bluuebunny/crossref_metadata_2025_split huggingface dataset:
The contractile proteins of dictyostelium discoideum
Authors: Spudich James A. , Clarke Margaret 
Published: May 2005

AbstractWe have purified actin and my osin‐like proteins from amoebae of Dictyostelium discoideum. These proteins are very similar in their physical and enzymatic properties to muscle actin and myosin. Most importantly, they form thin and thick filaments, respectively, and Dictyostelium actin activates Dictyostelium myosin ATPase activity. Actin from these amoebae appears to be identical in size to muscle actin. The Dictyostelium myosin consists of two heavy chains of about 210,000 daltons and two classes of light chains, about 18,000 and 16,000 daltons. The heavy chains are slightly larger than those of muscle myosin. Biochemical and structural studies of membrane association of the contractile complex suggests that some of the amoeba actin is membrane‐bound and acts as an attachment point for myosin and other actin filaments.
Assembly of lipid‐containing viruses
Authors: Compans Richard W. , Meier‐Ewert Herbert , Palese Peter 
Published: May 2005

AbstractEnveloped viruses which form by budding at the cell surface possess a membrane consisting of a lipid bilayer and a small number of virus‐coded polypeptides. Since viral polypeptides become integral components of the plasma membrane during assembly, the process of synthesis and incorporation into membranes of these proteins may reflect the pathway of plasma membrane assembly. Electron microscopic studies have suggested that viral envelope proteins are incorporated into discrete, localized regions of the plasma membrane which serve as recognition sites for the viral nucleocapsid. In influenza virus‐infected cells, viral polypeptides are associated with cytoplasmic membranes as well as the plasma membrane. The major envelope glycoprotein appears to be synthesized in rough endoplasmic reticulum, and to migrate to smooth membranes after synthesis. Glycosylation is initiated in rough membranes and progresses further in smooth membranes. Unlike the glycoproteins, the major nonglycosylated polypeptide appears to be inserted directly into the plasma membrane. In the presence of 2‐deoxyglucose or high concentrations of glucosamine, aberrant viral glycoproteins are detected which appear to be unglycosylated or partially glycosylated; these are associated with membranes and incorporated into virus particles of reduced infectivity. Therefore normal glycosylation is not essential for incorporation of viral glycoproteins into cellular membranes or virus particles, but is required for normal biological activity. The role of the viral neuraminidase in assembly and release has been studied using mutants defective in neuraminidase at restrictive temperature. Under these conditions virus formation occurs, but the progeny form large aggregates at the cell surface. Colloidal iron hydroxide staining indicates that such virus particles contain neuraminic acid, and these residues appear to serve as receptors leading to the extensive aggregation.
Interactions between collagen chains and fiber formation
Authors: Fessler John H. , Tandberg William D. 
Published: May 2005

AbstractThe temperature‐dependent dissociation of neutral salt‐soluble collagen into its component chains was measured in 0.6–1.6 M urea solutions at pH 7.3. The temperature‐dependent association of the same radiocactively labeled collagen into fibers was measured in 0–0.4 M urea solutions, pH 7.3. The effect of urea on the temperature, Tm(G), for half dissociation into chains was small, and the value extrapolated to zero urea concentration was 39°C. In contrast, the effect of urea on the temperature, Tm(F), for half association into fibers was large, and the value at zero urea concentration was 30°C.We conclude that while body temperature provides excellent conditions for the matching of collagen chains to form molecules, the conditions are not optimal for the formation of highly ordered fibers. The large effects of 0.1 M urea suggest that other factors in vivo may help to destabilize mismatched molecular association during fiber growth. Alternately this might be facilitated by parts of the extension peptides of procollagen.
Assembly of the tail of bacteriophage T4
Authors: Kikuchi Yoshiko , King Jonathan 
Published: May 2005

