Pentameric Complex Research Articles

Depolarization-induced PKC activities suppress myogenin-driven AChR γ gene promoter activity

NICOTINIC acetylcholine receptor ( AChR) is a pentameric complex (a2Py/e8), which is composed of four different subunits; its expression is regulated by neurally evoked electrical activity. Before innervation in skeletal muscle during embryonic stage, AChR is expressed on the fiber surface; after synaptic formation, the expression of AChR is restricted to the motor endplate. Denervation of the neonate or blockage of the electrical activity by sodium channel inhibitor enhances the AChR levels over the entire plasma membrane markedly. Conversely, reinnervation or electrostimulation of denervated "supersensitive" muscle leads to down-regulation of the AChR gene expression in extrajunction. T o understand the coupling mechanism of membrane depolarization and transcriptional activation of AChR genes is becoming more and more interesting. Klarsfeld et a l . ['I demonstrated that calcium and PKC are involved in the inhibition of AChR a subunit biosynthesis induced by electrical activity. Calcium influx through 1, type calcium channel on depolarized membrane rapidly inactivates the AChR geneL2]. Membrane depolarization activates nuclear PKC, which almost instantly leads to the inhibition of AChR genes[31 . I t appears as if a factor required for the expression of these genes is inactivated by phosphorylation. The kinetics of receptor gene expression following denervation and electrostimulation suggests that this factor should have autocatalytic properties[41. So myogenin, a member of myogenic regulatory factors, is a plausible candidate, and this hypothesis is supported by ~ f t i m i e ' ~ ' and other researchers. We have studied the effects of calcium and PKC on the coupling mechanism of excitationtranscription of AChR subunit gene using 50 mmol/L potassium instead of electrostimulation of CzClz cells, and have successfully mimicked the roles of myogenin in this signaling' pathway.

Structure of the transmembrane cysteine residues in phospholamban.

Phospholamban, a 52-residue membrane protein, associates to form a pentameric complex of five long alpha-helices traversing the sarcoplasmic reticulum membrane of cardiac muscle cells. The transmembrane domain of the protein is largely hydrophobic, with only three cysteine residues having polar side chains, yet it functions as a Ca2+-selective ion channel. In this report, infrared spectroscopy is used to probe the conformation of the three cysteine side chains and to establish whether the free S-H groups form intrahelical hydrogen bonds in the pentameric complex. Vibrational spectra of a transmembrane peptide were obtained which corresponded to the transmembrane domain of wild-type phospholamban and three peptides each containing a cysteine --> alanine substitution. The observed S-H frequencies argue that each of the sulfhydryl groups is hydrogen-bonded to an i-4 backbone carbonyl oxygen. Electrostatic calculations on a model of phospholamban based on molecular dynamics and mutagenesis studies, show that the sulfhydryl groups may significantly contribute to the electrostatic potential field of the protein.

Functional Expression of Soluble Human Interleukin-11 (IL-11) Receptor α and Stoichiometry of in Vitro IL-11 Receptor Complexes with gp130

The interleukin-6 (IL-6) family of cytokines activates signaling through the formation of either gp130 homodimers, as for IL-6, or gp130-leukemia inhibitory factor receptor (LIFR) heterodimers as for ciliary neurotrophic factor (CNTF), leukemia inhibitory factor, oncostatinM, and cardiotrophin-1. Recent in vitro studies with IL-6 and CNTF have demonstrated that higher order hexameric receptor complexes are assembled in which signaling chain dimerization is accompanied by the dimerization of both the cytokine molecule and its specific receptor alpha subunits (IL-6Ralpha or CNTFRalpha, respectively). IL-11 is a member of the IL-6 family and known to require gp130 but not LIFR for signaling. In this study we investigate the functional and biochemical composition of the IL-11 receptor complex. The human IL-11 receptor alpha-chain was cloned from a human bone marrow cDNA library. IL-11Ralpha was shown to confer IL-11 responsiveness to human hepatoma cells either by cDNA transfection or by adding a soluble form of the receptor (sIL11Ralpha) expressed in the baculovirus system to the culture medium. In vitro immunoprecipitation experiments showed that sIL11Ralpha specifically binds IL-11 and that binding is enhanced by gp130. Similarly to IL-6 and CNTF, gp130 is able to induce dimerization of the IL-11.IL-11Ralpha subcomplex, the result of which is the formation of a pentameric receptor complex. However, in contrast to the other two cytokines, IL-11 was unable to induce either gp130 homodimerization or gp130/LIFR heterodimerization. These results strongly suggest that an as yet unidentified receptor beta-chain is involved in IL-11 signaling.

