Expression of fusion proteins of the nicotinic acetylcholine receptor from mammalian muscle identifies the membrane-spanning regions in the alpha and delta subunits

We have investigated the topology of the alpha and delta subunits of the nicotinic acetylcholine receptor (AChR) from mammalian muscle synthesized in an in vitro translation system supplemented with dog pancreatic microsomes. Fusion proteins were expressed in which a carboxy-terminal fragment of bovine prolactin was attached downstream of each of the major putative transmembrane domains, M1-M4 and MA, in the AChR subunits. The orientation of the prolactin domain relative to the microsomal membrane was then determined for each protein by a proteolysis protection assay. Since the prolactin domain contains no information which either directs or prevents its translocation, its transmembrane orientation depends solely on sequences within the AChR subunit portion of the fusion protein. When subunit-prolactin fusion proteins with the prolactin domain fused after either M2 or M4 were tested, prolactin-immunoreactive peptides that were larger than the prolactin domain itself were recovered. No prolactin-immunoreactive peptides were recovered after proteolysis of fusion proteins containing prolactin fused after M1, M3, or MA. These results support a model of AChR subunit topology in which M1-M4, but not MA, are transmembrane domains and the carboxy terminus is extracellular.     

Location of signal sequences for membrane insertion of the Na+,K+-ATPase alpha subunit.

To study the membrane insertion of the Na+,K+-ATPase (EC 3.6.1.37) alpha subunit with six to eight transmembrane segments, mRNAs encoding the entire alpha subunit and its four different domains were prepared and translated in rabbit reticulocyte lysate with rough microsomal membranes. On the basis of the resistance of the membrane-inserted products to alkali extraction and the failure to insert the translation products into N-ethylmaleimide-treated membranes, it is suggested that at least two signal sequences are contained within the four transmembrane segments from the amino terminus of the alpha subunit.     

Cell-associated episialin is a complex containing two proteins derived from a common precursor.

cDNA for the epithelial sialomucin episialin encodes a transmembrane molecule with a large extracellular domain, which mainly consists of repeats of 20 amino acids. Here we confirm the existence of a previously proposed proteolytic cleavage of episialin that occurs in the endoplasmic reticulum (Hilkens, J., and Buijs, F. (1988) J. Biol. Chem. 263, 4215-4222) and show that a similar cleavage takes place in in vitro translation systems. Using in vitro translation of truncated mRNAs, we map the cleavage site to a region located between 71 and 53 amino acids upstream of the transmembrane domain. Analysis of a mutant, in which this region has been deleted, indicates that the cleavage sites used in vitro and in vivo are identical or in close proximity. Both cleavage products remain associated although they are not linked through disulfide bonds. Therefore, the subunit derived from the N terminus, which represents the actual mucin-like domain, remains indirectly anchored to the cell membrane as a result of its interaction with the C-terminal subunit.     

Amino acid sequence of the human fibronectin receptor

The amino acid sequence deduced from cDNA of the human placental fibronectin receptor is reported. The receptor is composed of two subunits: an alpha subunit of 1,008 amino acids which is processed into two polypeptides disulfide bonded to one another, and a beta subunit of 778 amino acids. Each subunit has near its COOH terminus a hydrophobic segment. This and other sequence features suggest a structure for the receptor in which the hydrophobic segments serve as transmembrane domains anchoring each subunit to the membrane and dividing each into a large ectodomain and a short cytoplasmic domain. The alpha subunit ectodomain has five sequence elements homologous to consensus Ca2+-binding sites of several calcium-binding proteins, and the beta subunit contains a fourfold repeat strikingly rich in cysteine. The alpha subunit sequence is 46% homologous to the alpha subunit of the vitronectin receptor. The beta subunit is 44% homologous to the human platelet adhesion receptor subunit IIIa and 47% homologous to a leukocyte adhesion receptor beta subunit. The high degree of homology (85%) of the beta subunit with one of the polypeptides of a chicken adhesion receptor complex referred to as integrin complex strongly suggests that the latter polypeptide is the chicken homologue of the fibronectin receptor beta subunit. These receptor subunit homologies define a superfamily of adhesion receptors. The availability of the entire protein sequence for the fibronectin receptor will facilitate studies on the functions of these receptors.     

