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The effect of multiple dispersed copies of the origin-of-assembly sequence from TMV RNA on the morphology of pseudovirus particles assembled in vitro.

Supercoiled plasmid DNA was used as a template to transcribe long concatameric RNA molecules containing several dispersed copies of the origin-of-assembly sequence (OAS) from tobacco mosaic virus (TMV) RNA. When incubated with TMV coat protein "disks" in vitro, these RNAs spontaneously assembled into TMV-like pseudovirus particles. However, as each OAS initiated assembly more or less simultaneously, the concatameric RNA species generated complex nucleoprotein structures of predictable morphology. Similar structures were proposed some time ago (M. E. Taliansky, I. B. Kaplan, L. V. Yarvekulg, T. I. Atabekova, A. A. Agranovsky, and J. G. Atabekov, 1982, Virology 118, 309-316) to account for the RNase-sensitive phenotype of a ts mutant of TMV, Ni2519. These results extend the utility of our RNA-packaging vector system and confirm many of the predictions based on the current model for the self-assembly of TMV.

Base Sequence↗

In vitro assembly of poliovirus 14 S subunits: identification of the assembly promoting activity of infected cell extracts.

Purified poliovirus 14 S subunits are assembled into empty capsids in vitro only if their concentration exceeds a 1.6 nM threshold. This also holds true for the 14 S subunits in unpurified extracts of infected cells. Such an extract may promote the assembly of extraneous 14 S subunits, but only if it contributes enough 14 S subunits to raise their total concentration over the threshold. As a result of assembly, the concentration of 14 S subunits in an infected cell extract decreases exponentially, with a half life of 15 min at 37 degrees. When purified 14 S subunits of serotype 1 are mixed with extracts of cells infected with type 2 or 3, chimeric empty capsids are formed, thus showing the pooling of endogenous and extraneous 14 S subunits. In conclusion, the assembly promoting activity of infected cell extracts amounts to nothing more than the supply of endogenous 14 S subunits.

Antibodies, Monoclonal↗

Lack of complete cooperativity of ribosome assembly in vitro and its possible relevance to in vivo ribosome assembly and the regulation of ribosomal gene expression.

Earlier studies have shown that the reconstitution of Escherichia coli 50S as well as 30S ribosomal subunits from component rRNA and ribosomal protein (r-protein) molecules in vitro is not completely cooperative and binding of more than one r-protein to a single 16S rRNA (or 23S rRNA) molecule is required to initiate a successful 30S (or 50S) ribosome assembly reaction. We first confirmed this conclusion by carrying out 30S subunit reconstitution in the presence of a constant amount of 16S rRNA together with various amounts of total 30S r-proteins (TP30) and by analyzing the physical state of reconstituted particles rather than by assaying protein synthesizing activity of the particles as was done in the earlier studies. As expected, under conditions of excess rRNA, the efficiency of 30S subunit reconstitution per unit amount of TP30 decreased greatly with the decrease in the ratio of TP30 to rRNA, indicating the lack of complete cooperativity in the assembly reaction. We then asked the question whether the cooperativity of ribosome assembly is complete in vivo. We treated exponentially growing E coli cells with low concentrations of chloramphenicol which is known to inhibit protein synthesis without inhibiting rRNA synthesis, creating conditions of excess synthesis of rRNA relative to r-proteins. Several concentrations of chloramphenicol (ranging from 0.4 to 4.0 micrograms/ml) were used so that inhibition of protein synthesis ranged from 40 to 95%. Under these conditions, we examined the synthesis of RNA, ribosomal proteins and 50S ribosomal subunits as well as the synthesis of total protein. We found that the synthesis of 50S subunits was not inhibited as much as the synthesis of total protein at lower concentrations of chloramphenicol, but the degree of inhibition of 50S subunit synthesis increased sharply with increasing concentrations of chloramphenicol and was in fact greater than the degree of inhibition of total protein synthesis at chloramphenicol concentrations of 2 micrograms/ml or higher. The inhibition of 50S subunit synthesis was significantly greater than the inhibition of r-protein synthesis at all chloramphenicol concentrations examined. These data are consistent with the hypothesis that the cooperativity of ribosome assembly in vivo is also not complete as is the case for in vitro ribosome reconstitution, but are difficult, if not impossible, to explain on the basis of the complete cooperativity model.(ABSTRACT TRUNCATED AT 400 WORDS)

Centrifugation, Density Gradient↗

In vitro assembly of mosaic hepatitis B virus capsid-like particles (CLPs): rescue into CLPs of assembly-deficient core protein fusions and FRET-suited CLPs.

