalize beta-migrating very low density lipoprotein, a lipoprotein that contains apoprotein E as well as apoprotein B-100. These data support the hypothesis that the seven repeated sequences in the receptor constitute the LDL binding domain. The data further indicate that the sixth repeat is required for binding of LDL, but not beta-migrating very low density lipoprotein, and that deletion of a single cysteine-rich repeat can alter the binding specificity of the LDL receptor.

Deletion in cysteine-rich region of LDL receptor impedes transport to cell surface in WHHL rabbit

The Watanabe heritable hyperlipidemic (WHHL) rabbit, an animal with familial hypercholesterolemia, produces a mutant receptor for plasma low-density lipoprotein (LDL) that is not transported to the cell surface at a normal rate. Cloning and sequencing of complementary DNA's from normal and WHHL rabbits, shows that this defect arises from an in-frame deletion of 12 nucleotides that eliminates four amino acids from the cysteine-rich ligand binding domain of the LDL receptor. A similar mutation, detected by S1 nuclease mapping of LDL receptor messenger RNA, occurred in a patient with familial hypercholesterolemia whose receptor also fails to be transported to the cell surface. These findings suggest that animal cells may have fail-safe mechanisms that prevent the surface expression of improperly folded proteins with unpaired or improperly bonded cysteine residues.

Exon-Alu recombination deletes 5 kilobases from the low density lipoprotein receptor gene, producing a null phenotype in familial hypercholesterolemia

Among patients with familial hypercholesterolemia, half of the mutant alleles at the low density lipoprotein (LDL) receptor locus produce no immunologically detectable protein. To determine the molecular basis for one such null allele, we have cloned an abnormally short restriction fragment from the genomic DNA of one patient. The DNA sequence revealed a 5-kilobase deletion that joins a coding sequence in exon 13 to an Alu repetitive element in intron 15. The deletion joint is flanked by two inverted repeats that could potentially form a double stem-loop structure that might have predisposed to this deletion. A similar double stem-loop structure can be drawn for a previously described deletion in the LDL receptor gene and for a deletion in the beta-globin gene cluster. We speculate that such double stem-loop structures might contribute to the formation of large deletions in the human genome.

The J.D. mutation in familial hypercholesterolemia: amino acid substitution in cytoplasmic domain impedes internalization of LDL receptors

Genomic DNA encompassing the terminal exons of the gene for the low density lipoprotein (LDL) receptor was isolated from J.D., a patient with familial hypercholesterolemia whose receptor fails to cluster in coated pits. The DNA sequence revealed a substitution of a cysteine codon for a tyrosine codon at residue 807 in the cytoplasmic domain of the receptor. We reproduced this substitution through oligonucleotide-directed mutagenesis of the normal human receptor cDNA. Upon transfection into receptor-deficient hamster cells, the cDNA specified a receptor that bound LDL normally, but entered the cell slowly. Electron microscopy showed that this receptor was distributed diffusely over the cell surface, whereas the receptor produced by the normal cDNA was concentrated in coated pits. These results support the hypothesis that cytoplasmic domains direct receptors to coated pits, thereby determining the high rate of receptor internalization in animal cells.

Deduced primary structure of the alpha subunit of the GTP-binding stimulatory protein of adenylate cyclase

A bovine adrenal cDNA clone encoding the entire alpha subunit of the GTP-binding regulatory protein that stimulates adenylate cyclase (Gs) was isolated and sequenced. This cDNA directed the synthesis of the larger, 52-kDa form of the polypeptide in COS cells, even though the clone appeared to encode a 46-kDa protein. Comparison of the deduced amino acid sequence of Gs alpha with the alpha subunit of another G protein, transducin, revealed striking homologies.

The effect of social support and the work environment upon burnout among nurses

Burnout among hospital-based nurses appears to be a serious problem affecting the delivery of health care. Findings from previous empirical research indicate that burnout among these nurses results from reactions to adversities inherent in the hospital work environment, and that burnout can lead nurses to change jobs and/or abandon the practice of nursing. This paper presents and discusses research findings on the effects of various aspects of the hospital work environment on burnout among nurses, and, in addition, evaluates the effects of social support in reducing and/or mitigating the relationship between negative aspects of the work environment and burnout. A multiple regression approach is employed to test the hypothesized model. The data were collected from a sample of nurses (n = 310) employed at Fitzsimons Army Medical Center (FAMC), Aurora, Colorado. The major determinants of burnout were found to be low job enhancement (autonomy, task orientation, clarity, innovation, and physical comfort); work pressure; and lack of supervisor support, along with the interaction term involving the combined effects of job enhancement and supervisor support. These predictors, in conjunction with demographic and job-related variables explained 53% of the variance in emotional exhaustion, a central component of the burnout syndrome.