Assembly of the B Subunit Pentamer of Escherichia coli Heat-labile Enterotoxin: KINETICS AND MOLECULAR BASIS OF RATE-LIMITING STEPS IN VITRO

The B subunits of Escherichia coli heat-labile enterotoxin (EtxB) and cholera toxin (CtxB) assemble in vivo into exceptionally stable homopentameric complexes, which maintain their quaternary structure in a range of conditions that would normally be expected to cause protein denaturation. Recently, we showed that the simultaneous protonation of two of the COOH-terminal carboxylates in pentameric EtxB was required to cause its disassembly at pH values below 2.0 (Ruddock, L., Ruston, S. P., Kelly, S. M., Price, N. C., Freedman, R. B., and Hirst, T. R.(1995) J. Biol. Chem. 270, 29953-29958). Here, we investigate the influence of environmental parameters on the kinetics of reassembly of acid-generated EtxB monomers in vitro. Such monomers were found to undergo a further acid-mediated conformational change, with an activation energy of 76 +/- 2 J.mol-1.K-1, consistent with isomerization of the cis-proline residue at position 93, and which prevented subsequent EtxB reassembly. By using rapid neutralization of acid-generated monomers, a high proportion of the B-subunits adopted an assembly-competent conformation, which resulted in up to 75% of the protein reassembling into a stable pentameric complex, indistinguishable from native EtxB pentamers. The rate-limiting step in reassembly, over a concentration range of 50-200 microg/ml, was shown to be due to an intramolecular event, which exhibited a pH dependence with a pKa of 7.0. Modification of EtxB with amine-specific probes revealed that the protonation state of the NH2-terminal alanine residue was responsible for the pH dependence of reassembly. The implications of these findings for the biogenesis of Escherichia coli enterotoxin and related enterotoxins in vivo, are considered.

Reconstitution of a pentameric complex of dimeric transforming growth factor beta ligand and a type I, II, III receptor in baculoviral-infected insect cells.

Two transmembrane serine-threonine kinases (type I and II receptors), a membrane-anchored proteoglycan (type III), and a homodimeric ligand participate in the transforming growth factor beta type one (TGF beta 1) signal transduction complex. The expression of recombinant receptors in insect cells co-infected with up to three recombinant baculoviruses was employed to study interactions among the ectodomains of the three types of receptors and the TGF beta 1 ligand in absence of uncontrollable extrinsic factors in mammalian cells. Multi-subunit complexes were assembled in intact cells and purified on glutathione-conjugated beads for analysis by tagging one of the subunits with glutathione S-transferase (GST). Intrinsic ligand-independent interactions were observed among receptor subunits as follows: type III-III, type I-I, type III-I, and type II-I. The homeotypic complex of type II-II receptors and the heterotypic type III-II interaction was ligand dependent. The type I, but not the type III, subunit displaced about 50% of the type II component in either ligand-dependent homomeric type II-type II complexes or heteromeric type III-type II complexes to form type II-I or type III-II-I oligomers, respectively. The type II subunit displaced type I subunits in oligomers of the type I subunit. Specificity of type I receptors may result from differential affinity for the type II receptor rather than specificity for ligand. A monomeric subunit of the TGF beta 1 ligand bound concurrently to type III and type II or type III and type I receptors, but failed to concurrently bind to the type II and type I subunits. The binding of TGF beta 1 to the type I kinase subunit appears to require an intact disulfide-linked ligand dimer in the absence of a type III subunit. The combined results suggest a pentameric TGF beta signal transduction complex in which one unit each of the type III, type II, and type I components is assembled around the two subunits of the dimeric TGF beta ligand. An immobilized GST-tagged subunit of the receptor complex was utilized to assemble multi-subunit complexes in vitro and to study the phosphorylation events among subunits in the absence of extrinsic cell-derived kinases. The results revealed that (a) a low level of ligand-independent autophosphorylation occurs in the type I kinase; (b) a high level of autophosphorylation occurs in the type II kinase; (c) both the type III and type I subunits are trans-phosphorylated by the type II subunit; and (d) the presence of both type I and II kinases complexed with the type III subunit and dimeric TGF beta 1 ligand in a pentameric complex causes maximum phosphorylation of all three receptor subunits.