Hepatitis B virus core protein self-assembles into icosahedral, highly immunogenic capsid-like particles (CLPs) that can serve as molecular platforms for heterologous proteins. Insertion into the centrally located c/e1 epitope leads to surface display, fusion to the C terminus to internal disposition of the foreign domains. However, symmetry-defined space restrictions on the surface and particularly inside the CLPs limit the size of usable heterologous fusion partners. Further, CLPs carrying differing foreign domains are desirable for applications such as multivalent vaccines, and for structure probing by distance sensitive interactions like fluorescence resonance energy transfer (FRET). Here, we report an in vitro co-assembly system for such mosaic-CLPs allowing successful CLP formation with a per se assembly-deficient fusion protein, and of CLPs from two different fluoroprotein-carrying fusions that exert FRET in an assembly-status dependent way.

Capsid↗

Self-assembled drug delivery systems. 1. Properties and in vitro/in vivo behavior of acyclovir self-assembled nanoparticles (SAN).

Self-assembled drug delivery systems (SADDS) were designed in the paper. They can be prepared from the amphiphilic conjugates of hydrophilic drugs and lipids through self-assembling into small-scale aggregates in aqueous media. The outstanding characteristic of SADDS is that they are nearly wholly composed of amphiphilic prodrugs. The self-assembled nanoparticles (SAN) as one of SADDS had been prepared from the lipid derivative of acyclovir (SGSA) in the previous paper. They were further studied on the properties and the in vitro/in vivo behavior in this paper. The SAN kept the physical state stable upon centrifugation or some additives including some inorganic salts, alkaline solutions, surfactants and liposomes except for HCl solution, CaCl(2) solution and animal plasma. Autoclave and bath heat for sterilization hardly influenced the SAN. However, gamma-irradiation strongly destroyed the structure of SAN and SGSA was degraded. SGSA in SAN showed good stability in weak acidic or neutral buffers although it was very sensitive to alkaline solutions and carboxylester enzymes, the half-lives (t(1/2)) of which in the buffer at pH 7.4, the alkaline solution at pH 12.0, pig liver carboxylester enzyme solution, rabbit plasma, and rabbit liver tissue homogenate were 495, 21, 4.7, 25 and 8.7 h, respectively. Compared with SGSA in a disordered state, the specific bilayer structures of SAN could protect SGSA from hydrolysis through hiding the sensitive ester bonds. The SAN showed hemolytic action because the amphiphilic SGSA could insert into rabbit erythrocyte membranes. Both the high concentration of SGSA in samples and the long incubation time improved hemolysis. No hemolysis was observed if the additional volume of the SAN was less than 10% of rabbit whole blood in spite of the high concentration of SGSA. Plasma proteins could interfere the interaction between the SAN and erythrocytes by binding the SAN. The in vitro antiviral activity of acyclovir SAN was limited possibly because of the weak hydrolysis of SGSA in Vero cells, and the SAN showed a little cell toxicity possible due to the amphiphilicity of SGSA. A macrophage cell line of QXMSC1 cells showed uptake of the SAN but not significantly. The SAN were rapidly removed from blood circulation after bolus iv administration to rabbits with the very short distribution t(1/2) (1.5 min) and the elimination t(1/2) (47 min). The SAN were mainly distributed in liver, spleen and lung after iv administration, and SGSA was eliminated slowly in these tissues (t(1/2), about 7 h). It would appear that the nanosized SAN were trapped by the mononuclear phagocyte system. SADDS including SAN combine prodrugs, molecular self-assembly with nanotechnology, and hopefully become novel drug delivery approaches.

Acyclovir↗

Structural model for the cooperative assembly of HIV-1 Rev multimers on the RRE as deduced from analysis of assembly-defective mutants.

