Search PubMed⌕ Search

Biomedical subjects

E Hartmann

Publications and source records attributed to E Hartmann.

At least 91 records · Page 5Linked to original sources
PubMed1995

Two different subunits of importin cooperate to recognize nuclear localization signals and bind them to the nuclear envelope.

BACKGROUND: Selective protein import into the cell nucleus occurs in two steps: binding to the nuclear envelope, followed by energy-dependent transit through the nuclear pore complex. A 60 kD protein, importin, is essential for the first nuclear import step, and the small G protein Ran/TC4 is essential for the second. We have previously purified the 60kD importin protein (importin 60) as a single polypeptide. RESULTS: We have identified importin 90, a 90 kD second subunit that dissociates from importin 60 during affinity chromatography on nickel (II)-nitrolotriacetic acid-Sepharose, a technique that was originally used to purify importin 60. Partial amino-acid sequencing of Xenopus importin 90 allowed us to clone and sequence its human homologue; the amino-acid sequence of importin 90 is strikingly conserved between the two species. We have also identified a homologous budding yeast sequence from a database entry. Importin 90 potentiates the effects of importin 60 on nuclear protein import, indicating that the importin complex is the physiological unit responsible for import. To assess whether nuclear localization sequences are recognized by cytosolic receptor proteins, a biotin-tagged conjugate of nuclear localization signals linked to bovine serum albumin was allowed to form complexes with cytosolic proteins in Xenopus egg extracts; the complexes were then retrieved with streptavidin-agarose. The pattern of bound proteins was surprisingly simple and showed only two predominant bands: those of the importin complex. We also expressed the human homologue of importin 60, Rch1p, and found that it was able to replace its Xenopus counterpart in a functional assay. We discuss the relationship of importin 60 and importin 90 to other nuclear import factors. CONCLUSIONS: Importin consists of a 60 and a 90 kD subunit. Together, they constitute a cytosolic receptor for nuclear localization signals that enables import substrates to bind to the nuclear envelope.

Amino Acid Sequence↗
PubMed1995

Purification and characterization of a cAMP-binding protein of Volvox carteri f. nagariensis Iyengar.

Two cAMP-binding proteins, cbp1 and cbp2, were purified from the cytoplasm of the green alga Volvox carteri. Both proteins have a native molecular mass of 90 kDa as determined by gel filtration. cbp2 was purified to apparent electrophoretic homogeneity, having a subunit molecular mass of 42 kDa as determined by SDS/PAGE. The cbp1 preparation contains a 42-kDa and a 44-kDa band. The cAMP-binding activity is not associated with protein kinase activity. Tryptic peptides of cbp2 were sequenced by automated Edman degradation. Two pairs of peptides differ in one amino acid only, thus pointing to the presence of isoforms of cbp2. Both binding proteins differed from the cAMP-specific phosphodiesterases of V. carteri with respect to charge, molecular mass and binding affinity to N6-cAMP-agarose. Reverse-phase chromatography of the bound ligand revealed that the two binding proteins hydrolyse cAMP to 5' AMP. The binding specificity of purified cbp1 and cbp2 was probed by a set of modified cAMP derivatives. Both proteins bind cAMP strictly specifically in the anti conformation; position 1 and 6 of the adenine moiety and at least one of the exocyclic O atoms of the ribose cyclic phosphate moiety are essential. 3-Isobutyl-1-methylxanthine is an effective inhibitor of binding but the natural methylxyanthines are not. At present it is not clear whether cbp1 and cbp2 are individual proteins or isoforms of one another.

Amino Acid Sequence↗
PubMed1995

Transport route for synaptobrevin via a novel pathway of insertion into the endoplasmic reticulum membrane.