Phosphorylation of the beta subunit of casein kinase II in human A431 cells. Identification of the autophosphorylation site and a site phosphorylated by p34cdc2

To examine the phosphorylation of casein kinase II in cells, the enzyme was isolated by immunoprecipitation from metabolically labeled human epidermal carcinoma A431 cells using polyclonal antipeptide antibodies specific for either the alpha subunit or the beta subunit of the enzyme. When isolated from 32P-labeled cells, the beta subunit was found to be significantly labeled on serine residues whereas only minimal labeling was associated with the alpha subunit. In vitro, the beta subunit of purified bovine casein kinase II was autophosphorylated, also on serine residues. Cleavage of the beta subunit, that had been autophosphorylated in vitro, at tryptophan 9 and tryptophan 12 using N-chlorosuccinimide demonstrated that the autophosphorylation site is located near the amino terminus of the protein, most likely at serine 2 and serine 3. Two-dimensional maps of phosphopeptides generated by digestion of the beta subunit with endoproteinase Glu-C indicted that the majority of the phosphate that was incorporated into the protein in cells was at sites that were indistinguishable from the sites that were autophosphorylated in vitro. In addition to phosphorylation at the autophosphorylation site, the beta subunit is also phosphorylated at an additional site, serine 209, in intact cells. This residue, which is near the carboxyl terminus of the protein, can be phosphorylated in vitro by p34cdc2.     

Identifying the binding site(s) for antidepressants on the Torpedo nicotinic acetylcholine receptor: [3H]2-azidoimipramine photolabeling and molecular dynamics studies

Radioligand binding, photoaffinity labeling, and docking and molecular dynamics were used to characterize the tricyclic antidepressant (TCA) binding sites in the nicotinic acetylcholine receptor (nAChR). Competition experiments indicate that the noncompetitive antagonist phencyclidine (PCP) inhibits [3H]imipramine binding to resting (closed) and desensitized nAChRs. [3H]2-azidoimipramine photoincorporates into each subunit from the desensitized nAChR with approximately 25% of the labeling specifically inhibited by TCP (a PCP analog), whereas no TCP-inhibitable labeling was observed in the resting (closed) state. For the desensitized nAChR and within the alpha subunit, the majority of specific [3H]2-azidoimipramine labeling mapped to a approximately 20 kDa Staphylococcus aureus V8 protease fragment (alphaV8-20; Ser173-Glu338). To further map the labeling site, the alphaV8-20 fragment was further digested with endoproteinase Lys-C and resolved by Tricine SDS-PAGE. The principal labeled fragment (11 kDa) was further purified by rpHPLC and subjected to N-terminal sequencing. Based on the amino terminus (alphaMet243) and apparent molecular weight, the 11 kDa fragment contains the channel lining M2 segment. Finally, docking and molecular dynamics results indicate that imipramine and PCP interact preferably with the M2 transmembrane segments in the middle of the ion channel. Collectively, these results are consistent with a model where PCP and TCA bind to overlapping sites within the lumen of the Torpedo nAChR ion channel.     

Isolation of a proteolytically derived domain of the insulin receptor containing the major site of cross-linking/binding