The functional efficacy of the HIV-1 Rev protein is highly dependent on its ability to assemble onto its HIV-1 RNA target (the RRE) as a multimeric complex. To elucidate the mechanism of multimeric assembly, we have devised two rapid and broadly applicable strategies for examining cooperative interactions between proteins bound to RNA, one based on cooperative translational repression of a two-site reporter and the other on gel shift analysis with crude E. coli extracts. Using these strategies, we have identified two distinct surfaces of Rev (head and tail) that are critical for different steps in multimeric assembly. Our data indicate that Rev assembles cooperatively on the RRE via a series of symmetrical tail-to-tail and head-to-head protein-protein interactions. The insights into molecular architecture suggested by these findings have enabled us to derive a structural model for Rev and its multimerization on the RRE.

Allosteric Site↗

Role of the carboxy terminus of polypeptide D1 in the assembly of a functional water-oxidizing manganese cluster in photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: assembly requires a free carboxyl group at C-terminal position 344.

The D1 polypeptide of the photosystem II (PSII) reaction center is synthesized as a precursor polypeptide which is posttranslationally processed at the carboxy terminus. It has been shown in spinach that such processing removes nine amino acids, leaving Ala344 as the C-terminal residue [Takahashi, M., Shiraishi, T., & Asada, K. (1988) FEBS Lett. 240, 6-8; Takahashi, Y., Nakane, H., Kojima, H., & Satoh, K. (1990) Plant Cell Physiol. 31, 273-280]. We show here that processing on the carboxy side of Ala344 also occurs in the cyanobacterium Synechocystis 6803, resulting in the removal of 16 amino acids. By constructing a deletion strain of Synechocystis 6803 that lacks the three copies of the psbA gene encoding D1, we have developed a system for generating psbA mutants. Using this system, we have constructed mutants of Synechocystis 6803 that are modified in the region of the C-terminus of the D1 polypeptide. Characterization of these mutants has revealed that (1) processing of the D1 polypeptide is blocked when the residue after the cleavage site is changed from serine to proline (mutant Ser345Pro) with the result that the manganese cluster is unable to assemble correctly; (2) the C-terminal extension of 16 amino acid residues can be deleted with little consequence either for insertion of D1 into the thylakoid membrane or for assembly of D1 into a fully active PSII complex; (3) removal of only one more residue (mutant Ala344stop) results in a loss of assembly of the manganese cluster; and (4) the ability of detergent-solubilized PSII core complexes (lacking the manganese cluster) to bind and oxidize exogenous Mn2+ by the secondary donor, Z+, is largely unaffected in the processing mutants (the Ser345Pro mutant of Synechocystis 6803 and the LF-1 mutant of Scenedesmus obliquus) and the truncation mutant Ala344stop. Our results are consistent with a role for processing in regulating the assembly of the photosynthetic manganese cluster and a role for the free carboxy terminus of the mature D1 polypeptide in the ligation of one or more manganese ions of the cluster.

Amino Acid Sequence↗

Dual function C-terminal domain of dynamin-1: modulation of self-assembly by interaction of the assembly site with SH3 domains.

Impairment of endocytosis by mutational targeting of dynamin-1 GTPases can result in paralysis and embryonic lethality. Dynamin-1 assembles at coated pits where it functions to cleave vesicles from donor membranes. Receptor endocytosis is modulated by SH3 (src homology 3) domain proteins, which directly bind to dynamin C-terminal proline motif sequences, affecting both the dynamin GTPase activity and its recruitment to coated pits. We have determined that dynamin-dynamin interactions, which are required for dynamin helix formation, involve these same SH3 domain-binding C-terminal proline motif sequences. Consequently, SH3 domain proteins induce the in vitro disassembly of dynamin helices. Our results therefore suggest the the dual function of the dynamin C-terminus (involving amino acids 800-840) permits direct regulation of dynamin assembly and function through interaction with SH3 domain proteins. Additionally, the N-terminal GTPase domain plays an important role in assembly. Finally, we show that the central PH (pleckstrin homology) domain exerts a strong inhibitory effect on the capacity for dynamin-1 self-assembly.

Dynamin I↗

Electrochemical assembly and potential-dependent plasmon absorption of au nanoclusters covered with a 4-aminothiophenol self-assembled monolayer.