Synaptobrevin/vesicle-associated membrane protein is one of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is proposed to provide specificity for the targeting and fusion of vesicles with the plasma membrane. It belongs to a class of membrane proteins which lack a signal sequence and contain a single hydrophobic segment close to their C-terminus, leaving most of the polypeptide chain in the cytoplasm (tail-anchored). We show that in neuroendocrine PC12 cells, synaptobrevin is not directly incorporated into the target organelle, synaptic-like vesicles. Rather, it is first inserted into the endoplasmic reticulum (ER) membrane and is then transported via the Golgi apparatus. Its insertion into the ER membrane in vitro occurs post-translationally, is dependent on ATP and results in a trans-membrane orientation of the hydrophobic tail. Membrane integration requires ER protein(s) different from the translocation components needed for proteins with signal sequences, thus suggesting a novel mechanism of insertion.

Adenosine Triphosphate↗
PubMed1995

Reorganization of microfilaments in protonemal tip cells of the moss Ceratodon purpureus during the phototropic response.

The F-actin distribution in caulonemal tip cells of the moss Ceratodon purpureus was examined by rhodamine-phalloidin staining. Gravitropically-growing caulonemal tip cells of the moss possess a distinct alignment of microfilaments (MFs) in their apices. Axially oriented actin bundles run from subapical regions to the apex where they converge towards a central area of the tip, although bundles are absent from the central area itself thus forming a collar-like structure. During a unilateral red light irradiation the actin strands of the apical dome become reoriented towards the irradiated apical flank and still surround an area free of MFs, the point of prospective outgrowth. This process is closely correlated with the morphological effect of bulging and precedes the light-directed outgrowth. The collar structure is essential for the tubular growth form. In darkness, under the influence of antimicrotubule agents the structure is decomposed, the actin strands drift along the cell flanks and finally accumulate in randomly distributed areas where further growth takes place. The microtubules (MTs) are not involved in the phytochrome-mediated reorientation of the microfilaments. Unilateral red light suppresses the distorting effect of antimicrotubule drugs and restores the collar structure with a pronounced light-directed orientation. Instead, the MTs seem to be responsible for restricting the reorientation to the cell tip. This notion is based on the observation that the small area in the apical dome, which is normally the exclusive location of the light-regulated MF rearrangement, extends towards the cell base when MT inhibitors are applied before the unilateral red light irradiation. This in turn leads to a non-tubular expansion of the light-directed cell flank.

Actin Cytoskeleton↗
PubMed1995

Guilty by insolubility--does a protein's detergent insolubility reflect a caveolar location?

Several recent papers have described a simple approach to isolate caveolae and proteins involved in apical sorting in epithelial cells that is based on their detergent insolubility. These publications have excited such diverse fields of cell biology as intracellular protein transport, signal transduction and, in particular, research into caveolae. In this article, Kurzchalia, Hartmann and Dupree argue that a more critical evaluation of this detergent insolubility is needed before a subcellular location or function can be ascribed to a protein.

Journal Article↗
PubMed1994

Isolation of a protein that is essential for the first step of nuclear protein import.

We have purified a cytosolic protein from Xenopus eggs that is essential for selective protein import into the cell nucleus. The purified protein, named importin, promotes signal-dependent binding of karyophilic proteins to the nuclear envelope. We have cloned, sequenced, and expressed a corresponding cDNA. Importin shows 44% sequence identity with SRP1p, a protein associated with the yeast nuclear pore complex. Complete, signal-dependent import into HeLa nuclei can be reconstituted by combining importin purified from Xenopus eggs or expressed in E. coli with Ran/TC4. Evidence for additional stimulatory factors is provided.

Adenosine Triphosphate↗
PubMed1994

The N-terminal region of the alpha-subunit of the TRAP complex has a conserved cluster of negative charges.

The alpha-subunit of the TRAP complex (TRAP alpha) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane. Here, we demonstrate the widespread occurrence of TRAP alpha in eukaryotes as indicated by its existence in man, fish and plants. Despite the fact that the sequence homology is much lower than for other proteins in the translocation site, the overall topology, the location of the glycosylation sites and, most interestingly, the distribution of charges are conserved. These data indicate that the TRAP complex has a ubiquitous function.

