Two forms of elongation factor 1 alpha (EF-1 alpha O and 42Sp50), present in oocytes, but absent in somatic cells of Xenopus laevis.

We have purified and partially sequenced the EF-1 alpha protein from Xenopus laevis oocytes (EF-1 alpha O). We show that the two cDNA clones isolated by Coppared et al. (Coppard, N. J., K. Poulsen, H. O. Madsen, J. Frydenberg, and B. F. C. Clark. 1991. J. Cell Biol. 112:237-243) do not encode 42Sp50, as claimed by these authors, but two very similar forms of EF-1 alpha O (EF-1 alpha O and EF-1 alpha O1). 42Sp50 is the major protein component of a 42S nucleoprotein particle that is very abundant in previtellogenic oocytes of X. laevis, 42Sp50 differs from EF-1 alpha O not only by its amino acid sequence, but also by several properties already reported. In particular, 42Sp50 has a low EF-1 alpha activity. It is distributed uniformly in the cytoplasm of previtellogenic oocytes, in contrast to EF-1 alpha O which is concentrated in a small region of the cytoplasm, known as the mitochondrial mass or Balbiani body.

SERCA1 truncated proteins unable to pump calcium reduce the endoplasmic reticulum calcium concentration and induce apoptosis.

By pumping calcium from the cytosol to the ER, sarco/endoplasmic reticulum calcium ATPases (SERCAs) play a major role in the control of calcium signaling. We describe two SERCA1 splice variants (S1Ts) characterized by exon 4 and/or exon 11 splicing, encoding COOH terminally truncated proteins, having only one of the seven calcium-binding residues, and thus unable to pump calcium. As shown by semiquantitative RT-PCR, S1T transcripts are differentially expressed in several adult and fetal human tissues, but not in skeletal muscle and heart. S1T proteins expression was detected by Western blot in nontransfected cell lines. In transiently transfected cells, S1T homodimers were revealed by Western blot using mildly denaturing conditions. S1T proteins were shown, by confocal scanning microscopy, to colocalize with endogenous SERCA2b into the ER membrane. Using ER-targeted aequorin (erAEQ), we have found that S1T proteins reduce ER calcium and reverse elevation of ER calcium loading induced by SERCA1 and SERCA2b. Our results also show that SERCA1 variants increase ER calcium leakage and are consistent with the hypothesis of a cation channel formed by S1T homodimers. Finally, when overexpressed in liver-derived cells, S1T proteins significantly induce apoptosis. These data reveal a further mechanism modulating Ca(2+) accumulation into the ER of nonmuscle cells and highlight the relevance of S1T proteins to the control of apoptosis.

Structural requirements of 5S rRNA for nuclear transport, 7S ribonucleoprotein particle assembly, and 60S ribosomal subunit assembly in Xenopus oocytes.

Structural requirements of 5S rRNA for nuclear transport and RNA-protein interactions have been studied by analyzing the behavior of oocyte-type 5S rRNA and of 31 different in vitro-generated mutant transcripts after microinjection into the cytoplasm of Xenopus oocytes. Experiments reveal that the sequence and secondary and/or tertiary structure requirements of 5S rRNA for nuclear transport, storage in the cytoplasm as 7S ribonucleoprotein particles, and assembly into 60S ribosomal subunits are complex and nonidentical. Elements of loops A, C, and E, helices II and V, and bulged and hinge nucleotides in the central domain of 5S rRNA carry the essential information for these functional activities. Assembly of microinjected 5S rRNA into 60S ribosomal subunits was shown to occur in the nucleus; thus, the first requirement for subunit assembly is nuclear targeting. The inhibitory effects of ATP depletion, wheat germ agglutinin, and chilling on the nuclear import of 5S rRNA indicate that it crosses the nuclear envelope through the nuclear pore complex by a pathway similar to that used by karyophilic proteins.

Hepatitis B virus-related insertional mutagenesis implicates SERCA1 gene in the control of apoptosis.