AbstractThe protein products of at least 21 phage genes are needed for the formation of the tail of bacteriophage T4. Cells infected with amber mutants defective in these genes are blocked in the assembly process. By characterizing the intermediate structures and unassembled proteins accumulating in mutant‐infected cells, we have been able to delineate most of the gene‐controlled steps in tail assembly. Both the organized structures and unassembled proteins serve as precursors for in vitro tail assembly.We review here studies on the initiation, polymerization, and termination of the tail tube and contractile sheath and the genetic control of these processes. These studies make clear the importance of the baseplate; if baseplate formation is blocked (by mutation) the tube and sheath subunits remain essentially unaggregated, in the form of soluble subunits.Seventeen of the 21 tail genes specify proteins involved in baseplate assembly. The genes map contiguously in two separate clusters, one of nine genes and the other of eight genes. Recent studies show that the hexagonal baseplate is the end‐product of two independent subassembly pathways. The proteins of the first gene cluster interact to form a structure which probably represents one‐sixth of the outer radius. The products of the other gene cluster interact to form the central part of the baseplate.Most of the phage tail precursor proteins appear to be synthesized in a non‐aggregating form; they are converted to a reactive form upon incorporation into preformed substrate complexes, without proteolytic cleavage. Thus reactive sites are limited to growing structures.
The formation of filamentous structures from iodinated neurotubules
Authors: Gaskin Felicia , Litman David J. , Cantor Charles R. , Shelanski Michael L. 
Published: May 2005

AbstractThe porcine neurotubule and its basic subunit were found to be modified in vitro by iodination of amino acids (principally tyrosine) using lactoperoxidase. Iodide ion, H2O2, or lactoperoxidase singly or in any pairwise combination had virtually no effect on neurotubules. However, when all three reagents were present, permitting covalent iodination, it was found that at 0.1 iodotyrosines per tubulin dimer the microtubules unravel to form structures which morphologically resemble strands of protofilaments twisted or wound around each other. These abnormal tubules are stable at room temperature and 4°C. Both monomers of tubulin are labeled to approximately the same extent. Iodinated tubulin (0.1 iodotyrosines/dimer) is unable to assemble in vitro under normal assembly conditions. Heavily iodinated microtubules (8 iodines per tubulin dimer) are similar in morphology to the slightly iodinated structures.
Interaction of the 5′‐ends of 28S RNA in dimerization of hamster ribosomes
Authors: Silverman David J. , Lubin Martin 
Published: May 2005

AbstractFree ribosomes extracted from hamster cells and 28S RNA purified from these ribosomes are known to form dimers. We find that spleen phosphodiesterase inhibits ribosomal dimer formation, but only when a free 5′‐hydroxyl end group, produced by the action of alkaline phosphatase, is present. Hence, formation of dimer ribosomes probably involves interaction at or near the phosphorylated 5′‐ends of 28S RNA.Dimer RNA molecules show a modal length, when measured on electron micrographs, of 2.1μm, which is about double the length of 28S RNA. Electron micrographs of 115S dimer ribosomes often show profiles consistent with our interpretation that in dimers the 28S RNA chains are loosely linked by their 5′‐ends.
Transient kinetic and isotopic tracer studies of the myosin adenosine triphosphatase reaction
Authors: Bagshaw C. R. , Trentham D. R. 
Published: May 2005

AbstractFrom transient kinetic studies of the Mg2+‐dependent adenosine triphosphatase of myosin subfragment 1, prepared from rabbit skeletal muscle, a seven‐step mechanism has been proposed. Features of this mechanism include two‐step processes for ATP and ADP binding in which the binary complex isomerizes in addition to a rapid nucleotide association step. In the case of ATP a large negative standard free energy change is associated with the isomerization. An overall rate‐limiting isomerization of the myosin‐product complex prior to product release has been identified. Studies on the mechanism of cleavage of ATP bound to the active site indicate the process is readily reversible and can account for the observation that more than one oxygen of the product phosphate arises from water. This proposal has been substantiated by the finding that the oxygen atoms of the γ‐phosphoryl group of bound ATP also undergo extensive exchange with water.
Structure and function of cholera toxin and hormone receptors
Authors: Bennett V. , Craig S. , Hollenberg M. D. , O'Keefe E. , Sahyoun N. , Cuatrecasas P. 
Published: May 2005

