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.     

Studies on the role of amino acids 38-45 in the expression of a functional thyrotropin receptor.

We previously reported that deletion or substitution of a unique eight-amino acid tract (residues 38-45) in the extracellular domain of the human TSH receptor led to the loss of specific ligand binding to the surface of transfected cells. In the present study we analyzed this region in more detail. Using site-directed mutagenesis of the TSH receptor cDNA, we substituted amino acid residues 38-45, either in three overlapping groups of four amino acids each or individually. The resultant TSH receptor mutant cDNAs were stably transfected into Chinese hamster ovary cells, and the cells were tested for their TSH-binding ability. Our data demonstrate that amino acid residues 38-40 and 42-45 in this region of the human TSH receptor can be substituted without alteration in receptor function and are, therefore, not critical in forming or maintaining the TSH-binding site. However, substitution of Cys41, either alone or together with adjacent amino acids, leads to the loss of TSH binding to its receptor. These data suggest a central role for the amino acid in position 41 in preserving the biological function of the TSH receptor.     

The use of monoclonal antibodies to localize the low density lipoprotein receptor-binding domain of apolipoprotein B

Human apolipoprotein (apo) B-100 is composed of 4536 amino acids. It is thought that the binding of apoB to the low density lipoprotein (LDL) receptor involves an interaction between basic amino acids of the ligand and acidic residues of the receptor. Three alternative models have been proposed to describe this interaction: 1) a single region of apoB is involved in receptor binding; 2) groups of basic amino acids from throughout the apoB primary structure act in concert in apoB receptor binding; and 3) apoB contains multiple independent binding regions. We have found that monoclonal antibodies (Mabs) specific for a region that spans a thrombin cleavage site at apoB residue 3249 (T2/T3 junction) totally blocked LDL binding to the LDL receptor. Mabs specific for epitopes outside this region had either no or partial ability to block LDL binding. In order to define the region of apoB directly involved in the interaction with the LDL receptor we have tested 22 different Mabs for their ability to bind to LDL already fixed to the receptor. A Mab specific for an epitope situated between residues 2835 and 2922 could bind to its epitope on LDL fixed to its receptor whereas a second epitope between residues 2980 and 3084 is inaccessible on receptor-bound LDL. A series of epitopes near residue 3500 of apoB is totally inaccessible, and another situated between residues 4027 and 4081 is poorly accessible on receptor-bound LDL. In contrast, an epitope that is situated between residues 4154 and 4189 is fully exposed. Mabs specific for epitopes upstream and downstream of the region 3000-4000 can bind to receptor-bound LDL with a stoichiometry close to unity. Our results strongly suggest that the unique region of apoB directly involved in the LDL-receptor interaction is that of the T2/T3 junction.     

A novel trypsin-like serine protease (hepsin) with a putative transmembrane domain expressed by human liver and hepatoma cells

Recombinant clones with cDNA inserts coding for a new serine protease (hepsin) have been isolated from cDNA libraries prepared from human liver and hepatoma cell line mRNA. The total length of the cDNA is approximately 1.8 kilobases and includes a 5' untranslated region, 1251 nucleotides coding for a protein of 417 amino acids, a 3' untranslated region, and a poly(A) tail. The amino acid sequence coded by the cDNA for hepsin shows a high degree of identity to pancreatic trypsin and other serine proteases present in plasma. It also exhibits features characteristic of zymogens to serine proteases in that it contains a cleavage site for protease activation and the highly conserved regions surrounding the His, Asp, and Ser residues that participate in enzyme catalysis. In addition, hepsin lacks a typical amino-terminal signal peptide. Hydropathy analysis of the protein sequence, however, revealed a very hydrophobic region of 27 amino acids starting 18 residues downstream from the apparent initiator Met. This region may serve as an internal signal sequence and a transmembrane domain. This putative transmembrane domain could be involved in anchoring hepsin to the cell membrane and orienting it in such a manner that its carboxyl terminus, containing the catalytic domain, is extracellular.     

Characterization of primary amino acid sequence of human complement control protein factor I from an analysis of cDNA clones.