Radiolabeled insulin was affinity cross-linked to purified insulin receptor with six separate bifunctional N-hydroxysuccinimide esters of different lengths. Results were qualitatively identical for each cross-linker in that insulin was predominantly cross-linked through its B chain to the receptor's alpha subunit. The maximum efficiencies of cross-linking were 10-15% for the most effective reagents, and this value was dependent upon the concentration and length of the cross-linker. In an effort to locate the cross-linking site, monoiodoinsulin was cross-linked to affinity-purified insulin receptor with disuccinimidyl suberate. Limited proteolysis of the hormone/receptor adduct with Staphylococcus aureus V8 protease, chymotrypsin, or thermolysin in an SDS-containing buffer rapidly generated a 55-kDa, insulin-labeled fragment as shown by SDS-polyacrylamide gel electrophoresis. We reported earlier that the 55-kDa chymotryptic fragment contained multiple internal disulfide bonds as evidenced by its shifting mobility on an SDS gel after dithiothreitol treatment [Boni-Schnetzler et al. (1987) J. Biol. Chem. 262, 8395-8401]. Here we show that the 55-kDa fragment is also formed by proteolysis of the receptor in the absence of prior insulin cross-linking. This fragment was prepared in amounts sufficient for sequence analysis and was purified by passage successively over gel permeation and reverse-phase HPLC columns. The sequence of the fragment's amino terminus corresponds to that of the amino terminus of the receptor's alpha subunit. This fragment also reacts with an antibody raised against a synthetic peptide corresponding to residues 242-253 of the receptor's alpha subunit.(ABSTRACT TRUNCATED AT 250 WORDS)     

Membrane topology mapping of vitamin K epoxide reductase by in vitro translation/cotranslocation

Vitamin K epoxide reductase (VKOR) catalyzes the conversion of vitamin K 2,3-epoxide into vitamin K in the vitamin K redox cycle. Recently, the gene encoding the catalytic subunit of VKOR was identified as a 163-amino acid integral membrane protein. In this study we report the experimentally derived membrane topology of VKOR. Our results show that four hydrophobic regions predicted as the potential transmembrane domains in VKOR can individually insert across the endoplasmic reticulum membrane in vitro. However, in the intact enzyme there are only three transmembrane domains, residues 10-29, 101-123, and 127-149, and membrane-integration of residues 75-97 appears to be suppressed by the surrounding sequence. Results of N-linked glycosylation-tagged full-length VKOR shows that the N terminus of VKOR is located in the endoplasmic reticulum lumen, and the C terminus is located in the cytoplasm. Further evidence for this topological model of VKOR was obtained with freshly prepared intact microsomes from insect cells expressing HPC4-tagged full-length VKOR. In these experiments an HPC4 tag at the N terminus was protected from proteinase K digestion, whereas an HPC4 tag at the C terminus was susceptible. Altogether, our results suggest that VKOR is a type III membrane protein with three transmembrane domains, which agrees well with the prediction by the topology prediction program TMHMM.     

Membrane tethering enables an extracellular domain of the acetylcholine receptor alpha subunit to form a heterodimeric ligand-binding site

The first step of assembly of the nicotinic acetylcholine receptor (AChR) of adult skeletal muscle is the specific association of the alpha subunit with either delta or epsilon subunits to form a heterodimer with a ligand-binding site. Previous experiments have suggested that het erodimer formation in the ER arises from interaction between the luminal, NH2-terminal domains of the subunits. When expressed in COS cells with the delta subunit, however, the truncated NH2-terminal domain of the subunit folded correctly but did not form a heterodimer. Association with the delta subunit occurred only when the NH2-terminal domain was retained in the ER and was tethered to the membrane by its own M1 transmembrane domain, by the transmembrane domain of another protein, or by a glycolipid link. In each case, the ligand-binding sites of the resulting heterodimers were indistinguishable from that formed when the full-length alpha subunit was used. Attachment to the membrane may promote interaction by concentrating or orienting the subunit; alternatively, a membrane-bound factor may facilitate subunit association.     

A topological analysis of subunit alpha from Escherichia coli F1F0-ATP synthase predicts eight transmembrane segments