AbstractThe enterotoxin from Vibrio cholerae is a protein of 100,000 mol wt which stimulates adenylate cyclase activity ubiquitously. The binding of biologically active 125I‐labeled choleragen to cell membranes is of extraordinary affinity and specificity. The binding may be restricted to membrane‐bound ganglioside GMI. This ganglioside can be inserted into membranes from exogenous sources, and the increased toxin binding in such cells can be reflected by an increased sensitivity to the biological effects of the toxin. Features of the toxin‐activated adenylate cyclase, including conversion of the enzyme to a GTP‐sensitive state, and the increased sensitivity of activation by hormones, suggest analogies between the basic mechanism of action of choleragen and the events following binding of hormones to their receptors. The action of the toxin is probably not mediated through intermediary cytoplasmic events, suggesting that its effects are entirely due to processes involving the plasma membrane. The kinetics of activation of adenylate cyclase in erythrocytes from various species as well as in rat adipocytes suggest a direct interaction between toxin and the cyclase enzyme which is difficult to reconcile with catalytic mechanisms of adenylate cyclase activation. Direct evidence for this can be obtained from the comigration of toxin radioactivity with adenylate cyclase activity when toxin‐activated membranes are dissolved in detergents and chromatographed on gel filtration columns. Agarose derivatives containing the “active” subunit of the toxin can specifically adsorb adenylate cyclase activity, and specific antibodies against the choleragen can be used for selective immunoprecipitation of adenylate cyclase activity from detergentsolubilized preparations of activated membranes. It is proposed that toxin action involves the initial formation of an inactive toxin‐ganglioside complex which subsequently migrates and is somehow transformed into an active species which involves relocation within the two‐dimensional structure of the membrane with direct pertubation of adenylate cyclase molecules (virtually irreversibly). These studies suggest new insights into the normal mechanisms by which hormone receptors modify membrane functions.
The exchange of erythrocyte membrane phospholipids with rat liver extracts in vitro
Authors: Steck Theodore L. , Wackman Nancy , Tarlov Alvin R. 
Published: May 2005

AbstractIntact rat or human erythrocytes and their isolated (ghost) membranes were incubated with the high speed supernatant fraction of homogenates derived from 32P‐labeled rat livers. Phospholipid molecules were transferred between the red cell membranes and the liver extracts, as reflected by the convergence of their specific radioactivities with time. Whereas ghosts usually approached isotopic equilibrium with the liver supernatant fraction during a few hours of incubation at 37° C, the exchange of phospholipids by intact cells was no more than one‐half, even after 18 hr. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and sphingomyelin were all exchanged in both intact cells and ghosts, albeit to different extents. (A control experiment, incubating 32P‐labeled rat erythrocytes or ghosts with unlabeled rat liver extracts, also demonstrated the exchange of all four major phospholipids.) These data may signify that the phospholipids on the cytoplasmic side of the membrane of intact erythrocytes do not exchange with the phospholipids in exogenous liver extracts. If so, all four major phospholipid classes would appear to be present to some extent at both membrane surfaces. The first inference is in agreement with several other studies on this membrane, while the second inference is not.
Lesion‐Induced synaptogenesis in brain: A study of dynamic changes in neuronal membrane specializations
Authors: Cotman Carl W. 
Published: May 2005

AbstractWhen incoming fibers to a given brain region are damaged and degenerate, the remaining undamaged fibers can, in some cases, form new synapses, and restore physiologically functional circuitry. Synaptic membrane events underlie this reconstruction: the connection between membranes is broken and reformed. In order to understand these membrane events, it is necessary to know the molecular composition of the synapse and the nature of the interaction between pre‐ and postsynaptic membranes. The synaptic membranes are probably joined by proteins extending from their surfaces. The postsynaptic membrane has on its outer surface an array of lectin receptors, probably glycoproteins. On its inner surface, juxtaposed to the bilayer, the membrane has an electron‐dense structure called the postsynaptic density which, from studies on the isolated structure, is composed of a few polypeptides. On the basis of the molecular composition and structure of CNS synapses and ultrastructural studies of the lesion‐induced synaptogenesis, some of the underlying dynamic events at synaptic membranes are inferred. New synapses are formed either by reutilization of the old contact sites or by generation of new ones. The protein and carbohydrates in the cleft are enzymatically degraded and a new synapse is generated in response to ingrowing fibers by the addition or reutilization of the specialized proteins of postsynaptic membrane, which differentiate a small segment of the postsynaptic membrane.
Substrate heterogeneity of component a of the human erythrocyte membrane
Authors: Roses Allen D. 
Published: May 2005