A cDNA clone of the mRNA coding for the human complement system control protein Factor I has been isolated. The predicted amino acid sequence obtained from the DNA sequence demonstrates a protein consisting of a heavy chain (Mr 35,400) linked to a light chain (Mr 27,600), both of which contain three sites for N-linked glycosylation. The light chain has clear homology with other serine proteinases, most notably in the region of the catalytically active and structurally important amino acids and shares some of the features characteristic of the plasminogen activators. The heavy chain has a clear 'mosaic' nature typical of the plasma serine proteinases; in particular it contains class A and class B LDL (low-density lipoprotein) receptor repeats with conserved cysteine residues similar to those found in other complement proteins.     

Site-specific mutations in the COOH-terminus of placental alkaline phosphatase: a single amino acid change converts a phosphatidylinositol- glycan-anchored protein to a secreted protein

Placental alkaline phosphatase (PLAP) is anchored in the plasma membrane by a phosphatidylinositol-glycan moiety (PI-glycan). PI-glycan is added posttranslationally to the nascent peptide chain after the removal of 29 amino acids from the COOH-terminus. The contribution of selected COOH-terminal amino acids to the signal for PI-glycan addition was tested by creating a fusion protein with the COOH-terminus of PLAP and a secreted protein and by mutagenesis of specific PLAP COOH-terminal amino acids. The cDNA encoding the COOH-terminus of PLAP was fused in frame to the cDNA for human clotting Factor X and expressed in transfected COS-1 cells. Fusion proteins containing 32 amino acids of the PLAP COOH-terminus were modified by PI-glycan addition. Thus, the signal for PI-glycan modification must reside in these amino acids. Next, the region between the hydrophobic domain and the cleavage site was examined for additional determinants. Mutations of the hydrophilic residues in the spacer region demonstrated that these amino acids do not contribute to the signal for PI-glycan addition. Deletion of amino acids in the spacer region prevented the addition of PI-glycan suggesting that the length of the spacer domain or the amino acids around the cleavage site are important determinants. Finally, we demonstrated that interruption of the hydrophobic domain by a charged residue prevents PI-glycan addition and results in a protein that is secreted into the medium. The finding that a single Leu to Arg substitution in the hydrophobic domain converts a PI-glycan anchored, membrane protein to a secreted protein suggests that an essential signal for the correct sorting of PI-glycan anchored proteins versus secreted proteins resides in the hydrophobic domain. Substitution of a charged amino acid for a hydrophobic amino acid may be a mechanism for producing membrane bound and secreted forms of the same protein.     

Use of antipeptide antibodies to demonstrate external orientation of the NH2-terminus of the low density lipoprotein receptor in the plasma membrane of fibroblasts

The low density lipoprotein (LDL) receptor is a member of a class of receptors that bind macromolecules at the cell surface and facilitate their cellular uptake by receptor-mediated endocytosis. The orientation of the LDL receptor in the plasma membrane is unknown. In the current studies the sequence of amino acids at the NH2-terminus of the bovine adrenal LDL receptor was determined, and a synthetic peptide corresponding to amino acids 1-16 was prepared. Antibodies against this peptide were raised in rabbits and were shown by immunoblotting analysis to react specifically with the bovine LDL receptor. The anti- receptor peptide antibodies also bound to the LDL receptor on the outer surface of the plasma membrane of intact human fibroblasts, as visualized by indirect immunofluorescence. Specificity of this binding reaction was confirmed by the observation that the anti-receptor peptide antibodies did not bind to mutant fibroblasts from a patient with homozygous familial hypercholesterolemia that lack LDL receptors. These data demonstrate that the LDL receptor is oriented in the plasma membrane with its NH2-terminus facing the extracellular surface.     

Molecular cloning and characterization of rat estrogen receptor cDNA.

A cDNA clone of rat uterus estrogen receptor (ER) has been isolated and sequenced. This clone contains a complete open reading frame encoding 600 amino acid residues which is 5 and 11 amino acids larger than the corresponding molecules of human and chicken, respectively. The molecular weight of this protein is calculated to be 67,029. When this clone was ligated to the pSV2 vector and transfected into COS7 cells, a protein was produced that had the same affinity to estrogen as rat uterus ER. This sequence shows 88% homology with human ER; 528 amino acids are identical and 14 amino acids are conservative substitutions. The comparison of rat, human and chicken ER sequences indicate the presence of three highly conserved regions suggesting that these regions play important roles in ER function. The putative DNA-binding domain is completely identical in rat, human and chicken. The C-terminal half region which is thought to be the estrogen binding domain is also highly conserved and is rich in hydrophobic amino acid residues. Southern blot analysis of genomic DNA with ER cDNA as a probe has shown that related sequences are present in the genome.     