Deletion of clustered O-linked carbohydrates does not impair function of low density lipoprotein receptor in transfected fibroblasts

A single exon in the gene for the receptor for plasma low density lipoprotein (LDL) encodes a region of clustered serine and threonine residues that is immediately external to the membrane-spanning sequence. This region has been proposed as the site of clustered O-linked carbohydrate chains. In the current studies we have deleted the 144 base pairs (48 amino acids) that encode this serine- and threonine-rich region from the cDNA for the human LDL receptor. Upon transfection into receptor-deficient hamster fibroblasts, this mutated cDNA encoded a shortened receptor that no longer showed an anomalously high molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeling with [3H]glucosamine confirmed the lack of clustered O-linked sugars and further revealed that the shortened receptor and the normal receptor both contained isolated O-linked carbohydrate chains attached to the NH2-terminal portion of the protein. The ratio of clustered to isolated O-linked sugar chains in the normal receptor was estimated to be approximately 4-6 to 1. Despite the loss of clustered O-linked carbohydrate, the LDL receptor encoded by the deletion-bearing cDNA bound and internalized LDL normally. It also recycled normally and exhibited a normal half-life. We conclude that: 1) the serine- and threonine-rich region of the LDL receptor is the site for addition of clustered O-linked carbohydrates; 2) the receptor contains a small number of isolated chains of O-linked carbohydrates in addition to the clustered chains; and 3) the clustered O-linked carbohydrates are not essential for LDL receptor function in cultured hamster fibroblasts.

Molecular cloning of bovine LDL receptor cDNAs

Using methods described above, a partial cDNA clone for the bovine LDL receptor has been isolated. DNA sequence analysis and Northern blotting experiments are used to confirm the identity of pLDLR-1. Further DNA sequence analysis of pLDLR-1 reveals that the partial cDNA insert encodes 264 amino acids corresponding to the carboxy-terminal 25% of the bovine LDL receptor. Antipeptide antibodies directed against regions of the predicted protein sequence specifically recognize the purified bovine receptor. These findings provide an independent confirmation of the identity of pLDLR-1.

The LDL receptor in familial hypercholesterolemia: use of human mutations to dissect a membrane protein

Since the discovery of the LDL receptor 13 years ago, a multidisciplinary approach to its study has revealed much about this important cell-surface protein. Most recently, we have developed tools in the form of full-length cDNAs and cloned genomic DNAs necessary to understand the molecular genetics of this locus. The frequent occurrence of mutations in the LDL receptor gene in patients with FH provides a fertile ground on which to explore the parts of the receptor that are necessary for its function. The analysis of four large deletions has revealed an unexpectedly universal involvement of Alu repeats in their generation. These studies indicate that repetitive DNAs can destabilize a gene through homologous recombination. Inasmuch as the LDL receptor gene is a mosaic of exons shared with at least five other proteins, it is possible that early exon-shuffling events involved recombination between these repetitive elements. Is it possible that the very plasticity that permitted evolution of the LDL receptor also accounts for its frequent disruption by mutation? Further study may help to answer this question. Mutations that disrupt the structure of the protein have been identified. The biochemical and cellular consequences of these mutations reveal crucial aspects of receptor structure. The receptor is clearly divided into quasi-independent domains with discrete functions. Mutations that disrupt the cytoplasmic domain alter the ability of the LDL receptor to cluster in coated pits, but they do not disrupt ligand binding or produce major effects on intracellular transport. Some of the mutations in the external domain disrupt binding but do not affect transport or internalization.(ABSTRACT TRUNCATED AT 250 WORDS)

Human genes involved in cholesterol metabolism: chromosomal mapping of the loci for the low density lipoprotein receptor and 3-hydroxy-3-methylglutaryl-coenzyme A reductase with cDNA probes

Cellular cholesterol metabolism is regulated primarily through the coordinate expression of two proteins, the low density lipoprotein (LDL) receptor and 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (EC 1.1.1.34). We have used cDNA probes for the human genes encoding these proteins to determine the precise chromosomal location of the two loci. By in situ hybridization we have regionally mapped the LDL receptor gene, LDLR, to the short arm of chromosome 19 in bands p13.1-p13.3. This result concurs with and extends a previous study in which LDLR was mapped to chromosome 19 by screening somatic cell hybrids with a species-specific monoclonal antibody. We have assigned the HMG-CoA reductase gene, HMGCR, to chromosome 5 by Southern blotting of DNA from a somatic cell hybrid panel and to bands 5q13.3-q14 by in situ hybridizations of the cDNA probe to human metaphase cells with normal and rearranged chromosomes.