We have used the Hepatitis B Virus DNA genome as a probe to identify genes clonally mutated in vivo, in human liver cancers. In a tumor, HBV-DNA was found to be integrated into the gene encoding Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA), which pumps calcium, an important intracellular messenger for cell viability and growth, from the cytosol to the endoplasmic reticulum. The HBV X gene promoter cis-activates chimeric HBV X/SERCA1 transcripts, with splicing of SERCA1 exon 11, encoding C-terminally truncated SERCA1 proteins. Two chimeric HBV X/SERCA1 proteins accumulate in the tumor and form dimers. In vitro analyses have demonstrated that these proteins localize to the ER, determine its calcium depletion and induce cell death. We have also shown that these biological effects are related to expression of the SERCA, rather than of the viral moiety. This report involves for the first time the expression of mutated SERCA proteins in vivo in a tumor cell proliferation and in vitro in the control of cell viability. Oncogene (2000).

Interaction of membrane proteins and lipids with solubilizing detergents.

Detergents are indispensable in the isolation of integral membrane proteins from biological membranes to study their intrinsic structural and functional properties. Solubilization involves a number of intermediary states that can be studied by a variety of physicochemical and kinetic methods; it usually starts by destabilization of the lipid component of the membranes, a process that is accompanied by a transition of detergent binding by the membrane from a noncooperative to a cooperative interaction already below the critical micellar concentration (CMC). This leads to the formation of membrane fragments of proteins and lipids with detergent-shielded edges. In the final stage of solubilization membrane proteins are present as protomers, with the membrane inserted sectors covered by detergent. We consider in detail the nature of this interaction and conclude that in general binding as a monolayer ring, rather than as a micelle, is the most probable mechanism. This mode of interaction is supported by neutron diffraction investigations on the disposition of detergent in 3-D crystals of membrane proteins. Finally, we briefly discuss the use of techniques such as analytical ultracentrifugation, size exclusion chromatography, and mass spectrometry relevant for the structural investigation of detergent solubilized membrane proteins.

Isolation of rhodopsin by the combined action of cardiotoxin and phospholipase A2 on rod outer segment membranes.

Freeze-fracture electron microscopy was used to follow morphological changes induced by Naja mossambica mossambica venom VII4 cardiotoxin in rod outer segment membrane preparations. The extent of the morphological changes depend on the purity of the cardiotoxin. Pure cardiotoxin had no detectable effect upon the preparation, but, when contaminated with venom phospholipase A2, led to a rapid disintegration of the membrane vesicles. With trace amounts (up to about 0.5% of the cardiotoxin) of phospholipase A2, the membrane vesicles disintegrated into smooth lamellae and particles in solution. These two components were separated by centrifugation. The pellet, which showed the presence of smooth lamellae and aggregated particles, was composed of unbleached rhodopsin, initial membrane lipids, lysolipids and cardiotoxin. The supernatant, which showed only the presence of dispersed particles, was composed of unbleached rhodopsin, lysolipids and cardiotoxin. With cardiotoxin contained larger amounts of phospholipase A2 (more than 0.5% of the cardiotoxin), membrane vesicles were disintegrated into large aggregates of amorphous material, composed of bleached rhodopsin, initial membrane lipids, lysolipids and cardiotoxin. These results confirm our previous observation on the release of integral membrane proteins from membrane vesicles by the action of cardiotoxin containing traces of phospholipase A2 (Gulik-Krzywicki, T., Balerna, M., Vincent, J.P. and Lazdunski, M. (1981) Biochim. Biophys. Acta 643, 101-114) and suggest it possible use for isolation and purification of integral membrane proteins.

Properties of detergent-solubilized and membranous (Ca2+ + Mg2+)-activated ATPase from sarcoplasmic reticulum as studied by sulfhydryl reactivity and ESR spectroscopy. Effect of protein-protein interactions.