AbstractComponent a of the erythrocyte membrane is a specific substrate for endogenous protein kinase activity and its phosphorylation is significantly decreased under assay conditions in myotonic muscular dystrophy (Roses, A. D., and Appel, S. H., J. Membr. Biol. 20:51–58 (1975)). We have demonstrated substrate heterogeneity of two fractions of component a separated by concanavalin A (Con‐A) sepharose chromatography. The fraction of component a that is retarded by Con A and eluted with α‐methyl‐D‐glucoside does not accept the transfer of phosphate from [γ‐32P] ATP as a substrate for endogenous protein kinase activity. The nonretarded fraction contains > 90% of the radioactive label. These experiments also confirm the carbohydrate heterogeneity of component a (Findley, J. B. C., J. Biol. Chem. 249:4398 (1974)).
Avian tumor virus interactions with chicken fibroblast plasma membranes
Authors: Moldow Charles F. , McGrath Michael , Santen Lenore Van 
Published: May 2005

AbstractA method is described which will rapidly measure the binding of avian tumor viruses (ATV) to plasma membrane receptors. With this procedure it may be shown that Rous sarcoma virus pseudotypes bind to protease‐labile, heat‐stable structures on the surface of chicken embryo fibroblast (CEF) plasma membranes. The binding sites for ATV subgroups A and B appear distinct, and membranes from genetically resistant CEF bind as well those of sensitive CEF.
Estimation of cell surface associated protease activity and its application to lymphocytes
Authors: Tökés Zoltán A. , Kiefer Hansruedi 
Published: May 2005

AbstractA new method has been developed to estimate proteolytic activity available at the cell surface. Radioiodinated protein substrates are covalently linked to modified polystyrene‐divinylbenzene beads with various diameters. These beads are presented to viable cells. Secreted enzyme activity is estimated when no contact occurs between beads and cells. Surface associated proteolytic activity is estimated by the increased rate of iodinated peptide release due to a contact between beads and cells.This method was applied to various lymphocyte preparations. In the absence of serum, mouse spleen lymphocytes produce three‐ to fourfold higher proteolytic activity than lymph node cells. This activity is completely inhibited by serum diluted 1:10. Since the proteolysis is so marked in the case of spleen cells, one must conclude that lymphocytes removed from the serum and treated in buffered mediums at 37° C have enzymatically altered surface properties.Cell surface associated enzyme activity was measured using rat lymph node lymphocytes with less than 0.1% contamination by granulocytes. This predominantly thymus derived, T cell population had 30% increase in proteolysis due to contact between cells and solid‐phase localized substrate of casein. The released enzymatic activity was inhibited by diisopropylfluorophosphate, but its effect on the surface associated enzyme activity remains questionable since it perturbs several membrane functions.
Cell shape changes and transmembrane receptor uncoupling induced by tertiary amine local anesthetics
Authors: Nicolson Garth L. , Poste George 
Published: May 2005