Polymorphism and evolution of Alu sequences in the human low density lipoprotein receptor gene

Two clusters of Alu sequences in the human low density lipoprotein (LDL) receptor gene have been analyzed in detail. One Alu cluster is present within the intron separating exons 15 and 16 of the gene and contains a polymorphic Pvu II site. The presence or absence of this site gives rise to two allelic fragments of 14 and 16.5 kilobases, respectively, in genomic Southern blots using cloned cDNA probes. This DNA polymorphic site is caused by a single adenine to guanine transition within an Alu repetitive element. The second cluster of Alu sequences is located in exon 18 of the LDL receptor gene. Southern blotting of primate DNAs suggests that this cluster became associated with the gene about 30 million years ago. Comparison of bovine DNA sequences, which lack this Alu cluster, with those of the human indicates that the Alu sequences inserted in exon 18 in two independent events.

Internalization-defective LDL receptors produced by genes with nonsense and frameshift mutations that truncate the cytoplasmic domain

Certain mutant alleles at the low density lipoprotein (LDL) receptor locus produce receptors that bind LDL normally, but fail to cluster in coated pits and therefore cannot transport LDL into cells. We prepared genomic DNA libraries from cells of two individuals with this phenotype (internalization-defective familial hypercholesterolemia) and isolated the segment of the gene encoding the COOH-terminal cytoplasmic domain of the receptor. One mutant gene contains a single base substitution that changes a tryptophan codon (TGG) to a termination codon (TGA). This produces a receptor with only two amino acids in the cytoplasmic domain. The second mutant gene contains a four-base duplication, producing a frameshift that alters the reading frame. The cytoplasmic tail of this receptor has six of the normal amino acids plus eight additional amino acids. These data suggest that the signal for targeting the LDL receptor to coated pits resides in the cytoplasmic domain of the molecule.

Cassette of eight exons shared by genes for LDL receptor and EGF precursor

The amino acid sequences of the human low-density lipoprotein (LDL) receptor and the human precursor for epidermal growth factor (EGF) show 33 percent identity over a stretch of 400 residues. This region of homologous is encoded by eight contiguous exons in each respective gene. Of the nine introns that separate these exons, five are located in identical positions in the two protein sequences. This finding suggests that the homologous region may have resulted from a duplication of an ancestral gene and that the two genes evolved further by recruitment of exons from other genes, which provided the specific functional domains of the LDL receptor and the EGF precursor.

The LDL receptor gene: a mosaic of exons shared with different proteins

The multifunctional nature of coated pit receptors predicts that these proteins will contain multiple domains. To establish the genetic basis for these domains (LDL) receptor. This gene is more than 45 kilobases in length and contains 18 exons, most of which correlate with functional domains previously defined at the protein level. Thirteen of the 18 exons encode protein sequences that are homologous to sequences in other proteins: five of these exons encode a sequence similar to one in the C9 component of complement; three exons encode a sequence similar to a repeat sequence in the precursor for epidermal growth factor (EGF) and in three proteins of the blood clotting system (factor IX, factor X, and protein C); and five other exons encode nonrepeated sequences that are shared only with the EGF precursor. The LDL receptor appears to be a mosaic protein built up of exons shared with different proteins, and it therefore belongs to several supergene families.

Mutation in LDL receptor: Alu-Alu recombination deletes exons encoding transmembrane and cytoplasmic domains

The molecular size of the plasma LDL (low density lipoprotein) receptor synthesized by cultured fibroblasts from a patient with the internalization-defective form of familial hypercholesterolemia (FH 274) was smaller by 10,000 daltons than the size of the normal LDL receptor. The segment of the gene encoding the truncated portion of the FH 274 receptor was cloned into bacteriophage lambda. Comparison of the nucleotide sequences of the normal and FH 274 genes revealed a 5-kilobase deletion, which eliminated the exons encoding the membrane-spanning region and the carboxyl terminal cytoplasmic domain of the receptor. The deletion appeared to be caused by a novel intrastrand recombination between two repetitive sequences of the Alu family that were oriented in opposite directions. The truncated receptors lack membrane-spanning regions and cytoplasmic domains; they are largely secreted into the culture medium, but a small fraction remains adherent to the cell surface. The surface-adherent receptors bind LDL, but they are unable to cluster in coated pits, thus explaining the internalization-defective phenotype.

The human LDL receptor: a cysteine-rich protein with multiple Alu sequences in its mRNA

The nucleotide sequence of a cloned 5.3 kilobase cDNA for the human low density lipoprotein receptor revealed five domains in the 839 amino acid protein: 322 NH2-terminal amino acids, extremely rich in disulfide-bonded cysteine residues (15%) and including an 8-fold repeat of 40 residues that may contain the LDL binding site; 350 residues homologous to the precursor of mouse epidermal growth factor; a region immediately outside the plasma membrane, rich in serine and threonine and the site of O-linked glycosylation; 22 hydrophobic amino acids, spanning the plasma membrane; and 50 COOH-terminal amino acids, projecting into the cytoplasm. The mRNA for the receptor contains a 3' untranslated region of 2.5 kilobases that includes multiple copies of the Alu family of repetitive DNAs. Transfection of simian COS cells with the human LDL receptor cDNA linked to the SV40 early promoter resulted in expression of functional cell surface receptors.