(1) Sulfhydryl reactivity and electron spin resonance spectra of nitroxide maleimide spin labels, covalently attached to sarcoplasmic reticulum ATPase, were examined on both detergent-solubilized and membranous material. Monomeric and oligomeric ATPases were prepared by the use of dodecyloctaethylene glycol monoether as a solubilizing detergent. (2) Immediately after solubilization, the reaction curve of nonomeric ATPase with 5,5'-dithiobis(2-nitrobenzoate) was characterized by positive cooperativity (S-shaped as a function of time). In contrast, the SH reactivity of both oligomeric and membranous ATPases obeyed usual first-order kinetics and could be analyzed in terms of three classes of reactive site. All enzymatically active ATPase preparations responded to addition of ADP with a decrease in SH reactivity. During enzymatic inactivation of monomeric ATPase, the SH-modification rate was dramatically enhanced with loss of cooperative features. Ca2+ removal from the high-affinity sites stimulated SH reactivity before inactivation had taken place. (3) ESR spectroscopy indicated less motional constraints on monomeric than on oligomeric and membranous ATPases. Arrhenius plots of ESR spectral parameters suggest a conformational transition in both membranous and solubilized ATPases at about 22 degrees C. The transition was also present in EGTA-, but not in heat-inactivated ATPase. Although SH reactivity of monomeric ATPase was dramatically enhanced by EGTA inactivation, the results of ESR, circular dichroism and analytical ultracentrifugation experiments indicate limited conformational changes induced by EGTA treatment. (4) The data indicate marked differences in the properties of monomeric ATPase on the one hand and oligomeric and membranous enzymes on the other hand. They are consistent with previous functional evidence for the presence of ATPase in an associated state in the membrane (Møller, J.V., Lind, K.E. and Andersen, J.P. (1980) J. Biol. Chem. 255, 1912-1920).

Freeze-fracture study of water-soluble, standard proteins and of detergent-solubilized forms of sarcoplasmic reticulum Ca2+-ATPase.

Conventional freeze-fracturing electron microscopy was used to study water-soluble proteins and different forms of Ca2+-ATPase-detergent complexes. Freeze-fracture images of solutions containing proteins larger than myoglobin showed the presence of distinct, randomly dispersed particles on smooth fracture surfaces. The distribution of sizes of these particles was closely to Gaussian, with a mean size which was correlated to the Stokes diameter. Monomeric Ca2+-ATPase from sarcoplasmic reticulum, solubilized by deoxycholate or a non-ionic detergent, showed a bimodal distribution of particle sizes. Even more complex distributions were found for dimeric and trimeric preparations of Ca2+-ATPase. The results can be interpreted on the assumption that the Ca2+-ATPase molecule is elongated, with an overall length of about 110 A and a width in its largest part of about 75 A. It is concluded on the basis of the presented results that freeze-fracture electron microscopy can be successfully used for morphological studies of protein molecules in solution.

Effects of temperature and benzyl alcohol on the structure and adenylate cyclase activity of plasma membranes from bovine adrenal cortex.

Adenylate cyclase activation by corticotropin (ACTH), fluoride and forskolin was studied as a function of membrane structure in plasma membranes from bovine adrenal cortex. The composition of these membranes was characterized by a very low cholesterol and sphingomyelin content and a high protein content. The fluorescent probes 1,6-diphenylhexa-1,3,5-triene (DPH) and a cationic analogue 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene (TMA-DPH) were, respectively, used to probe the hydrophobic and polar head regions of the bilayer. When both probes were embedded either in the plasma membranes or in liposomes obtained from their lipid extracts, they exhibited lifetime heterogeneity, and in terms of the order parameter S, hindered motion. Under all the experimental conditions tested, S was higher for TMA-DPH than for DPH but both S values decreased linearly with temperature within the range of 10 to 40 degrees C, in the plasma membranes and the liposomes. This indicated the absence of lipid phase transition and phase separation. Addition to the membranes of up to 100 mM benzyl alcohol at 20 degrees C also resulted in a linear decrease in S values. Membrane perturbations by temperature changes or benzyl alcohol treatment made it possible to distinguish between the characteristics of adenylate cyclase activation with each of the three effectors used. Linear Arrhenius plots showed that when adenylate cyclase activity was stimulated by forskolin or NaF, the activation energy was similar (70 kJ.mol-1). Fluidification of the membrane with benzyl alcohol concentrations of up to 100 mM at 12 or 24 degrees C produced a linear decrease in the forskolin-stimulated activity, that led to its inhibition by 50%. By contrast, NaF stabilized adenylate cyclase activity against the perturbations induced by benzyl alcohol at both temperatures. In the presence of ACTH, biphasic Arrhenius plots were characterized by a well-defined break at 18 degrees C, which shifted at 12.5 degrees C in the presence of 40 mM benzyl alcohol. These plots suggested that ACTH-sensitive adenylate cyclase exists in two different states. This hypothesis was supported by the striking difference in the effects of benzyl alcohol perturbation when experiments were performed below and above the break temperature. The present results are consistent with the possibility that clusters of ACTH receptors form in the membrane as a function of temperature and/or lipid phase fluidity.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological changes of phosphatidylcholine bilayers induced by melittin: vesicularization, fusion, discoidal particles.