Role of the Nuclear Transport Factor p10 in Nuclear Import

The nuclear import factor p10 was cloned from Saccharomyces cerevisiae and found to be essential. The protein p10 can bind directly to several peptide repeat-containing nucleoporins. It also binds to the guanosine triphosphatase (GTPase) Ran in its guanosine diphosphate (GDP)-bound form and to karyopherin beta. Assembly of the karyopherin heterodimer on immobilized nucleoporin yielded cooperative binding of p10 and Ran-GDP. Addition of GTP to this pentameric complex led to dissociation of karyopherin (chi, presumably via in situ formation of Ran-GTP from Ran-GDP. Thus, p10 appears to coordinate the Ran-dependent association and dissociation reactions underlying nuclear import.

Mapping the lipid-exposed surfaces of membrane proteins

Phospholamban forms a stable complex of five long transmembrane helices. We show that the relative rotational orientation of the helices in the pentameric complex can be distinguished by S-H to S-D exchange of cysteine sulphydryl groups located in the transmembrane segment of the protein and exposed to the lipid environment. Of the three cysteine residues in phospholamban, two residues (Cys 36 and Cys 46) are oriented towards the helix interface and protected from exchange, while the third cysteine (Cys 41) is oriented towards the lipid interface and undergoes exchange with water diffused into the bilayer. Distinguishing the external and internal faces of a membrane protein by sulphydryl exchange provides a general approach for determining the three-dimensional fold of membrane proteins and enhances model building efforts to generate high-resolution structures.

The Large Ribosomal Subunit Stalk as a Regulatory Element of the Eukaryotic Translational Machinery

Publisher Summary This chapter reviews the structural and functional properties of different eukaryotic stalk components, focusing on the Saccharomyces cerevisiae system for which an extensive analysis has been carried out. One of the most characteristic structural features of the large ribosomal subunit is the stalk—a highly flexible lateral protuberance that, in bacterial particles, is formed by a pentameric protein complex of two dimers of proteins, L7 and L12, and one copy of protein L10. Proteins L7 and L12—the N-terminal acetylated and nonacetylated forms of a unique, strongly acidic polypeptide—form in solution stable dimers that have been proposed to have an elongated structure with a globular carboxyl domain and a rigid amino end joined by a flexible “hinge.” The corresponding components of the stalk and the GTPase domain in eukaryotic ribosomes have been identified and characterized. They show characteristics that resemble closely those of their bacterial counterparts, and it is generally accepted that they must play an analogous functional role in the basic protein synthesis machinery, facilitating also the interaction of elongation factors.

Analysis of tacrolimus (FK 506) in relation to therapeutic drug monitoring.