The membrane topology of subunit alpha from the Escherichia coli F1F0-ATP synthase was studied using a gene fusion technique. Fusion proteins linking different amino-terminal fragments of the alpha subunit with an enzymatically active fragment of alkaline phosphatase were constructed by both random transposition of TnphoA and site-directed mutagenesis. Those proteins with high levels of alkaline phosphatase activity are predicted to define periplasmic domains of alpha, and this was confirmed by testing for cell growth in minimal medium supplemented with polyphosphate (P greater than 75) as the sole source of phosphate. The enzymatic activity of some fusion proteins was shown to be sensitive to glucose present in the growth medium. Results from subcellular fractionation experiments suggest that these fusion proteins may be inactive even though they have a periplasmic alkaline phosphatase. The enzymatic activity appears dependent upon proteolytic release of the alkaline phosphatase moiety from its alpha subunit membrane anchor and suggests the target of glucose repression may be a protease present in the periplasm. For the topological analysis of the alpha subunit, a total of 28 unique fusion proteins were studied and the results were consistent with a model of alpha containing eight transmembrane segments, including periplasmic amino and carboxyl termini. Surprisingly, separate periplasmic domains were identified near amino acids 200, 233, and 270. These results suggest the flanking membrane spans are only 10-15 amino acids in length and not able to span a standard 30 A bilayer in an alpha-helical conformation. These short spans may have interesting mechanistic implications for the function of F0, because they contain several amino acids which appear critical for proton translocation. Finally, a fusion of alkaline phosphatase at amino acid 271, the carboxyl-terminal residue, but not at amino acid 260, was able to complement the strain RH305 (uncB-) for growth on succinate and suggests the last 11 amino acids of the alpha subunit are critical to the function of F1F0-ATP synthase.     

A single negatively charged residue affects the orientation of a membrane protein in the inner membrane of Escherichia coli only when it is located adjacent to a transmembrane domain

The orientation of membrane proteins is determined by the asymmetric distribution of charged residues in the sequences flanking the transmembrane domains. For the inner membrane of Escherichia coli, numerous studies have shown that an excess of positively charged residues defines a cytoplasmic domain of a membrane protein ("positive inside" rule). The role of negatively charged residues in establishing membrane protein topology, however, is not completely understood. To investigate the influence of negatively charged residues on this process in detail, we have constructed a single spanning chimeric receptor fragment comprising the N terminus and first transmembrane domain of the heptahelical G protein-coupled vasopressin V(2) receptor and the first cytoplasmic loop of the beta(2)-adrenergic receptor. When fused to alkaline phosphatase (PhoA), the receptor fragment inserted into the inner membrane of E. coli with its N terminus facing the cytoplasm (N(in)-C(out) orientation), although both membrane-flanking domains had rather similar topogenic determinants. The orientation of the receptor fragment was changed after the introduction of single glutamate residues into the N terminus. Orientation inversion, however, was found to be dependent on the location of the glutamate substitutions, which had to lie within a narrow window up to 6 residues distant from the transmembrane domain. These results demonstrate that a single negatively charged residue can play an active role as a topogenic determinant of membrane proteins in the inner membrane of E. coli, but only if it is located adjacent to a transmembrane domain.     

The human insulin receptor cDNA: the structural basis for hormone-activated transmembrane signalling

A cloned approximately 5 kb cDNA (human placenta) contains the coding sequences for the insulin receptor. The nucleotide sequence predicts a 1382 amino acid precursor. The alpha subunit comprises the N-terminal portion of the precursor and contains a striking cysteine-rich "cross-linking" domain. The beta-subunit (the C-terminal portion of the precursor) contains a transmembrane domain and, in the intracellular region, the elements of a tyrosine phosphokinase: an ATP-binding site and a possible tyrosine autophosphorylation site or sites. The overall structure is reminiscent of the EGF receptor; the cross-linking domain of the alpha subunit and several regions of the beta subunit exhibit sequence homology with the EGF receptor. The phosphokinase domain also exhibits homology with some oncogenic proteins that have tyrosine phosphokinase activity, in particular, a striking homology with v-ros. Southern blotting experiments suggest that the coding region spans more than 45 kb. The insulin receptor gene is located on chromosome 19.     

Identification of amino acid residues responsible for the alpha5 subunit binding selectivity of L-655,708, a benzodiazepine binding site ligand at the GABA(A) receptor