Morphological changes induced by the melittin tetramer on bilayers of egg phosphatidylcholine and dipalmitoylphosphatidylcholine have been studied by quasi-elastic light scattering, gel filtration and freeze-fracture electron microscopy. It is concluded that melittin similarly binds and changes the morphology of both single and multilamellar vesicles, provided that their hydrocarbon chains have a disordered conformation, i.e., at temperatures higher than that of the transition, Tm. When the hydrocarbon chains are ordered (gel phase), only small unilamellar vesicles are morphologically affected by melittin. However after incubation at T greater than Tm, major structural changes are detected in the gel phase, regardless of the initial morphology of the lipids. Results from all techniques agree on the following points. At low melittin content, phospholipid-to-peptide molar ratios, Ri greater than 30, heterogeneous systems are observed, the new structures coexisting with the original ones. For lipids in the fluid phase and Ri greater than 12, the complexes formed are large unilamellar vesicles of about 1300 +/- 300 A diameter and showing on freeze-fracture images rough fracture surfaces. For lipids in the gel phase, T less than Tm after passage above Tm, and for 5 less than Ri less than 50, disc-like complexes are observed and isolated. They have a diameter of 235 +/- 23 A and are about one bilayer thick; their composition corresponds to one melittin for about 20 +/- 2 lipid molecules. It is proposed that the discs are constituted by about 1500 lipid molecules arranged in a bilayer and surrounded by a belt of melittin in which the mellitin rods are perpendicular to the bilayer. For high amounts of melittin, Ri less than 2, much smaller and more spherical objects are observed. They are interpreted as corresponding to lipid-peptide co-micelles in which probably no more bilayer structure is left. It is concluded that melittin induces a reorganization of lipid assemblies which can involve different processes, depending on experimental conditions: vesicularization of multibilayers; fusion of small lipid vesicles; fragmentation into discs and micelles. Such processes are discussed in connexion with the mechanism of action of melittin: the lysis of biological membranes and the synergism between melittin and phospholipases.

Characterization of a high-affinity complex between the bacterial outer membrane protein FhuA and the phage T5 protein pb5.

Binding of bacteriophage T5 to Escherichia coli cells is mediated by specific interactions between the receptor-binding protein pb5 (67.8 kDa) and the outer membrane iron-transporter FhuA. A histidine-tagged form of pb5 was overproduced and purified. Isolated pb5 is monomeric and organized mostly as beta-sheets (51%). pb5 functionality was attested in vivo by its ability to impair infection of E. coli cells by phage T5 and Phi80, and to prevent growth of bacteria on iron-ferrichrome as unique iron source. pb5 was functional in vitro, since addition of an equimolar concentration of pb5 to purified FhuA prevented DNA release from phage T5. However, pb5 alone was not sufficient for the conversion of FhuA into an open channel. Direct interaction of pb5 with FhuA was demonstrated by isolating a pb5/FhuA complex using size-exclusion chromatography. The stoichiometry, 1 mol of pb5/1 mol of FhuA, was deduced from its molecular mass, established by analytical ultracentrifugation after determination of the amount of bound detergent. SDS-PAGE and differential scanning calorimetry experiments highlighted the great stability of the complex: (i) it was not dissociated by 2% SDS even when the temperature was raised to 70 degrees C; (ii) thermal denaturation of the complex occurred at 85 degrees C, while pb5 and FhuA were denatured at 45 degrees C and 74 degrees C, respectively. The stability of the complex renders it suitable for high-resolution structural studies, allowing future analysis of conformational changes into both FhuA and pb5 upon adsorption of the virus to its host.

Cloning of the newt Pleurodeles waltlii chromosomal DNA.