Tacrolimus (FK506) is a macrolide lactone which is used as an immunosuppressant agent for organ transplantation. Patient monitoring during clinical trials initially involved use of extraction/enzyme immunoassays (ELISA) for plasma and whole blood concentrations of FK506. Therapeutic concentrations of FK506 are approximately 0.4-1.2 ng/ml in plasma and 5-20 ng/ml in whole blood. Blood:plasma ratios are variable among patients and range from 11 to 114. A validation and quality assurance program was established to standardize procedures and assure concordance of ELISA analytic results from clinical sites participating in liver/kidney trials. HPLC/ELISA methods have been used for assessing assay specificity. Comparison of the ELISA method with a HPLC-MS procedure (TexMS) shows good correlation, but indicates metabolite inclusion in the ELISA. This method may be of value in conditions of altered FK-506 metabolism. Commercialization of the immunoassay has been made by Incstar with the ELISA and Abbott Laboratories with an IMx procedure; the two methods are generally comparable. Additional methods offering clinical promise are radioreceptor and pentamer complex ELISA assays. There are diverse methods offering sensitive quantitation but with varying specificity for tacrolimus in patient plasma and whole blood.

Analysis of the conserved glycosylation site in the nicotinic acetylcholine receptor: potential roles in complex assembly

Background: Assembly of the functional nicotinic acetylcholine receptor (nAChR) is dependent on a series of exquisitely coordinated events including polypeptide synthesis and processing, side-chain elaboration through post-translational modifications, and subunit oligonaerization. A 17-residue sequence that includes a cystine disulfide and an N-linked glycosylation site is conserved in the extracellular domain of each of the nAChR subunits, and is involved in intersubunit interactions that are critical for assembly of intact, pentameric complexes. A polypeptide representing the relevant sequence from the α-subunit of the nAChR (Ac-Tyr-Cys-Glu-Ile-Ile-Val-Thr-His-Phe-Pro-Phe-Asp-Gln-Gln-Asn-Cys-Thr-NH 2) is small enough to allow detailed structural analysis, which may provide insight into the role of glycosylation in the maturation process that leads to ion-channel assembly. We therefore investigated the effect of N-linked glycosylation on the structure of this heptadecapeptide. Results: Thermodynainic analysis shows that glycosylation alters disulfide formation in the loop peptide, shifting the equilibrium in favor of the disulfide. Spectroscopic studies reveal that the cis/trans amide isomer ratio of the proline is also affected by the modification, with a resultant shift in the equilibrium in favor of the trans isomer, even though the proline is several residues removed from the glycosylation site. Two-dimensional NMR analysis of the glycopeptide does not indicate the presence of any specific interactions between the carbohydrate and the peptide. Conclusions: These studies demonstrate that glycosylation can have a significant influence on disulfide formation and proline isomerization in a local peptide sequence. As both these processes are considered slow steps in protein folding, it is evident that N-linked glycosylation has important indirect roles that influence the folding of the receptor subunit and assembly of the pentameric complex.

Role of the Endoplasmic Reticulum Chaperone Calnexin in Subunit Folding and Assembly of Nicotinic Acetylcholine Receptors

The nicotinic acetylcholine receptor (AChR) is a pentameric complex assembled from four different gene products by mechanisms that are inadequately understood. In this study we investigated the role of the endoplasmic reticulum (ER)-resident molecular chaperone calnexin in AChR subunit folding and assembly. We have shown that calnexin interacts with nascent AChR alpha-subunits (AChR-alpha) in muscle cell cultures and in COS cells transfected with mouse AChR-alpha. In chick muscle cells maximal association of labeled alpha-subunits with calnexin was observed immediately after a 15-min pulse with [35S]methionine/cysteine and subsequently declined with a t1/2 of approximately 20 min. The decrease in association with calnexin was concomitant with the folding of the alpha-subunit to achieve conformational maturation shortly before assembly. Brefeldin A did not inhibit AChR subunit assembly or the dissociation of calnexin from the assembling subunits, confirming that the ER is the site of AChR assembly and that calnexin dissociation is not affected under conditions in which the exit of assembled AChR from the ER is blocked. These results indicate that calnexin participates directly in the molecular events that lead to AChR assembly.

