Nuclear protein transport

Current strategies and findings in clinically relevant post-translational modification-specific proteomics

Mass spectrometry-based proteomics has considerably extended our knowledge about the occurrence and dynamics of protein post-translational modifications (PTMs). So far, quantitative proteomics has been mainly used to study PTM regulation in cell culture models, providing new insights into the role of aberrant PTM patterns in human disease. However, continuous technological and methodical developments have paved the way for an increasing number of PTM-specific proteomic studies using clinical samples, often limited in sample amount. Thus, quantitative proteomics holds a great potential to discover, validate and accurately quantify biomarkers in body fluids and primary tissues. A major effort will be to improve the complete integration of robust but sensitive proteomics technology to clinical environments. Here, we discuss PTMs that are relevant for clinical research, with a focus on phosphorylation, glycosylation and proteolytic cleavage; furthermore, we give an overview on the current developments and novel findings in mass spectrometry-based PTM research.

Mitochondrial targeting of mouse NQO1 and CYP1B1 proteins

Four dioxin-inducible enzymes--NAD(P)H: quinone oxidoreductase-1 (NQO1) and three cytochromes P450 (CYP1A1, CYP1A2 & CYP1B1)--are implicated in both detoxication and metabolic activation of various endobiotics and xenobiotics. NQO1 is generally regarded as a cytosolic enzyme; whereas CYP1 proteins are located primarily in endoplasmic reticulum (ER), CYP1A1 and CYP1A2 proteins are also targeted to mitochondria. This lab has generated Cyp1a1(mc/mc) and Cyp1a1(mtt/mtt) knock-in mouse lines in which CYP1A1 protein is targeted exclusively to ER (microsomes) and mitochondria, respectively. Comparing dioxin-treated Cyp1(+/+) wild-type, Cyp1a1(mc/mc), Cyp1a1(mtt/mtt), and Cyp1a1(-/-), Cyp1b1(-/-) and Nqo1(-/-) knockout mice, in the present study we show that [a] NQO1 protein locates to cytosol, ER and mitochondria, [b] CYP1B1 protein (similar to CYP1A1 and CYP1A2 proteins) traffics to mitochondria as well as ER, and [c] NQO1 and CYP1B1 targeting to mitochondrial or ER membranes is independent of CYP1A1 presence in that membrane.

HIV Rev self-assembly is linked to a molten-globule to compact structural transition

By regulating the differential expression of proviral pre mRNA in the host cell, Rev plays a crucial role in the HIV-1 life cycle. The capacity of Rev to function is intimately linked to its ability to self-associate. Nevertheless, little is known about the exact role of self-association in the molecular mechanism defining its biological activity. A prerequisite knowledge is a definition of the molecular events undertaken by Rev during the process of self-assembly. Thus, this study was initiated to monitor the structure of Rev as a function of protein concentration. Rev undergoes a structural transition as a consequence of self-assembly. This structural transition was monitored by three spectroscopic methods. The accessibility of the single tryptophan in Rev monomer to acrylamide quenching increases with decreasing protein concentration. At very low concentration of Rev, the tryptophan accessibility is close to that of an unfolded Rev. As evaluated by circular dichroism, the secondary structure of Rev is protein concentration dependent as evidenced by an increase in the magnitude of ellipticity with increasing protein concentration. Further, results from ANS binding studies indicate that the ANS binding sites in Rev experience an apparent increase in hydrophobicity as the Rev concentration was increased. These concentration dependent changes seem to reach a maximum above 5 microM Rev monomer concentration. In order to define the mode of Rev self-association sedimentation velocity and equilibrium experiments were conducted. There are evidently two consecutive progressive association processes. At protein concentrations below 0.5 mg/ml, the data from sedimentation studies can be fitted to a single isodesmic model. Simulation of velocity sedimentation profile indicates that free Rev monomer that has not entered into the association processes can best be described to exhibit a value of S(20,w) that is substantially smaller than 1.4 S, a value needed to fit the rest of the data. The latter value is consistent for a Rev monomer with the expected molecules weight and if it were to assume a compact globular shape. These spectroscopic and hydrodynamic results imply that monomeric Rev is in a molten globule state, which becomes more compact upon self-association.