Pleurodeles waltlii genomic DNA has been cloned using several phage lambda vectors. We have isolated approx. 600 000 clones, which correspond to about 20% of the total DNA sequences of this organism. This constitutes the first large gene library of a Urodele. The low yield of cloning was attributable to the abundance of highly repetitive sequences, since recombinations in the bacterial host could lead to the loss of clones. Indeed, the existence of highly repetitive sequences was directly demonstrated by hybridization between recombinants and the total genome, and some of the cloned DNA was found to be unstable. We suggest new methods for cloning the highly repetitive sequences.

Purification of two thermostable components of messenger ribonucleoprotein particles (mRNPs) from Xenopus laevis oocytes, belonging to a novel class of RNA-binding proteins.

We have purified and partially sequenced two proteins from Xenopus laevis previtellogenic oocytes, belonging to messenger ribonucleoprotein particles (mRNPs). The purification procedure rests on the thermostability of these proteins, which remain soluble after heating the cell extracts at 80 degrees C. The thermostable proteins can be identified with two of the most abundant components (mRNP3 and mRNP4) of the mRNPs, described by Darnbrough and Ford (1981) [Eur. J. Biochem. 118, 415-424]. mRNP3 and mRNP4 are homologous to each other, but to no other protein of known sequence. The abundance and semi-periodic distribution of proline residues in mRNP3 and mRNP4 sequences suggest that these RNA-binding proteins adopt an unusual type of conformation.

Secondary structure of the membrane-bound domains of H+,K+-ATPase and Ca2+-ATPase, a comparison by FTIR after proteolysis treatment of the native membranes.

The sarcoplasmic reticulum Ca2+-ATPase and the gastric H+,K+-ATPase were cleaved under three different proteolysis conditions. After elimination of the protease and of the cleaved peptides, the vesicles containing the membrane-bound peptides of the ATPases were studied by Fourier transform attenuated total reflection infrared spectroscopy. In the harsher proteolysis conditions, the membrane-associated domain of the Ca2+-ATPase represented about 20% of the protein and was mainly constituted of alpha-helices. Polarized infrared spectroscopy showed that these alpha-helices were mainly oriented perpendicular to the membrane. However, only 10-20% of the H+,K+-ATPase was cleaved. The remaining, membrane-associated domain of the protein contained about 30% of alpha-helices and 30% of beta-sheet structures. The alpha-helices adopted a mainly transmembrane orientation. While the data on the Ca2+-ATPase are in general agreement with the current model of the protein, our results indicate that caution must be used in choosing this protein as a general structural model for all P-type ATPases. The protease-resistant, membrane-associated domain of the H+K+-ATPase is indeed much larger than predicted and also contained beta-sheet structures.

High-performance liquid chromatography of the main polypeptide (MP26) of lens fiber plasma membranes solubilized with n-octyl beta-D-glucopyranoside.

The main polypeptide isolated from lens fiber membrane has been solubilized in octyl glucoside and studied by gel filtration in high-performance liquid chromatography (HPLC). The combination of S20,w value obtained from analytical ultracentrifugation and Stokes radius determined by HPLC of the soluble fraction indicates that more than 90% of the protein is monomeric. The solubilization of the protein seems to be dependent upon the presence of the NH2 and COOH terminal sequences, since proteolytic degradation of MP26 which removes these terminal sequences is less soluble than the uncleaved polypeptide. Moreover, there is a higher amount of oligomer after proteolysis. Fatty acid analysis by gas chromatography shows that the insoluble membrane fraction from both cortical and nuclear fibers comprises a special class of long (C22) saturated fatty acids (behenic acid).

Does intrinsic fluorescence reflect conformational changes in the Ca2+-ATPase of sarcoplasmic reticulum?

We have investigated the kinetics of the intrinsic fluorescence drop observed when ATP is added to purified sarcoplasmic reticulum ATPase in a potassium-free medium containing magnesium and calcium, at pH 6 and 20 degrees C. Under these conditions, analysis of the fluorescence drop is complex. Several events contributed to the rate of the fluorescence drop initiated by turnover, including phosphorylation, conformational transition of the phosphorylated complex, and dephosphorylation. On the other hand, when 75% of total fluorescence was quenched by energy transfer to the membrane-bound ionophore A23187, the observed turnover-dependent drop in residual fluorescence mainly reflected the conformational transition of the phosphorylated ATPase. Combination of fast kinetics with the quenching of selected tryptophan residues is suggested to be a promising tool for the study of proteins containing many of these residues.