Structural Model of the Phospholamban Ion Channel Complex in Phospholipid Membranes

Phospholamban is a 52 amino acid residue membrane protein involved with the regulation of calcium levels across sarcoplasmic reticulum membranes in cardiac muscle cells. The N-terminal 30 amino acid residues of the protein are largely hydrophilic and include two sites whose phosphorylation is thought to dissociate an inhibitory complex between phospholamban and Ca2+ATPase. The C-terminal 22 amino acid residues are largely hydrophobic, anchor the protein in the membrane and are responsible for Ca2+selective ion conductance. Specific interactions between the transmembrane domains stabilize a pentameric protein complex. We have obtained circular dichroism (CD), transmission Fourier transform infrared (FTIR) and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of the full-length protein and have compared these results to those from a 28 residue peptide that includes the transmembrane domain. Both proteins reconstituted into phospholipid membranes are largely α-helical by CD and FTIR. Polarized ATR-FTIR measurements show that both the cytosolic and transmembrane helices are oriented perpendicular to the membrane plane with a tilt of 28(±6)° with respect to the membrane normal. This tilt angle is in close agreement to that calculated from a model for the transmembrane domain of phospholamban suggested by mutagenesis and molecular modeling. Phosphorylation does not significantly change the secondary structure or orientation of the protein. The pentameric complex is modeled as a left-handed coiled-coil of five long helices (40(±3) residues) that extend across the membrane from the lumenal carboxy terminus to the phosphorylation site in the cytoplasm. The helix bundle forms a perpendicular ion pore that may begin at a distance (17 to 29 Å) from the membrane surface. Based on the above, we propose a mechanism by which phospholamban regulates Ca2+levels across membranes that takes into account both its selective ion conductance and inhibitory association with the Ca2+pump.

GTP consumption of elongation factor Tu during translation of heteropolymeric mRNAs.

The stoichiometry of elongation factor Tu (EF-Tu) and GTP in the complex with aminoacyl-tRNA and the consumption of GTP during peptide bond formation on the ribosome were studied in the Escherichia coli system. The ribosomes were programmed either with two different heteropolymeric mRNAs coding for Met-Phe-Thr-Ile ... (mMFTI) or Met-Phe-Phe-Gly ... (mMFFG) or with poly(U). The composition of the complex of EF-Tu, GTP, and Phe-tRNA(Phe) was studied by gel chromatography. With equimolar amounts of factor and Phe-tRNA(Phe), a pentameric complex, (EF-Tu.GTP)2.Phe-tRNA(Phe), was observed, whereas the classical ternary complex, EF-Tu.GTP.Phe-tRNA(Phe), was found only when Phe-tRNA(Phe) was in excess. Upon binding of the purified pentameric complex to ribosomes carrying fMet-tRNA(fMet) in the peptidyl site and exposing a Phe codon in the aminoacyl site, only one out of two GTPs of the pentameric complex was hydrolyzed per Phe-tRNA bound and peptide bond formed, regardless of the mRNA used. In the presence of EF-G, the stoichiometry of one GTP hydrolyzed per peptide bond formed was found on mMFTI when one or two elongation cycles were completed. In contrast, on mMFFG, which contains two contiguous Phe codons, UUU-UUC, two GTP molecules of the pentameric complex were hydrolyzed per Phe incorporated into dipeptide, whereas the incorporation of the second Phe to form tripeptide consumed only one GTP. Thus, generally one GTP is hydrolyzed by EF-Tu per aminoacyl-tRNA bound and peptide bond formed, and more than one GTP is hydrolyzed only when a particular mRNA sequence, such as a homopolymeric stretch, is translated. The role of the additional GTP hydrolysis is not known; it may be related to frameshifting of peptidyl-tRNA during translocation.

Stoichiometry for the elongation factor Tu.aminoacyl-tRNA complex switches with temperature.