Expression and regulation of golli products of myelin basic protein gene during in vitro development of oligodendrocytes

The myelin basic protein (MBP) gene produces two families of proteins, the classic MBPs, important for myelination of the CNS, and the golli proteins, whose biological role in oligodendrocytes (OLs) is still unknown. The goals of this work were to study the in vitro pattern of expression of the golli products during OL differentiation and to compare it with that of the classic MBP products of the gene. Mouse primary glial cultures were analyzed at the mRNA and protein levels with an array of techniques. We found that OLs express golli mRNA primarily during intermediate stages of differentiation, which was confirmed by immunocytochemical analysis. Golli expression was low in proliferating OL progenitors as well as in terminally mature OLs. Golli proteins were found associated with the OL cell soma and nuclei and, to a lesser extent, with the cellular processes. We also found that golli proteins are not targeted to myelin in vitro and in vivo, in contrast to the classic MBPs. Finally, we found that golli expression is regulated during OL development and can be manipulated by growth factors such as basic fibroblast growth factor, neurotrophin-3, and retinoic acid.

1H NMR structural characterization of a nonmitogenic, vasodilatory, ischemia-protector and neuromodulatory acidic fibroblast growth factor

A shortened genetically engineered form of acidic fibroblast growth factor (aFGF), that includes amino acids 28-154 of the full-length sequence (154 residues) plus Met in substitution of Leu27, does not induce cell division even though it is recognized by the cell membrane receptor, triggers the early mitogenic events, and retains the neuromodulatory, vasoactive, and cardio- and neuroprotective properties of the native full-length molecule. Taken together, these properties make this truncated aFGF a promising compound in the treatment of a wide assortment of neurological and cardiovascular pathologies where aFGF mitogenic activity is dispensable. Differences in biological activities between the shortened aFGF and the wild-type form have been attributed to lack of stability, and to the specific amino acid sequence missing at the N-terminus. Here we show that this shortened aFGF form has a three-dimensional structure even more stable than the wild-type protein at the mitogenic assay conditions; that this structure is similar to that of the wild type except at site 1 of interaction with the cell membrane receptor; that its lack of mitogenic activity cannot be attributed to the specific missing sequence; and that the vasodilatory activity of aFGF seems impaired by alterations of the three-dimensional structure of site 2 of interaction with the cell membrane receptor.

Apoptosis-related functional features of the DNaseI-like family of nucleases

Rat DNaseYb and its human homolog DHP2 are members of a new family of DNaseI-like endonucleases. They contain all the conserved amino acid residues to engage a DNaseI-like catalytic activity and the same molecular mechanisms of DNA hydrolysis. The sequence similarity can be extended to other families of nucleases, such as FEN-1, DNA polymerases, RNaseH and exonuclease III, involved in the ion-dependent hydrolysis of nucleic acids. Their unique features include the NLS signals that place them in the nuclei and a high content of positively charged amino acid residues that results in their high affinity for the substrate. Their properties are consistent with a role in the early stage DNA degradation during apoptosis. The caspase-DFF45/CIDE-CPAN pathway is most likely involved in the second stage of internucleosomal DNA degradation. However, cells express constitutively multiple transcripts encoding DNA degrading enzymes and related molecules, hence they have the molecular diversity to engage the self-destructive pathway appropriate to a given trigger.

Identification of a nuclear targeting domain in the insertion between helices C and D in protease inhibitor-10

Protease inhibitor 10 (PI-10), an intracellular ovalbumin-serpin, contains a series of basic amino acids in the loop between helices C and D that exhibit homology to known nuclear targeting signals. Transfection of HeLa cells with plasmids encoding enhanced green fluorescent protein (EGFP) coupled to PI-10 revealed an intense fluorescence of the nucleus. Immunoblotting demonstrated a single Mr 80,000 EGFP.PI-10 complex in isolated nuclei. Mutation of four basic amino acids in the interhelical loop to alanines (i.e. K74A, K75A, R76A, K77A) resulted in the fluorescent complex being confined to the cytoplasm. Further evidence for a nuclear targeting signal in this region was provided by localization of the fluorescent label to the nucleus in cells transfected with a plasmid encoding EGFP fused to the 25 amino acids comprising the interhelical loop of PI-10 (i.e. Arg-63 to Glu-87), whereas a cytoplasmic distribution was noted for the construct encoding EGFP coupled to the mutated interhelical loop. These data raise the possibility that PI-10 may play a role in regulating protease activity within the nucleus, a property unique in the field of serpin biology.