Amino acid sequence of human protein Z, a vitamin K-dependent plasma glycoprotein

Protein Z is a vitamin K-dependent glycoprotein isolated and characterized from human and bovine plasma. A cDNA coding for human protein Z has been obtained by the isolation of phage clones from a liver cDNA library and in vitro amplification of two other liver libraries. Protein Z is synthesized with a prepro-leader sequence of 40 amino acids. The mature protein is composed of 360 residues including a Gla domain of 13 carboxyglutamic acid residues, two epidermal growth factor domains, and a carboxyl terminal region which is highly homologous to the catalytic domain of serine proteases. Human protein Z, however, contains an Asp instead of Ser and a Lys instead of His in the catalytic triad of the active site.     

Analysis of the acrolein-modified sites of apolipoprotein B-100 in LDL

We have reported that acrolein-conjugated low-density lipoprotein (Acro-LDL) uptake by scavenger receptor class A type 1 (SR-A1) can mediate macrophage foam cell formation. The purpose of this study was to determine which amino acid residues of apoB protein in LDL are conjugated with acrolein. Acro-apoB was prepared by incubation of LDL with acrolein (10 to 60 μM) at 37 °C for 7 days. Identification of acrolein-conjugated amino acid residues in apoB was performed using LC-MS/MS. The levels of acrolein-conjugated amino acid residues of apoB as well as crosslinking apoB increased in proportion to acrolein concentration. The level of LDL uptake by macrophages was parallel with the acrolein-conjugated monomer apoB. Acrolein-conjugated amino acid residues in apoB were C212, K327, K742, K949, K1087, H1923, K2634, K3237 and K3846. The NH₂-teriminal four amino acid residues (C212, K327, K742 and K949) were located at the scavenger receptor SR-A1 recognition site, suggesting that these four acrolein-conjugated amino acids are involved in the rapid uptake of Acro-LDL by macrophages. It is proposed that the rapid uptake of LDL by macrophages is dependent on acrolein conjugation of four amino acids residues at the scavenger receptor recognition site of apoB in LDL.     

Amino Acid Residues in the ω-Minus Region Participate in Cellular Localization of Yeast Glycosylphosphatidylinositol-Attached Proteins

The final destination of glycosylphosphatidylinositol (GPI)-attached proteins in Saccharomyces cerevisiae is the plasma membrane or the cell wall. Two kinds of signals have been proposed for their cellular localization: (i) the specific amino acid residues V, I, or L at the site 4 or 5 amino acids upstream of the GPI attachment site (the omega site) and Y or N at the site 2 amino acids upstream of the omega site for cell wall localization and (ii) dibasic residues in the region upstream of the omega site (the omega-minus region) for plasma membrane localization. The relationships between these amino acid residues and efficiencies of cell wall incorporation were examined by constructing fusion reporter proteins from open reading frames encoding putative GPI-attached proteins. The levels of incorporation were high in the constructs containing the specific amino acid residues and quite low in those containing two basic amino acid residues in the omega-minus region. With constructs that contained neither specific residues nor two basic residues, levels of incorporation were moderate. These correlations clearly suggest that GPI-attached proteins have two different signals which act positively or negatively in cell wall incorporation for their cellular localization.     

Differences in local conformation in human apolipoprotein B-100 of plasma low density and very low density lipoproteins as identified by cathepsin D