In bacterial protein synthesis binding of an aminoacyl-transferRNA (aa-tRNA) to the ribosomal acceptor site (A-site) is catalyzed by elongation factor Tu (EF-Tu). Two guanosine triphosphates (GTPs) are hydrolyzed on EF-Tu for every bound aa-tRNA. This was rationalized by the notion of an extended ternary complex, consisting of two EF-Tu.GTPs bound to a single aa-tRNA. In this work, we combine fast kinetics with RNase A protection experiments to measure the stoichiometry between EF-Tu.GTP and aa-tRNA at 37 degrees C, where the binding is weak. We find a 2:1 stoichiometry between EF-Tu.GTP and aa-tRNA at 37 degrees C, but at 0 degree C, under otherwise similar conditions, the stoichiometry of the complex is close to 1:1. These results suggest alternative pathways for aa-tRNA binding to ribosomes, since two GTPs are hydrolyzed per peptide bond at both temperatures. At 37 degrees C, aa-tRNA enters the A-site in a pentameric complex with two EF-Tu's on which two GTPs are hydrolyzed in synchrony, when cognate codon-anticodon contact is established. This pentameric model also explains how two GTPs can be hydrolyzed on EF-Tu, without rejection of 50% of the cognate aa-tRNAs in proofreading. At 0 degree C, in contrast, two ordinary ternary complexes may form a pentameric complex on, rather than off, the ribosome. When the two EF-Tu bound GTPs are hydrolyzed, one aa-tRNA enters the A-site, and the other dissociates to the free state.

The subunit structure of elongation factor 1 from Artemia. Why two alpha-chains in this complex?

Elongation factor 1 (EF-1) regulates the specific interaction of aminoacyl-tRNA with the ribosome during the elongation phase of protein biosynthesis. Although individual functions of its separate chains have been well defined, to date there is hardly information about the structure and function of the whole complex. We describe here the complete subunit structure of elongation factor 1, and discuss its change during development of Artemia. Elongation factor 1 consists of a pentameric complex, composed of four different subunits alpha, beta, gamma, and delta in a molar ratio of 2:1:1:1. Although one molecule of EF-1 alpha dissociates easily from the complex EF-1 alpha 2 beta gamma delta under the influence of aminoacyl-tRNA and GTP, the second molecule of EF-1 alpha was found to remain firmly attached. Thus, in eukaryotic protein synthesis, movement of transfer RNAs to the ribosome seems under the influence of two distinct molecules of EF-1 alpha, a result possibly related to the presumed consumption of two molecules of GTP by EF-Tu during the elongation step of prokaryotic protein synthesis.

CDNA-derived primary structure of the 25-kDa subunit of canine microsomal signal peptidase complex.

We purified the 25-kDa subunit of the pentameric signal peptidase complex (SPC 25) from canine pancreas microsomes and utilized amino acid sequence data from tryptic fragments for its molecular cloning and sequencing. Its primary structure (calculated molecular mass of 25,831 Da) shows the presence of two hydrophobic domains of which one is likely to serve as a transmembrane segment. Consistent with being the only SPC subunit with at least one intrachain disulfide bond, SPC 25 contains 4 Cys residues. SPC 25 does not contain a cleavable signal peptide, and it is proposed that the transmembrane segment doubles as signal sequence. The primary structure of SPC 25 is unrelated to any of the other three SPC subunits (SPC 22/23, SPC 21, and SPC 18) that have so far been molecularly cloned and sequenced. Neither is it similar to any protein in the data bases, except that it is 95% identical to an open reading frame of an incomplete and randomly isolated human cDNA clone.