Gold nanoclusters covered with 4-aminothiophenol (4-ATP) self-assembled monolayers (SAMs) were electrochemically assembled on an Au or ITO electrode. The assembly mechanism is discussed on the basis of results of electrochemical, FT-IR, and XPS measurements. The intensity of plasmon absorption of the gold nanocluster assembly was shown to be dependent on applied potential as a result of electrochemical doping/undoping of a counteranion in the polyaniline film.

Journal Article↗

Synthesis and assembly of thylakoid protein complexes: multiple assembly steps of photosystem II.

To study the synthesis and assembly of multisubunit thylakoid protein complexes, we performed [35S]Met pulse and chase experiments with isolated chloroplasts and intact leaves of spinach (Spinacia oleracea L.), followed by Blue Native gel separation of the (sub)complexes and subsequent identification of the newly synthesized and assembled protein subunits. PSII (photosystem II) core subunits were the most intensively synthesized proteins, particularly in vitro and at high light intensities in vivo, and could be sequestered in several distinct PSII subassemblies. Newly synthesized D1 was first found in the reaction centre complex that also contained labelled D2 and two labelled low-molecular-mass proteins. The next biggest PSII subassembly contained CP47 also. Then PsbH was assembled together with at least two other labelled chloroplast-encoded low-molecular-mass subunits, PsbM and PsbTc, and a nuclear-encoded PsbR. Subsequently, CP43 was inserted into the PSII complex concomitantly with PsbK. These assembly steps seemed to be essential for the dimerization of PSII core monomers. Intact PSII core monomer was the smallest subcomplex harbouring the newly synthesized 33 kDa oxygen-evolving complex protein PsbO. Nuclear-encoded PsbW was synthesized only at low light intensities concomitantly with Lhcb polypeptides and was distinctively present in PSII-LHCII (where LHC stands for light-harvesting complex) supercomplexes. The PsbH protein, on the contrary, was vigorously synthesized and incorporated into PSII core monomers together with the D1 protein, suggesting an intrinsic role for PsbH in the photoinhibition-repair cycle of PSII.

Dimerization↗

Nerve growth factor-induced neurite outgrowth in PC12 cells involves the coordinate induction of microtubule assembly and assembly-promoting factors.

Nerve growth factor (NGF) regulates the microtubule-dependent extension and maintenance of axons by some peripheral neurons. We show here that one effect of NGF is to promote microtubule assembly during neurite outgrowth in PC12 cells. Though NGF causes an increase in total tubulin levels, the formation of neurites and the assembly of microtubules follow a time course completely distinct from that of the tubulin induction. The increases in microtubule mass and neurite extension closely parallel 10- and 20-fold inductions of tau and MAP1, proteins shown previously to promote microtubule assembly in vitro. When NGF is removed from PC12 cells, neurites disappear, microtubule mass decreases, and both microtubule-associated proteins return to undifferentiated levels. These data suggest that the induction of tau and MAP1 in response to NGF promotes microtubule assembly and that these factors are therefore key regulators of neurite outgrowth.

Adrenal Gland Neoplasms↗

Regulation of desmosome assembly in MDCK epithelial cells: coordination of membrane core and cytoplasmic plaque domain assembly at the plasma membrane.

Desmosomes are major components of the intercellular junctional complex in epithelia. They consist of at least eight different cytoplasmic and integral membrane proteins that are organized into two biochemically and structurally distinct domains: the cytoplasmic plaque and membrane core. We showed previously that in MDCK epithelial cells major components of the cytoplasmic plaque (desmoplakin I and II; DPI/II) and membrane core domains (desmoglein I; DGI) initially enter a pool of proteins that is soluble in buffers containing Triton X-100, and then titrate into an insoluble pool before their arrival at the plasma membrane (Pasdar, M., and W. J. Nelson. 1988. J. Cell Biol. 106:677-685; Pasdar. M., and W. J. Nelson. 1989. J. Cell Biol. 109:163-177). We have now examined whether either the soluble or insoluble pool of these proteins represents an intracellular site for assembly and interactions between the domains before their assembly into desmosomes at the plasma membrane. Interactions between the Triton X-100-soluble pools of DPI/II and DGI were analyzed by sedimentation of extracted proteins in sucrose gradients. Results show distinct differences in the sedimentation profiles of these proteins, suggesting that they are not associated in the Triton X-100-soluble pool of proteins; this was also supported by the observation that DGI and DPI/II could not be coimmunoprecipitated in a complex with each other from sucrose gradient fractions. Immunofluorescence analysis of the insoluble pools of DPI/II and DGI, in cells in which desmosome assembly had been synchronized, showed distinct differences in the spatial distributions of these proteins. Furthermore, DPI/II and DGI were found to be associated with different elements of cytoskeleton; DPI/II were located along cytokeratin intermediate filaments, whereas DGI appeared to be associated with microtubules. The regulatory role of cytoskeletal elements in the intracellular organization and assembly of the cytoplasmic plaque and membrane core domains, and their integration into desmosomes on the plasma membrane is discussed.