Thesaurin a, the major protein of Xenopus laevis previtellogenic oocytes, present in the 42 S particles, is homologous to elongation factor EF-1 alpha.

We have purified in SDS X.laevis thesaurin a (Mr 50,000) which is part of the 42 S storage particles. Its N-terminal amino acid is blocked and several peptides obtained by V8 protease treatment were purified and sequenced. As expected from one of the functional roles of the 42 S particles (tRNA binding, protection against deacylation and exchange with the ribosome), the amino acid sequence of thesaurin a was found to be closely related to that of the elongation factor EF-1 alpha. We suggest that all three proteins involved in 5 S RNA and tRNA storage in previtellogenic oocytes, TFIIIA, thesaurin a and thesaurin b, have a dual function: storage and a role in transcription or in protein synthesis.

mRNP3 and mRNP4 are phosphorylatable by casein kinase II in Xenopus oocytes, but phosphorylation does not modify RNA-binding affinity.

mRNP3 and mRNP4 (also called FRGY2) are two mRNA-binding proteins which are major constituents of the maternal RNA storage particles of Xenopus laevis oocytes. The phosphorylation of mRNP3-4 has been implicated in the regulation of mRNA masking. In this study, we have investigated their phosphorylation by casein kinase II and its consequence on their affinity for RNA. Comparison of the phosphopeptide map of mRNP3-4 phosphorylated in vivo with that obtained after phosphorylation in vitro by purified Xenopus laevis casein kinase II strongly suggests that casein kinase II is responsible for the in vivo phosphorylation of mRNP3-4 in oocytes. The phosphorylation occurs on a serine residue in a central domain of the proteins. The affinity of mRNP3-4 for RNA substrates remained unchanged after the treatment with casein kinase II or calf intestine phosphatase in vitro. This suggests that phosphorylation of these proteins does not regulate their interaction with RNA but rather controls their interactions with other proteins.

[Protein-protein interactions in membranes: concepts and the example of calcium ATPase from the sarcoplasmic reticulum]. / Les interactions protéine-protéine dans les membranes: quelques concepts et l'exemple de l'ATPase calcique du reticulum sarcoplasmique.

The organization of polypeptide chains in the membrane has attracted widespread interest. This is particularly true for transport proteins: indeed, the existence of a quaternary structure may obviously have important implications for the mechanism of solute transport through the membrane. The problem arises from the fact that it is extremely difficult to demonstrate unambiguously that a protein is truly oligomeric in the membrane. In this paper various techniques are considered, either direct methods of investigation such as X-ray or neutron scattering, ESR, and radiation inactivation, or indirect methods (primarily the solubilization of the protein by non-denaturing detergents). In very few cases the existence of a 'structural' oligomer has been demonstrated. However, the question remains whether the oligomer also has a functional role, i.e., is it directly necessary for example to form a hydrophilic pathway for an ion, or indirectly to stabilize the enzyme structure or to allow a control to take place at a certain defined step of the transport cycle?.

Biochemical research on oogenesis: protein synthesis in whole cells and in cell-free extracts of Xenopus laevis immature ovaries.

Nearly all tRNA molecules in previtellogenic oocytes of Xenopus laevis are included in nucleoprotein particles sedimenting at 42S. The tRNA-binding sites of these particles have several properties in common with those of the ribosomes. This suggests that the 42S particles might behave like unprogrammed ribosomes and be the site of a template-independent polymerization of amino acids. We expected this reaction to be insensitive to protein synthesis inhibitors, such as cycloheximide and puromycin. We found that these antibiotics almost completely inhibit the incorporation of labeled amino acids into protein, when added to the incubation medium of whole ovaries or free oocytes. In cell-free extracts of ovaries, the incorporation of amino acids is partially insensitive to cycloheximide and puromycin. When such extracts are fractionated by sucrose density centrifugation and incubated with ATP, a major peak of amino acid incorporation can be detected, which nearly coincides with the 42S particle peak.