Amino Acid Sequence↗
PubMed1994

Protein translocation: common themes from bacteria to man.

Protein transport across the endoplasmic reticulum membrane in eukaryotes and across the cytoplasmic membrane in bacteria have turned out to be highly related. The core component of the translocation apparatus is the Sec61/SecYp complex; at least two of its subunits are conserved in evolution. The Sec61/SecYp complex is involved in both co- and post-translational transport pathways. The two modes require probably distinct additional components.

Bacteria↗
PubMed1994

Evolutionary conservation of components of the protein translocation complex.

Protein translocation into the mammalian endoplasmic reticulum requires the Sec61p complex, which consists of three membrane proteins. The alpha-subunit, the homologue of Sec61p of yeast, shows some similarity to SecYp, a key component of the protein export apparatus of bacteria. In Escherichia coli, SecYp is also associated with two other proteins (SecEp and band-1 protein). We have now determined the sequences of the beta- and gamma-subunits of the mammalian Sec61p complex. Sec61-gamma is homologous to SSS1p, a suppressor of sec61 mutants in Saccharomyces cerevisiae, and can functionally replace it in yeast cells. Moreover, Sec61-gamma and SSS1p are structurally related to SecEp of E. coli and to putative homologues in various other bacteria. At least two subunits of the Sec61/SecYp complex therefore seem to be key components of the protein translocation apparatus in all classes of organisms.

Amino Acid Sequence↗
PubMed1994

A novel pathway of peptide biosynthesis found in methanogenic Archaea.

The peptide subunits of the pseudomurein, the cell-wall peptidoglycan of some methanogens, are usually composed of glutamic acid, alanine and lysine. In order to get a more detailed picture of the biosynthetic pathway of the peptide subunit, we performed in vitro assays. Starting from glutamic acid a pentapeptide was obtained in seven steps: [formula: see text] The pentapeptide structure was identical to that of the peptide subunit of the intact pseudomurein except one additional alanine residue, which is split off during further processing. The pentapeptide synthesis starts with glutamic acid, which is phosphorylated at the N alpha-amino group. N alpha-phosphoryl-glutamic acid is transferred to a nucleotide-carrier, forming N alpha-UDP-glutamic acid. The further pentapeptide biosynthesis is achieved via a di-, tri- and tetrapeptide by stepwise addition of the corresponding amino acids.

Cell Wall↗
PubMed1994

Evidence for the existence of membrane-associated phytochrome in the cell.

Comparative fluorescence and photochemical studies of phytochrome in etiolated seedlings of maize and in soluble and membrane-containing fractions isolated from them were carried out. The membrane fractions prepared in the absence of Mg2+ from etiolated coleoptiles contained 13% of total photoreversible phytochrome, which was readily solubilized by mild detergents. Its molecular size was indistinguishable from soluble phytochrome and equal to nondegraded maize phytochrome. Low-temperature fluorescence studies with intact tissue found that the position of the emission maximum at 85 K (lambda max) and the extent of the phototransformation of the red-absorbing form (Pr) into the first stable photoproduct, lumi-R, at 85 K (gamma 1), varied in different parts of etiolated seedlings: lambda max and gamma 1 reached their maximum values in the tips of coleoptiles and roots, 686 nm and 0.30-0.40, whereas the lowest values, 682 nm and ca 0.05, were observed in the root base. These parameters correlated well with those obtained for the pigment in the soluble and membrane-containing fractions: 684 and 680 nm, and 0.33 and 0.06, respectively. The extent of the Pr phototransformation into the far red-absorbing form (Pfr) (gamma 2) did not differ much: values of 0.80-0.85 and 0.70-0.75 correlated with the high and low values of gamma 1. These variations of the parameters were interpreted in agreement with our previous observations in terms of two phytochrome A species whose relative concentrations vary depending on the experimental conditions--the longer wavelength bulk light-labile species with high gamma 1 (Pr'), and the shorter wavelength minor light-stable species with low gamma 1 (Pr").(ABSTRACT TRUNCATED AT 250 WORDS)

Cell Membrane↗
PubMed1993

Site-specific photocross-linking reveals that Sec61p and TRAM contact different regions of a membrane-inserted signal sequence.