L-655,708 is a ligand for the benzodiazepine site of the gamma-aminobutyric acid type A (GABA(A)) receptor that exhibits a 100-fold higher affinity for alpha5-containing receptors compared with alpha1-containing receptors. Molecular biology approaches have been used to determine which residues in the alpha5 subunit are responsible for this selectivity. Two amino acids have been identified, alpha5Thr208 and alpha5Ile215, each of which individually confer approximately 10-fold binding selectivity for the ligand and which together account for the 100-fold higher affinity of this ligand at alpha5-containing receptors. L-655,708 is a partial inverse agonist at the GABA(A) receptor which exhibited no functional selectivity between alpha1- and alpha5-containing receptors and showed no change in efficacy at receptors containing alpha1 subunits where amino acids at both of the sites had been altered to their alpha5 counterparts (alpha1Ser205-Thr,Val212-Ile). In addition to determining the binding selectivity of L-655,708, these amino acid residues also influence the binding affinities of a number of other benzodiazepine (BZ) site ligands. They are thus important elements of the BZ site of the GABA(A) receptor, and further delineate a region just N-terminal to the first transmembrane domain of the receptor alpha subunit that contributes to this binding site.     

Topology,glycosylation and conformational changes in the membrane domain of the vacuolar H+‐ATPase a subunit

Published topological models of the integral membrane a subunit of the vacuolar proton‐translocating ATPase complex have not been in agreement with respect to either the number of transmembrane helices within the integral membrane domain, or their limits and orientations within the lipid bilayer. In the present work we have constructed a predictive model of the membrane insertion of the yeast a subunit, Vph1p, from a consensus of seven topology prediction algorithms. The model was tested experimentally using epitope tagging, green fluorescent protein fusion, and protease accessibility analysis in purified yeast vacuoles. Results suggest that a consensus prediction of eight transmembrane helices with both the amino‐terminus and carboxyl‐terminus in the cytoplasm is correct. Characterization of two glycosylation sites within the homologous mouse a subunit membrane domain further corroborates this topology. Moreover, the model takes into account published data on cytoplasmic and luminal accessibility of specific amino acids. Changes in the degree of protease accessibility in response to the V‐ATPase substrate, MgATP, and the V‐ATPase‐specific inhibitor, concanamycin A, suggest that functional conformational changes occur in the large cytoplasmic loop between TM6 and TM7 of Vph1p. These data substantially confirm one topological model of the V‐ATPase a subunit and support the notion that conformational changes occur within the membrane domain, possibly involving previously proposed axial rotation and/or linear displacement of TM7 in the proton transport cycle. J. Cell. Biochem. 114: 1474–1487, 2013. © 2013 Wiley Periodicals, Inc.     

cDNA sequences from the alpha subunit of the fibronectin receptor predict a transmembrane domain and a short cytoplasmic peptide

The fibronectin receptor is a complex of two cell surface glycopeptides that mediate the binding of cells to fibronectin substrata. To study the structure of this receptor, we have isolated cDNA clones coding for the human fibronectin receptor alpha subunit from a lambda gt11 placental cDNA library. The cDNAs code for 229 amino acids from the COOH terminus of the alpha subunit. The deduced sequence has a hydrophobic region with properties characteristic of a membrane-spanning domain. From the membrane-spanning domain to the COOH terminus are 23-28 amino acids that are likely to constitute the cytoplasmic domain. These results establish the fibronectin receptor alpha subunit as an integral membrane protein.     

Membrane topology of Bves/Pop1A,a cell adhesion molecule that displays dynamic changes in cellular distribution during development

We investigated the membrane topology of Bves/Pop1A as a foundation to dissect the molecular basis and function of Bves/Pop1A trafficking during development. Bves contains two asparagine-linked glycosylation sites within the amino terminus and three putative membrane domains. Therefore, glycosylation assays were performed to determine if the amino terminus of Bves is delivered into the endoplasmic reticulum lumen and glycosylated. We establish that Bves from chick heart and transfected cells is glycosylated, implying that the amino terminus of cell surface molecules is extracellular. Three biochemically distinct approaches were utilized to determine the orientation of the carboxyl terminus of Bves. First, glycosylation of Bves at exogenous sites within the carboxyl terminus was only observed in a construct that lacked the third membrane domain, which presumably reversed the orientation of the carboxyl terminus. Second, co-expression of full-length Bves with soluble, carboxyl-terminal Bves constructs that reside in different subcellular compartments revealed that Bves-Bves interactions occur in the cytoplasm. Third, the immunoreactivity of endogenous Bves at the cell surface of epicardial cells was dramatically enhanced with detergent. These results suggest that the membrane topology of cell surface Bves/Pop1A is composed of an extracellular amino terminus, three transmembrane domains, and a cytoplasmic carboxyl terminus. We therefore hypothesize that the carboxyl terminus regulates the cellular distribution of Bves/Pop1A during coronary vessel development.     