ATP Synthase from Bovine Heart Mitochondria In Vitro Assembly of a Stalk Complex in the Presence of F1-ATPase and in its Absence

Four subunits of the F1F0-ATPase from bovine heart mitochondria have been produced by heterologous over-expression in Escherichia coli . They are the oligomycin sensitivity conferral protein (OSCP), coupling factor 6 (F6) and subunits b and d. Likewise, fragments b′, bI, bC, and bM (amino acid residues 79 to 214, 121 to 214, 165 to 214 and 79 to 164, respectively, of subunit b), and fragment ′ (subunit d lacking residue 1 to 14) have been produced in abundant quantities by bacterial expression. These subunits, and the fragments of subunits b and d, have been assayed singly and in various combinations by gel-filtration chromatography for their abilities to bind to bovine heart F1-ATPase. Only the OSCP was found to be capable of forming a stable binary complex with F1-ATPase. When fragments b′, bI or bC were added to F1-ATPase together with the OSCP, the ternary complexes F1·OSCP·b′, F1-OSCP-bI or F1·OSCP·bC were formed, but b′, bI and bC appeared to be present in sub-stoichiometric amounts. When F6 was added also, then the stoichiometric quaternary complexes F1·OSCP·b′·F6 and F1·OSCP·bI·F6 were obtained, as was a fourth quaternary complex containing approximately equivalent amounts of F1 and OSCP, and sub-stoichiometric quantities of bC and F6. Finally, three pentameric complexes F1·OSCP·b′·F6·d, F1·OSCP·b′·F6·′ and F1·OSCP·b·F6·d were isolated. In a further series of reconstitution experiments, the binary complexes b′·OSCP and b′·d, the ternary complex b′·′·F6, and the quaternary complex OSCP·b′·F6·d were obtained. The pre-formed quaternary complex produced a stoichiometric pentameric complex with F1-ATPase. It was shown by S-carboxymethylation of cysteine residues with iodo-[2-14C]acetic acid that bovine F1F0-ATPase and the reconstituted F1·stalk complex, F1·OSCP·b′·d·F6, each contained one copy per complex of subunits b (or b′), OSCP and d, and that the separate stalk complex contained the same three subunits in the approximate molar ratio 1 : 1 : 1. The ratio of b to d in purified F0 was 1 : 1. Finally, it was demonstrated that the binding of the various subunits to F1-ATPase increases the ATP hydrolase activity and diminishes its inactivation by exposure to cold. These assembly experiments help to define some of the inter-subunit interactions in the stalk region of the F1F0-ATPase complex, and they are an essential step forward towards the goal of extending the high-resolution structure of bovine F1-ATPase into the stalk.

ATP Synthase from Bovine Heart Mitochondria: In Vitro Assembly of a Stalk Complex in the Presence of F1-ATPase and in its Absence

Four subunits of the F1F0-ATPase from bovine heart mitochondria have been produced by heterologous over-expression in Escherichia coli . They are the oligomycin sensitivity conferral protein (OSCP), coupling factor 6 (F6) and subunits b and d. Likewise, fragments b′, bI, bC, and bM (amino acid residues 79 to 214, 121 to 214, 165 to 214 and 79 to 164, respectively, of subunit b), and fragment ′ (subunit d lacking residue 1 to 14) have been produced in abundant quantities by bacterial expression. These subunits, and the fragments of subunits b and d, have been assayed singly and in various combinations by gel-filtration chromatography for their abilities to bind to bovine heart F1-ATPase. Only the OSCP was found to be capable of forming a stable binary complex with F1-ATPase. When fragments b′, bI or bC were added to F1-ATPase together with the OSCP, the ternary complexes F1·OSCP·b′, F1-OSCP-bI or F1·OSCP·bC were formed, but b′, bI and bC appeared to be present in sub-stoichiometric amounts. When F6 was added also, then the stoichiometric quaternary complexes F1·OSCP·b′·F6 and F1·OSCP·bI·F6 were obtained, as was a fourth quaternary complex containing approximately equivalent amounts of F1 and OSCP, and sub-stoichiometric quantities of bC and F6. Finally, three pentameric complexes F1·OSCP·b′·F6·d, F1·OSCP·b′·F6·′ and F1·OSCP·b·F6·d were isolated. In a further series of reconstitution experiments, the binary complexes b′·OSCP and b′·d, the ternary complex b′·′·F6, and the quaternary complex OSCP·b′·F6·d were obtained. The pre-formed quaternary complex produced a stoichiometric pentameric complex with F1-ATPase. It was shown by S-carboxymethylation of cysteine residues with iodo-[2-14C]acetic acid that bovine F1F0-ATPase and the reconstituted F1·stalk complex, F1·OSCP·b′·d·F6, each contained one copy per complex of subunits b (or b′), OSCP and d, and that the separate stalk complex contained the same three subunits in the approximate molar ratio 1 : 1 : 1. The ratio of b to d in purified F0 was 1 : 1. Finally, it was demonstrated that the binding of the various subunits to F1-ATPase increases the ATP hydrolase activity and diminishes its inactivation by exposure to cold. These assembly experiments help to define some of the inter-subunit interactions in the stalk region of the F1F0-ATPase complex, and they are an essential step forward towards the goal of extending the high-resolution structure of bovine F1-ATPase into the stalk.