Artificial neural networks for computer-based molecular design

The theory of artificial neural networks is briefly reviewed focusing on supervised and unsupervised techniques which have great impact on current chemical applications. An introduction to molecular descriptors and representation schemes is given. In addition, worked examples of recent advances in this field are highlighted and pioneering publications are discussed. Applications of several types of artificial neural networks to compound classification, modelling of structure-activity relationships, biological target identification, and feature extraction from biopolymers are presented and compared to other techniques. Advantages and limitations of neural networks for computer-aided molecular design and sequence analysis are discussed.

DNaseY: a rat DNaseI-like gene coding for a constitutively expressed chromatin-bound endonuclease

A rat gene, designated DNaseY, encoding a 36 kDa endonuclease was identified and cloned. Sequence analysis of the cDNA showed it to be the rat homologue of human DNAS1L3. The DNaseY gene product had 42% identity to DNaseI, including conserved critical active site residues, the essential disulfide bridge, the calcium binding domain, and a signal peptide, as well as 2 of the 3 signature boxes. Significantly, DNaseY had 2 nuclear localization signals and was more basic (pI 9.5) than DNaseI (pI 4.8). The DNaseY gene contained a number of exons similar to that of DNaseI, separated by much larger introns, resulting in a gene of >17 kb compared to <4 kb gene of DNaseI. The 36 kDa DNaseY gene product was catalytically inactive but was converted to an active 33 kDa endonuclease following processing of the hydrophobic signal peptide. Antibody generated against peptides representing the predicted amino acid sequence of DNaseY cross-reacted with a 33 kDa nuclear protein which possessed endonucleolytic activity. The enzyme was active over a broad pH range (optimum pH 7-8), was Ca2+/Mg2+-dependent, was inhibited by Zn2+, and was capable of both single- and double-stranded DNA cleavage, producing DNA fragments with 3'-OH ends. Furthermore, the DNaseY gene was expressed constitutively in all cells and tissues tested, but it was not transcriptionally up-regulated in apoptotic cells. All these features were consistent with a role in the early stages of apoptotic DNA fragmentation.

Intracellular location of the Saccharomyces cerevisiae CDC6 gene product

The CDC6 gene product from Saccharomyces cerevisiae is required for transition from late G1 to S phase of the cell cycle. We have investigated the subcellular localization of the CDC6 protein in yeast to explore where Cdc6p exerts its gene function (s). Using affinity-purified sera we localized Cdc6p to the cytoplasm and the nuclear matrix by both subcellular fractionation and indirect immunofluorescence microscopy. The nuclear localization was confirmed to be in the nuclear scaffold by the low-salt extraction method. The Cdc6p cannot be detected in the mitochondrial or plasma membrane fractions. Using indirect immunofluorescence, we found that a subpopulation of Cdc6p migrated into the nucleus after G1/S transition and diminished after M phase, suggesting its temporal role in nuclear DNA replication. The predicted Cdc6p polypeptide contains a conserved nuclear localization, 27PLKRKKL33, similar to that of the SV40 large T antigen and other nuclear proteins. To test whether this peptide segment plays a role in mediating nuclear transport, we have carried out site-directed mutagenesis to alter the conserved 29Lys to Thr and Arg. The wild-type nuclear localization signal of Cdc6p was found to mediate the LacZ reporter gene fused to CDC6 efficiently to the nucleus, but not the mutated versions of the nuclear localization motif. The results suggested that 29Lys is important in mediating nuclear localization, the 29Thr and 29Arg mutant versions of the CDC6 gene were also unable to complement the cdc6 temperature-sensitive mutant. However, when these mutants were expressed from a multicopy plasmid, the mutated genes could complement the mutation. Similar results were obtained in the cdc6-disrupted cells. Taken together, we suggest that (i) Cdc6p is predominantly located in the cytoplasm, (ii) the nuclear entry of Cdc6p is cell cycle dependent, and (iii) nuclear entry of Cdc6p is mediated by its nuclear localization signal. The presence of Cdc6p in both the nucleus and the cytoplasm suggests a model that Cdc6p exerts its gene function in DNA replication and mitotic restraint in the cell cycle.