The signal recognition particle receptor alpha subunit assembles co-translationally on the endoplasmic reticulum membrane during an mRNA-encoded translation pause in vitro.

Many proteins, including the alpha subunit of the signal recognition particle receptor (SR alpha), are targeted within the cell by poorly defined mechanisms. A 140 residue N-terminal domain of SR alpha targets and anchors the polypeptide to the endoplasmic reticulum membrane by a mechanism independent of the pathway involving the signal recognition particle. To investigate the mechanism of membrane anchoring, translation pause sites on the SR alpha mRNA were used to examine the targeting of translation intermediates. A strong pause site at nucleotide 507 of the mRNA open reading frame corresponded with the shortest nascent SR alpha polypeptide able to assemble on membranes. An mRNA sequence at this pause site that resembles a class of viral -1 frameshift sequences caused translation pausing when transferred into another mRNA context. Site-directed mutagenesis of the mRNA greatly reduced translation pausing without altering the polypeptide sequence, demonstrating unambiguously a role for this mRNA sequence in translation pausing. SR alpha polypeptides synthesized from the non-pausing mRNA were impaired in co-translational membrane anchoring. Furthermore, co-translational membrane assembly of SR alpha appears to anchor polysomes translating SR alpha to membranes.     

Topology of the transposon Tn10-encoded tetracycline resistance protein within the inner membrane of Escherichia coli

The transposon Tn10-encoded tetracycline resistance protein TetA is an integral membrane protein responsible for the export of tetracycline from the cytoplasmic to the periplasmic side of the inner membrane of Gram-negative bacteria. From a plot of the average hydrophobicity along the sequence of this protein, a two-dimensional membrane topology with 12 transmembrane domains may be predicted. Using plasmid-bearing Escherichia coli maxicells we specifically radiolabeled the TetA protein. The amino terminus of this membrane protein was shown not to be processed, and its location on the inner side of the cytoplasmic membrane was demonstrated by a newly developed use of a chemical method. Spheroplasts and inside-out vesicles of the TetA protein synthesizing maxicells were subjected to limited digestion by proteases of different specificities. The TetA protein was not accessible to proteases from the periplasmic side. On the inner side of the cytoplasmic membrane, the carboxyl terminus and four sites accessible to endoproteases could be identified. The cleavage sites are proposed to be localized between amino acid residues 60-70, 110-130, 180-200, and at amino acid 327. These results allow the definition of a model for the two-dimensional topology of the TetA protein.     

Predicting the transmembrane secondary structure of ligand-gated ion channels

Recent mutational analyses of ligand-gated ion channels (LGICs) have demonstrated a plausible site of anesthetic action within their transmembrane domains. Although there is a consensus that the transmembrane domain is formed from four membrane-spanning segments, the secondary structure of these segments is not known. We utilized 10 state-of-the-art bioinformatics techniques to predict the transmembrane topology of the tetrameric regions within six members of the LGIC family that are relevant to anesthetic action. They are the human forms of the GABA alpha 1 receptor, the glycine alpha 1 receptor, the 5HT3 serotonin receptor, the nicotinic AChR alpha 4 and alpha 7 receptors and the Torpedo nAChR alpha 1 receptor. The algorithms utilized were HMMTOP, TMHMM, TMPred, PHDhtm, DAS, TMFinder, SOSUI, TMAP, MEMSAT and TOPPred2. The resulting predictions were superimposed on to a multiple sequence alignment of the six amino acid sequences created using the CLUSTAL W algorithm. There was a clear statistical consensus for the presence of four alpha helices in those regions experimentally thought to span the membrane. The consensus of 10 topology prediction techniques supports the hypothesis that the transmembrane subunits of the LGICs are tetrameric bundles of alpha helices.