Interaction of Tacrolimus (FK506) and Its Metabolites with FKBP and Calcineurin

Tacrolimus(FK506) is a strong immuno-suppressant and shows its activity through inhibiting IL-2 mRNA transcription by forming pentameric complex with intracellular receptor(FK506 binding protein 12 KDa or FKBP12), Ca 2+, calmodulin, and calcineurin. Here, we report the binding activity to FKBP12, the pentameric complex formation and Con-A response inhibiting activities of 7 metabolites. C15-demethylated metabolite(M-3) needed higher quantity to compete in Con-A assay and in pentamer formation assay, although it binds more strongly to FKBP12. The result suggests that the ability to form a pentameric complex is not a two step reaction with the first binding to FKBP12, but a single step reaction by components for the pentamer formation.

Role of phosphorylation in desensitization of acetylcholine receptors expressed in Xenopus oocytes

The nicotinic acetylcholine receptor (AChR) is a pentameric complex made up of four types of subunits in the stoichiometry alpha 2 beta gamma delta. These subunits have been shown to be differentially phosphorylated by cAMP-dependent protein kinase (PKA) protein kinase C, and a protein tyrosine kinase. A variety of studies have suggested that phosphorylation of the AChR in vitro and in vivo regulates the rate of desensitization of the receptor. In this study we have used site-specific mutagenesis and patch-clamp techniques to examine the role of phosphorylation in the regulation of desensitization of the AChR expressed in Xenopus oocytes Expression of wild-type AChR in Xenopus oocytes results in the constitutive phosphorylation of the AChR on the gamma and delta subunits. This phosphorylation is apparently due to the high basal level of PKA in oocytes since a specific peptide inhibitor of PKA completely eliminated phosphorylation of the AChR by oocyte extracts in vitro. The phosphorylation of the AChR in oocytes was not significantly enhanced by forskolin or cAMP analogs or by coexpression with the catalytic subunit of PKA, suggesting that the basal activity of PKA in oocytes is sufficient to phosphorylate the receptor to a high stoichiometry. Using site-specific mutagenesis, the sites of phosphorylation were determined to be serines 353 and 354 on the gamma subunit and serines 361 and 362 on the delta subunit. To examine the functional properties of wild-type and mutant receptors lacking phosphorylation sites, we used patch-clamp techniques to measure the responses of out-side-out patches to repetitive pulses of ACh using a rapid perfusion system. Wild-type and mutant receptors showed rapid concentration-dependent activation and desensitization to applied agonist. The time constant of desensitization of ensemble mean currents ranged from several hundred milliseconds at low ACh concentrations to 100-200 msec at saturating concentrations. The desensitization time constants for mutant receptors lacking all phosphorylation sites were significantly slower than wild-type phosphorylated receptors at all concentrations of ACh tested. In addition, mutant receptors that had the serine residues changed to glutamate residues in order to mimic the negative charge of the phosphorylated serine residue produced receptors that had desensitization rates approaching those of the wild-type phosphorylated receptor. These results provide further support that phosphorylation of the nicotinic ACh receptor regulates rate of desensitization.