Actin Cytoskeleton↗

Roles of LAP2 proteins in nuclear assembly and DNA replication: truncated LAP2beta proteins alter lamina assembly, envelope formation, nuclear size, and DNA replication efficiency in Xenopus laevis extracts.

Humans express three major splicing isoforms of LAP2, a lamin- and chromatin-binding nuclear protein. LAP2beta and gamma are integral membrane proteins, whereas alpha is intranuclear. When truncated recombinant human LAP2beta proteins were added to cell-free Xenopus laevis nuclear assembly reactions at high concentrations, a domain common to all LAP2 isoforms (residues 1-187) inhibited membrane binding to chromatin, whereas the chromatin- and lamin-binding region (residues 1-408) inhibited chromatin expansion. At lower concentrations of the common domain, membranes attached to chromatin with a unique scalloped morphology, but these nuclei neither accumulated lamins nor replicated. At lower concentrations of the chromatin- and lamin-binding region, nuclear envelopes and lamins assembled, but nuclei failed to enlarge and replicated on average 2. 5-fold better than controls. This enhancement was not due to rereplication, as shown by density substitution experiments, suggesting the hypothesis that LAP2beta is a downstream effector of lamina assembly in promoting replication competence. Overall, our findings suggest that LAP2 proteins mediate membrane-chromatin attachment and lamina assembly, and may promote replication by influencing chromatin structure.

Amino Acid Sequence↗

Assembly of Newly Imported Oxygen-Evolving Complex Subunits in Isolated Chloroplasts: Sites of Assembly and Mechanism of Binding.

We have examined the assembly of the nuclear-encoded subunits of the oxygen-evolving complex (OEC) after their import into isolated intact chloroplasts. We showed that all three subunits examined (OE33, OE23, and OE17) partition between the thylakoid lumen and a site on the inner surface of the thylakoid membrane after import in a homologous system (e.g., pea or spinach subunits into pea or spinach chloroplasts, respectively). Although some interspecies protein import experiments resulted in OEC subunit binding, maize OE17 did not bind thylakoid membranes in chloroplasts isolated from peas. Newly imported OE33 and OE23 were washed from the membranes at the same concentrations of urea and NaCl as the native, indigenous proteins; this observation suggests that the former subunits are bound productively within the OEC. Inhibition of neither chloroplast protein synthesis nor light- or ATP-dependent energization of the thylakoid membrane significantly affected these assembly reactions, and we present evidence suggesting that incoming subunits actively displace those already bound to the thylakoid membrane. Transport of OE33 took place primarily in the stromal-exposed membranes and proceeded through a protease-sensitive, mature intermediate. Initial binding of OE33 to the thylakoid membrane occurred primarily in the stromal-exposed membranes, from where it migrated with measurable kinetics to the granal region. In contrast, OE23 assembly occurred in the granal membrane regions. This information is incorporated into a model of the stepwise assembly of oxygen-evolving photosystem II.

Journal Article↗

Assembly of an FtsZ mutant deficient in GTPase activity has implications for FtsZ assembly and the role of the Z ring in cell division.