AbstractTertiary amine local anesthetics (dibucaine, Tetracaine, procaine, etc.) modify cell morphology, concanavalin A (Con A)‐mediated agglutinability and redistribution of Con A receptors. Con A agglutination of untransformed mouse 3T3 cells was enhanced at low concentrations of local anesthetics, and the dynamics of fluorescent‐Con A indicated that ligand‐induced clustering was increased in the presence of the drugs. In contast, these drugs inhibited Con A‐induced receptor capping on mouse spleen cells. These effects can be duplicated by combinations of vinblastine (or colchicine) and cytochalasin B suggesting that local anesthetics act on microtubule cell surface receptor mobility and distribution. It is proposed that tertiary amine local anesthetics displace plasma membrane‐bond Ca2+, resulting in disengagement of microfilament systems from the plasma membrane and increased cellular Ca2+ concentration to levels which disrupt microtubular organization. The possible involvement of cellular Ca2+ in cytoskeletal destruction by local anesthetics was investigated utilizing Ca2+‐specific ionophores A23187 and X537A. In media containing Ca2+ and cytochalasin B these ionophores caused effects similar to tertiary amine local anesthetics.
Does a bacterial elongation factor share a common evolutionary ancestor with actin?
Authors: Rosenbusch Jurg P. , Jacobson Gary R. , Jaton Jean‐Claude 
Published: May 2005

AbstractProtein synthesis elongation factor Tu from E. coli shares several physical, chemical, and functional properties with actin‐like proteins. Limited tryptic degradation indicartes that the two polypeptides have a similar molecular architecture. These observations suggest that they could have evolved from a common ancestor, although more information will be necessary to prove or disprove this hypothesis. A partial sequence, comprising 22 aminoacid residues from the aminoterminal end of the large tryptic fragment of elongation factor Tu is presented.
Characterization of a periplasmic protein related to sn‐glycerol‐3‐phosphate transport in escherichia coli
Authors: Argast Manfred , Schumacher Güunter , Boos Winfried 
Published: May 2005

AbstractThe cold osmotic shock procedure releases a protein (GLPT) from the cell envelope of Escherichia coli that is related to the transport of sn‐glycerol‐3‐phosphate in this organism. The evidence for this correlation is as follows: (1) GLPT is under the regulatory control of the glpR gene. (2) Some glpT mutants that were isolated as phosphonomycin resistant clones do not synthesize GLPT. Revertants of these mutants (growth on sn‐glycerol 3‐phosphate) again synthesize GLPT. (3) Some amber mutations in glpT reduce the amount of GLPT while suppressed strains produce normal amounts. (4) Transfer of a plasmid carrying the glpT genes into a strain lacking GLPT and sn‐glycerol‐3‐phosphate transport restores both functions in the recipient. Transport and GLPT synthesis in the plasmid carrying strain are increased 2‐ to 3‐fold over a fully induced wild‐type strain, but appear to be constitutive. GLPT is a soluble protein of molecular weight 160,000 composed of 4 identical subunits. The 160,000 molecular weight complex is stable in 1% sodium dodecylsulfate at room temperature. Upon boiling in 1% sodium dodecylsulfate GLPT dissociates into its subunits. Likewise, 8 M urea at room temperature dissociates GLPT into its subunits. Dialysis of dissociated GLPT against phosphate or Tris‐HCl buffer, pH 7.0, allows renaturation to the tetrameric form. The protein is acidic in nature (isoelectric point 4.4).In contrast to the typical transport‐related periplasmic‐binding proteins, no conditions could be found where pure GLPT exhibited binding activity toward its supposed substrate, sn‐glycerol‐3‐phosphate.In vivo new appearance of transport activity for sn‐glycerol‐3‐phosphate transport occurs only shortly before cell division. However, GLPT synthesis does not fluctuate during the cell cycle. The available evidence indicates a cell‐division‐dependent processing of GLPT in the cell envelope as a reason for the alteration in transport activity.Transport in whole cells is sensitive to the cold osmotic shock procedure, demonstrating the participation of an essential periplasmic component. However, isolated membrane vesicles that are devoid of periplasmic components, including GLPT, are fully active in sn‐glycerol‐3‐phosphate transport. Therefore, we conclude that GLPT is essential in overcoming a diffusion barrier for sn‐glycerol‐3‐phosphate established by the outer membrane. Attempts to isolate mutants that are transport negative in whole cells due to a defect in GLPT but are active in isolated membrane vesicles have failed so far. All GLPT mutants tested, whether or not they synthesize GLPT, are not active in isolated membrane vesicles.Iodination of whole cells with [125I] followed by osmotic shock reveals that several shock‐releasable proteins including GLPT become radioactively labeled. This indicates that some portions of GLPT are accessible to the external medium.