A peptide C-terminal to the second Zn finger of human vitamin D receptor is able to specify nuclear localization

A peptide of 27 amino acids, VDR(102-76), representing residues 76-102 immediately C-terminal to the second Zn finger of human vitamin D receptor (hVDR) was conjugated to fluorescein-labelled IgG using a bifunctional coupling reagent, m-maleimidobenzoyl n-hydroxysuccinimide. Upon microinjection into the cytoplasm of human osteosarcoma MG-63 cells, the chimeras accumulated in the nuclei. This transport was arrested by chilling or energy depletion. Two other peptides, VDR(80-67), spanning the N-terminal part of VDR(102-76), and VDR(108-97), spanning the C-terminal part of VDR(102-76), were not able to target the linked proteins to the nuclei. SV40(135-112), a peptide containing a well-characterized nuclear localization sequence (amino acids 112-135) of simian virus 40 (SV40) large T-antigen, caused complete nuclear accumulation under the same conditions. Wheat germ agglutinin, which inhibits SV40(135-112) transport, also inhibited the nuclear accumulation of VDR(102-76) as did energy depletion.

Putative nuclear localization signals (NLS) in protein transcription factors

We have recognized about ten distinct forms of strongly basic hexapeptides, containing at least four arginines and lysines, characteristic of nuclear proteins among all eukaryotic species, including yeast, plants, flies and mammals. These basic hexapeptides are considered to be different versions of a core nuclear localization signal, NLS. Core NLSs are present in nearly all nuclear proteins and absent from nearly all "nonassociated" cytoplasmic proteins that have been investigated. We suggest that the few (approximately 10%) protein factors lacking a typical NLS core peptide may enter the nucleus via their strong crosscomplexation with their protein factor partners that possess a core NLS. Those cytoplasmic proteins found to possess a NLS-like peptide are either tightly associated with cell membrane proteins or are integral components of large cytoplasmic protein complexes. On the other hand, some versions of core NLSs are found in many cell membrane proteins and secreted proteins. It is hypothesized that in these cases the N-terminal hydrophobic signal peptide of extracellular proteins and the internal hydrophobic domains of transmembrane proteins are stronger determinants for their subcellular localization. The position of core NLSs among homologous nuclear proteins may or may not be conserved; however, if lost from an homologous site it appears elsewhere in the protein. This search provides a set of rules to our understanding of the nature of core nuclear localization signals: (1) Core NLS are proposed to consist most frequently of an hexapeptide with 4 arginines and lysines; (2) aspartic and glutamic acid residues as well as bulky amino acids (F, Y, W) need not to be present in this hexapeptide; (3) acidic residues and proline or glycine that break the alpha-helix are frequently in the flanking region of this hexapeptide stretch; (4) hydrophobic residues ought not to be present in the core NLS flanking region allowing for the NLS to be exposed on the protein. In this study we attempt to classify putative core NLS from a wealth of nuclear protein transcription factors from diverse species into several categories, and we propose additional core NLS structures yet to be experimentally verified.

Cytoplasmic-nuclear trafficking of progesterone receptor. In vivo study of the mechanism of action of antiprogestins

The signal responsible for the nuclear localization of the progesterone receptor has been characterized. The study of the mechanism of this nuclear localization has revealed that the receptor continuously shuttles between the nucleus and the cytoplasm. The receptor diffuses into the cytoplasm and is constantly and actively transported back into the nucleus. Preliminary evidence suggests that the same mechanism exists for estradiol and glucocorticoid receptors. Experiments designed to study the traffic of steroid hormone receptors have been applied to the determination of the molecular mechanism of action of antisteroids. Using these techniques, we have shown that two major antiprogestins, RU486 and ZK98299, act at the same point in the cell as the hormone.

Cytoplasmic-nuclear trafficking of steroid hormone receptors

The nuclear localization of most steroid hormone receptors reflects a dynamic process: the receptor constantly diffuses out of the nucleus and is reimported by an active mechanism. The outward movement from the nucleus of the receptors and of other nuclear proteins is also mediated by the nuclear localization signals.