FtsZ, the ancestral homologue of eukaryotic tubulins, assembles into the Z ring, which is required for cytokinesis in prokaryotic cells. Both FtsZ and tubulin have a GTPase activity associated with polymerization. Interestingly, the ftsZ2 mutant is viable, although the FtsZ2 mutant protein has dramatically reduced GTPase activity due to a glycine-for-aspartic acid substitution within the synergy loop. In this study, we have examined the properties of FtsZ2 and found that the reduced GTPase activity is not enhanced by DEAE-dextran-induced assembly, indicating it has a defective catalytic site. In the absence of DEAE-dextran, FtsZ2 fails to assemble unless supplemented with wild-type FtsZ. FtsZ has to be at or above the critical concentration for copolymerization to occur, indicating that FtsZ is nucleating the copolymers. The copolymers formed are relatively stable and appear to be stabilized by a GTP-cap. These results indicate that FtsZ2 cannot nucleate assembly in vitro, although it must in vivo. Furthermore, the stability of FtsZ-FtsZ2 copolymers argues that FtsZ2 polymers would be stable, suggesting that stable FtsZ polymers are able to support cell division.

Bacterial Proteins↗

In vitro assembly of PB2 with a PB1-PA dimer supports a new model of assembly of influenza A virus polymerase subunits into a functional trimeric complex.

Influenza virus RNA-dependent RNA polymerase is a heterotrimeric complex of PB1, PB2, and PA. We show that the individually expressed PB2 subunit can be assembled with the coexpressed PB1-PA dimer in vitro into a transcriptionally active complex. Furthermore, we demonstrate that a model viral RNA promoter can bind to the PB1-PA dimer prior to assembly with PB2. Our results are consistent with a recently proposed model for the sequential assembly of viral RNA polymerase complex in which the PB1-PA dimeric complex and the PB2 monomer are transported into the nucleus separately and then assembled in the nucleus.

Cell Line↗

MEK-ERK inhibition corrects the defect in VLDL assembly in HepG2 cells: potential role of ERK in VLDL-ApoB100 particle assembly.

OBJECTIVE: Hepatic VLDL assembly is defective in HepG2 cells, resulting in the secretion of immature triglyceride-poor LDL-sized apoB particles. We investigated the mechanisms underlying defective VLDL assembly in HepG2 and have obtained evidence implicating the MEK-ERK pathway. METHODS AND RESULTS: HepG2 cells exhibited considerably higher levels of the ERK1/2 mass and activity compared with primary hepatocytes. Inhibition of ERK1/2 using the MEK1/MEK2 inhibitor, U0126 (but not the inactive analogue) led to a significant increase in apoB secretion. In the presence of oleic acid, ERK1/2 inhibition caused a major shift in the lipoprotein distribution with a majority of particles secreted as VLDL, an effect independent of insulin. In contrast, overexpression of constitutively active MEK1 decreased apoB and large VLDL secretion. MEK1/2 inhibition significantly increased both cellular and microsomal TG mass, and mRNA levels for DGAT-1 and DGAT-2. In contrast to ERK, modulation of the PI3-K pathway or inhibition of the p38 MAP kinase, had no effect on lipoprotein density profile. Modulation of the MEK-ERK pathway in primary hamster hepatocytes led to changes in apoB secretion and altered the density profile of apoB-containing lipoproteins. CONCLUSIONS: Inhibition of the overactive ras-MEK-ERK pathway in HepG2 cells can correct the defect in VLDL assembly leading to the secretion of large, VLDL-sized particles, similar to primary hepatocytes, implicating the MEK-ERK cascade in VLDL assembly in the HepG2 model. Modulation of this pathway in primary hepatocytes also regulates apoB secretion and appears to alter the formation of VLDL-1 sized particles.

Animals↗

In vitro matrix assembly induced by critical assembly concentration (CAC).

L-2 cells are an immortalized cell line derived from yolk sac parietal endoderm cells, which are responsible for the production of Reichert's membrane, a thick basement membrane produced during rat gestation. Although the L-2 cells secrete all the major components of the basal lamina, they do not assemble a robust matrix in cell culture. We hypothesized that the reason L-2 cells fail to assemble a matrix in cell culture is because the concentrations of matrix components necessary for this matrix assembly do not reach a critical association concentration (CAC) under standard cell culture conditions. To limit the diffusion of secreted molecules while maintaining a nutrient-rich environment for the cells to thrive, we developed a technique that uses a dialysis membrane to limit protein diffusion in a 2-well plate format. This technique permits L-2 cells to assemble a robust matrix in as little as 24 hr that continues to be formed for at least 72 hr. This technique may address some of the physical limitations imposed by cell culture and could be readily applied to other cell types and medium conditions.

Animals↗