The conformational changes of human apolipoprotein (apo) B-100 which accompany the conversion of plasma very low density lipoproteins (VLDL) to low density lipoproteins (LDL) were investigated by studying the accessibility of apoB-100 in LDL and VLDL to limited proteolysis with cathepsin D, an aspartyl proteinase involved in intracellular protein degradation. We characterized the proteolytic products of apoB-100 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by NH2-terminal sequence analysis to locate cleavage sites. The results identified at least 10 cleavage products generated from apoB-100 and showed differential accessibility of cleavage sites for cathepsin D in apoB-100 between LDL and VLDL. We identified a specific peptide region (residues 2660-2710), which is preferentially accessible to limited proteolysis by cathepsin D but inaccessible to limited proteolysis by 12 other enzymes tested. Within this peptide region, cathepsin D cleaved apoB-100 of LDL and VLDL preferentially at different sites, separated by 33-36 amino acids (2665-2666 or 2668-2669 (LDL) and 2701-2702 (VLDL]. In addition, we identified a cleavage site, located at residues 3272-3273, specific for cathepsin D, which is contained within the COOH-terminal enzyme-accessible peptide region (residues 3180-3280), which we have demonstrated using 12 endoproteases with various specificities. The previously identified NH2-terminal region (residues 1280-1320) appears to be resistant to limited cleavage by cathepsin D. However, a new site was revealed only approximately 66 kDA from the NH2 terminus. We conclude that differential accessibility and the shift of the novel scission site for cathepsin D by 33-36 amino acids indicate significant differences in local conformation at these sites in apoB-100 as VLDL are converted to LDL.     

The sequence and topology of human complement component C9.

A partial nucleotide sequence of human complement component C9 cDNA representing 94% of the coding region of the mature protein is presented. The amino acid sequence predicted from the open reading frame of this cDNA concurs with the amino acid sequence at the amino-terminal end of three proteolytic fragments of purified C9 protein. No long stretches of hydrophobic residues are present, even in the carboxy-terminal half of the molecule which reacts with lipid-soluble photoaffinity probes. Monoclonal antibody epitopes have been mapped by comparing overlapping fragments of C9 molecule to which the antibodies bind on Western blots. Several of these epitopes map to small regions containing other surface features (e.g., proteolytic cleavage sites and N-linked oligosaccharide). The amino-terminal half of C9 is rich in cysteine residues and contains a region with a high level of homology to the LDL receptor cysteine-rich domains. A model for C9 topology based on these findings is proposed.     

Expression and structure of the human NGF receptor

The nucleotide sequence for the human nerve growth factor (NGF) receptor has been determined. The 3.8 kb receptor mRNA encodes a 427 amino acid protein containing a 28 amino acid signal peptide, an extracellular domain containing four 40 amino acid repeats with six cysteine residues at conserved positions followed by a serine/threonine-rich region, a single transmembrane domain, and a 155 amino acid cytoplasmic domain. The sequence of the extracellular domain of the NGF receptor predicts a highly ordered structure containing a negatively charged region that may serve as the ligand-binding site. This domain is conserved through evolution. Transfection of a full-length cDNA in mouse fibroblasts results in stable expression of NGF receptors that are recognized by monoclonal antibodies to the human NGF receptor and that bind [125I]NGF.     

The glycine cleavage system. Molecular cloning of the chicken and human glycine decarboxylase cDNAs and some characteristics involved in the deduced protein structures

A cDNA encoding chicken glycine decarboxylase (pCP15b) was isolated using an antibody specific to this protein. Additional cDNAs were cloned with the aid of the genomic fragments obtained by using the pCP15b cDNA probe. No initiator methionine codon is found in the currently elucidated cDNA sequence, and an ATG codon in an exon is assigned to this role. The precursor glycine decarboxylase deduced from the 3514-base pair nucleotide sequence is comprised of 1,004 amino acids (Mr = 111,848). The 1,020 amino acid residues are encoded for the precursor form of human glycine decarboxylase (Mr = 112,869) in the 3,783-base long cDNA sequence of two 1.9-kilobase pair cDNAs with a pentanucleotide overlap. The pyridoxal phosphate binding site lysine and a glycine-rich region, which is suggested to be responsible for the attachment of the phosphate moiety of pyridoxal phosphate, are found in close proximity in both the chicken and human enzymes. This region essential for the enzyme action is suggested to be embedded in a segment rich in beta-turns and random coils and is surrounded by conserved and repetitive amino acid sequences. It is suggested that these structures are involved in the organization of the active site of glycine decarboxylase.     

Effects of truncations of the cytoplasmic tail of the luteinizing hormone/chorionic gonadotropin receptor on receptor-mediated hormone internalization.