Domain map of the LDL receptor: sequence homology with the epidermal growth factor precursor

The nucleotide sequence of a partial cDNA for the bovine low-density lipoprotein (LDL) receptor revealed an open reading frame of 264 amino acids that encodes the COOH-terminal 25% of the receptor protein. The sequence predicts a cytoplasmic domain of 50 amino acids at the COOH terminus, followed in order by a membrane-spanning region of 27 hydrophobic amino acids and an externally disposed stretch of 42 amino acids, that is rich in serine and threonine residues and appears to be the site of O-linked glycosylation. This orientation was confirmed by proteolysis experiments in which the relevant fragments were localized by blotting with anti-peptide antibodies and a galactose-specific lectin. The extracytoplasmic domain of the LDL receptor contains a region that is 38% identical with a 96 amino acid sequence in the precursor to mouse epidermal growth factor (EGF), a peptide hormone. This unexpected homology raises the possibility that proteins involved in growth stimulation (e.g., EGF precursor) and nutrient delivery (e.g., LDL receptor) may have a common evolutionary origin.

Nucleotide sequence of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase, a glycoprotein of endoplasmic reticulum

The nucleotide sequence of a 4.8-kilobase mRNA for hamster 3-hydroxy-3-methylglutaryl coenzyme A reductase, the endoplasmic reticulum enzyme that controls cholesterol biosynthesis, shows that it is a protein of 887 amino acids (molecular weight 97,092) which contains three potential sites for asparagine-linked glycosylation. The reductase is a transmembrane glycoprotein, but in contrast to many other transmembrane glycoproteins, it lacks a cleavable or hydrophobic NH2-terminal signal sequence.

cDNA cloning of the bovine low density lipoprotein receptor: feedback regulation of a receptor mRNA

The low density lipoprotein (LDL) receptor belongs to a class of migrant cell surface proteins that mediate endocytosis of macromolecular ligands. No cDNAs for this class of proteins have been isolated to date. In the current paper, we report the isolation of a cDNA clone for the LDL receptor from a bovine adrenal cDNA library. The library was constructed by the Okayama-Berg method from poly(A)+ RNA that had been enriched in receptor mRNA by immunopurification of polysomes. Mixtures of synthetic oligonucleotides encoding the amino acid sequence of two neighboring regions of a single cyanogen bromide fragment were used as hybridization probes to identify a recombinant plasmid containing the LDL receptor cDNA. This plasmid, designated pLDLR-1, contains a 2.8-kilobase (kb) insert that includes a sequence which corresponds to the known amino acid sequence of a 36-residue cyanogen bromide fragment of the receptor. pLDLR-1 hybridized to a mRNA of approximately equal to 5.5 kb in the bovine adrenal gland. This mRNA, like the receptor protein, was 9-fold more abundant in bovine adrenal than in bovine liver. pLDLR-1 cross-hybridized to a mRNA of approximately equal to 5.5 kb in cultured human epidermoid carcinoma A-431 cells. This mRNA was markedly reduced in amount when sterols were added to the culture medium, an observation that explains the previously observed feedback regulation of LDL receptor protein. Southern blot analysis of bovine genomic DNA with 32P-labeled pLDLR-1 revealed a simple pattern of hybridization, consistent with a single-copy gene containing introns.

The impact of patients with nonorganic physical findings on a controlled trial of transcutaneous electrical nerve stimulation and electroacupuncture

Fifty-four patients treated in a three-week in-patient rehabilitation program were randomly assigned to and accepted treatment with electroacupuncture (n = 17), TENS (low-intensity transcutaneous nerve stimulation, n = 18) and TENS-dead battery (placebo, n = 18). Outcome measures included estimates of pain (on a visual analogue scale) and disability by both physician and patient as well as physical measures of spine function. Two groups were constructed based on the absence of nonorganic physical findings (Valid group, n = 30) and the presence of two or more nonorganic physical findings out of a possible four (Invalid group, n = 10). Multivariate and univariate analyses of covariance were utilized to determine effects of treatment (acupuncture, TENS, placebo) and the effects of over-reporting (presence of excessive nonorganic physical findings). Statistically significant findings demonstrated that the acupuncture group enjoyed more relief of peak pain and more relief of pain on an average day at the three-month return assessment. Additionally, the acupuncture group demonstrated greater improvement in extension trunk strength at the discharge assessment. The Invalid group were found to have a contaminating effect on the acupuncture results. Analysis also demonstrated associations between nonorganic physical findings and both personality traits ("Conversion V" profile on MMPI) and retention of an attorney. Researchers conducting clinical trials in chronic low-back pain patients should control for contamination by the presence of over-reporters.