A chemically charged amber suppressor tRNA was used to introduce the photoactivatable amino acid (Tmd)Phe at a selected position within the signal sequence of the secretory protein preprolactin. This allowed the interactions of the NH2-terminal, the central, and the COOH-terminal regions of the signal sequence to be investigated during insertion into the membrane of the endoplasmic reticulum (ER). We found that different regions of the nascent chains were photocross-linked to different ER proteins. The TRAM protein (translocating chain-associating membrane protein) contacts the NH2-terminal region of the signal sequence while the mammalian Sec61p contacts the hydrophobic core of the signal sequence and regions COOH-terminal of this. These results suggest that the ER translocation complex is composed of heterologous protein subunits which contact distinct regions of nascent polypeptides during their membrane insertion.

Acylation↗
PubMed1993

A tetrameric complex of membrane proteins in the endoplasmic reticulum.

The translocation site (translocon), at which nascent polypeptides pass through the endoplasmic reticulum membrane, contains a component previously called 'signal sequence receptor' that is now renamed as 'translocon-associated protein' (TRAP). Two glycosylated subunits of the TRAP complex have been identified before (alpha and beta subunits). We now show that TRAP complex is actually comprised of four membrane proteins (alpha, beta, gamma, delta), present in a stoichiometric relation, which are genuine neighbours in intact microsomes. The amino acid sequences of the additional, non-glycosylated subunits were deduced from cloning of the corresponding cDNAs. The delta subunit spans the membrane only once and has its major portion, containing a disulfide bridge, at the lumenal side. The gamma subunit is predicted to span the membrane four times.

Amino Acid Sequence↗
PubMed1993

Inhibition of K1735-M2 melanoma cell invasion in vitro by retinoic acid.

Melanoma cell invasion in vitro was tested by means of confrontation cultures of melanoma multicellular spheroids with rounded fragments of embryonic chick heart tissue. Quantitative determination of invasion was performed using a computerized image analysis program, facilitating the evaluation of the efficacy of potentially anti-invasive compounds. Retinoic acid (RA; 1 microM) [corrected] considerably impaired K1735-M2 melanoma cell invasion, as demonstrated by various measuring parameters. Parameter TUMAREA, expressing the amount of tumor tissue, indicates a growth inhibitory effect and the invasion parameter STRCSTR shows that after treatment with RA the stromal component was better preserved than in untreated controls. Besides the inhibitory effect of RA on melanoma cell invasion in confrontation cultures, RA increased the dynamics of adhesion of melanoma cells to the extracellular matrix components type I collagen and laminin, and slightly impaired melanoma cell directional migration. Fluorescence microscopy using rhodamine-labeled phalloidin showed that RA also modulated the organization of the actin cytoskeleton by inducing the formation of actin-containing stress fibers. Our data show that 1 microM RA exhibited a pronounced anti-invasive effect on highly metastatic melanoma cells in vitro. Impairment of host tissue degradation, altered adhesion abilities, changes in the actin cytoskeleton, as well as the antiproliferative effect may all account for inhibition of melanoma cell invasion.

Actin Cytoskeleton↗
PubMed1993

A class of membrane proteins with a C-terminal anchor.

Integral membrane proteins are generally targeted to translocation-competent membranes by virtue of signal sequences located close to the N-terminus of the polypeptide chain. Membrane anchoring is caused by the signal sequence or other hydrophobic segments located after it in the amino acid sequence. However, some integral membrane proteins do not follow these rules. The members of one class of nonconformist membrane proteins have no signal sequence, but instead possess a hydrophobic segment near the C-terminus that orients them with their N-termini in the cytoplasm. Members of this class are found in many organelles and are probably inserted into membranes by an unusual mechanism.

Journal Article↗