The LH/CG receptor is a member of the family of G protein-coupled receptors and consists of a large N-terminal extracellular domain (which is responsible for binding hormone) attached to a region that spans the plasma membrane seven times, ending with an intracellularly located C-terminus. Binding of LH or human CG (hCG) to the LH/CG receptor causes a stimulation of adenylyl cyclase, presumably via activation of Gs. The binding of hormone also leads to its subsequent internalization by receptor-mediated endocytosis. In order to investigate the role of the cytoplasmic tail of this receptor in these events, we prepared a series of mutants in which progressively larger portions of the cytoplasmic tail were deleted. Deletion of 58 amino acids from the C-terminus, in which only 11 cytoplasmic residues remain, resulted in a receptor that was not expressed on the plasma membrane. Receptors rat LHR (rLHR)-t653 and rLHR-t631, in which 21 or 43 amino acids were removed, respectively, were properly expressed. These results suggest that a region(s) between residues 616 and 631 of the rLH/CG receptor are required for proper insertion and/or targeting of the receptor into the plasma membrane. Cells expressing rLHR-t653 or rLHR-t631 bound hCG with the same high affinity as cells expressing the full-length receptor, and basal levels of cAMP were the same among the cells. However, cells expressing the truncated receptors responded to hCG with approximately 2-fold greater levels of maximal cAMP accumulation than cells expressing the full-length receptor. Deletion of up to 43 amino acids from the C-terminus of the rLH/CG receptor had no deleterious effect on hCG internalization. In fact, mutants lacking 21 and 43 amino acids exhibited progressively faster rates of hCG internalization as compared to the full-length receptor. Once internalized, hCG was also degraded at a faster rate in cells expressing the truncated LH/CG receptors. Since hCG-stimulated cAMP stimulation and hCG internalization are retained by rLHR-t631, it can be concluded that the residues, not necessarily the same, required for these functions reside within the 26 amino acids of the cytoplasmic tail closest to the seventh transmembrane helix and/or residues within the intracellular loops. Our data show, however, that both hCG-stimulated cAMP production and hCG internalization are enhanced by the removal of the distal portion of the cytoplasmic tail.     

A heparin binding protein whose expression increases during differentiation of embryonal carcinoma cells to parietal endoderm cells: cDNA cloning and sequence analysis

A cDNA clone isolated from a lambda gt11 expression library of teratocarcinoma OTT6050 specifies for a glycoprotein with a molecular weight of about 44,000. The new glycoprotein was termed heparin binding protein-44 (HBP-44), since it was absorbed to a heparin-agarose column and was eluted from it by a buffer containing 1.5 M NaCl. HBP-44 mRNA was intensely expressed in PYS-2 parietal endoderm cells and in the kidney, and the RNA level increased about 10-fold during differentiation of F9 embryonal carcinoma cells to parietal endoderm cells. From the cDNA sequence, HBP-44 was concluded to be rich in charged amino acids, and large segments of the protein appeared to form alpha-helixes. The protein was considered to be anchored to the membrane by a cluster of hydrophobic amino acids present in the N-terminal region. Indeed, the N-terminal sequence of HBP-44 was homologous to asialoglycoprotein receptor, which is anchored to the membrane by the N-terminal region. Furthermore, a portion of the N-terminal region of HBP-44 was homologous to the leucine zipper domain. Except for the N-terminal region, HBP-44 had over-all homology with structural proteins such as myosin heavy chain. We propose that HBP-44 is extruded from plasma membranes and interacts with heparin and related molecules and that it is involved in the interactions of plasma membranes with basement membranes.     

Identification of low density lipoprotein receptor binding domains of human apolipoprotein B-100: a proposed consensus LDL receptor binding sequence of apoB-100

The human liver apoB-100 gene cloned in the lambda gt-11 expression vector expresses fusion proteins reacting with apoB antibodies. A fusion protein induced from a apoB-lambda gt-11 clone reacted with apoB-100 monoclonal antibodies known to block the binding of LDL to the LDL receptor. The fusion protein contains an amino acid sequence domain enriched in positively charged residues which is complementary to the negatively charged amino acids present in the consensus LDL receptor binding domain. This sequence of apoB-100 is proposed as a binding domain for the interaction with the LDL receptor. Comparison of derived amino acid sequences from the entire structure of apoB-100 molecule revealed several similar domains enriched in positively charged amino acids. A consensus sequence of the potential LDL binding domain was identified which contained positively charged amino acids at positions 1, 5 and 8 and a loop of 8-11 amino acids followed by two adjacent positively charged amino acids. These results are interpreted as indicating that there are several potential LDL receptor binding domains in apoB-100.