Differential intracellular compartmentalization of phosphotyrosine phosphatases in a glial cell line: TC-PTP versus PTP-1B

Intracellular levels of protein-phosphotyrosine are regulated by the protein-tyrosine phosphatase (PTP) family. Cellular compartmentalization may play an important role in modulating the function of these enzymes. The recent demonstration that PTP-1B is localized to the endoplasmic reticulum (Frangioni et al: Cell 68:545, 1992) is consistent with this proposition. In this study we have examined the intracellular distribution of TC-PTP in a glial cell line (C6). Using indirect immunofluorescence we have shown that this enzyme is distributed differently from PTP-1B and is mainly concentrated in the perinuclear region of these cells.

Nucleolar accumulation of core protein in cells naturally infected with Semliki Forest virus. Quantitative aspects

The core (C) protein of Semliki Forest virus (SFV) is known to exert several important functions with regard to both the virus and the host. This paper shows that migration of parental and progeny C protein to the nucleolus is a common feature in infected vertebrate and invertebrate cells. The amount of C protein accumulating to the nucleolus is small, always less than 1% of the intra- and extracellular C protein at various times post infection. Migration to the nucleolus is a fast process; 1.5 h post infection a prominent amount of parental C protein is already incorporated into nucleolar fractions.

The nuclear membrane

The nuclear membrane forms a major barrier within the cell, permitting levels of regulation not found in prokaryotes. The dynamics and diverse functions of the nuclear membrane and its associated structures are considered in this review. The role of the nuclear pore complex in selective transport across the nuclear membrane has been studied to a considerable degree; however, many crucial questions remain. Components of a signal transduction mechanism are associated with the nucleus, suggesting that nuclear functions may be influenced directly by this system. The involvement of the heat shock cognate protein Hsc70 in nuclear protein import is discussed, and a specific signal-presentation role for this protein is proposed.

Localization of the nucleolar protein NO38 in amphibian oocytes

To examine the role of primary amino acid sequence in the localization of proteins within the nucleus, we studied the nucleolar protein NO38 of amphibian oocytes. We synthesized NO38 transcripts in vitro, injected them into the oocyte cytoplasm, and followed the distribution of the translation products. The injected RNA contained a short sequence encoding an epitope derived from the human c-myc protein. We used an mAb against this epitope to detect translation products from injected RNAs by Western blots and by immunofluoresent staining of cytological preparations. When full-length transcripts of NO38 were injected into oocytes, the translation products accumulated efficiently in the germinal vesicle, and a major fraction was localized in the multiple nucleoli. To identify protein domains involved in this nucleolus-specific accumulation, we prepared a series of carboxy-terminal deletions of the cDNA. Oocytes injected with RNA encoding truncated forms of NO38 were examined for altered patterns of protein accumulation. We defined a domain of about 24 amino acids near the carboxy terminus that was essential for nucleolar localization of NO38. This domain is separated by more than 70 amino acids from two putative nuclear localization signals near the middle of the molecule. Hybrid constructs were made which encoded part of Escherichia coli beta-galactosidase or pyruvate kinase fused to a long segment of NO38 containing the essential domain. Injection of RNA from these constructs showed that the essential domain was not sufficient to target the hybrid proteins to the nucleolus. We suggest that nucleolar accumulation of NO38 requires more than a single linear domain.

Association of calmodulin with isolated nuclei from rat hepatocytes

Calmodulin plays an important role in regulating cell proliferation and intranuclear processes (J. Biol. Chem. 265: 18595, 1990). Therefore we studied the association of 125I-calmodulin with highly purified rat hepatocyte nuclear preparations which were characterized by marker enzymes and electron microscopy. Steady-state association of 125I-calmodulin was reached within 5 minutes. Half-maximal binding was achieved at approximately 7.1 microM. This association was partially Ca(2+)-dependent, but was not influenced by ATP, GTP or wheat germ agglutinin. Ultrastructural autoradiography showed specific association of 125I-calmodulin with peripheral and non-peripheral heterochromatin, nuclear membranes, and nucleoli. Specific binding (ratio of the grain density of 125I-calmodulin to Na125I) was greatest in the regions of the nucleoli and non-peripheral heterochromatin. The data indicate that exogenous calmodulin can associate with specific nuclear components in an energy-independent and Ca(2+)-dependent manner.

Nucleocytoplasmic shuttling of the progesterone receptor

The nuclear localization of the progesterone receptor is mediated by two signal sequences: one is constitutive and lies in the hinge region (between the DNA and steroid binding domains), the other is hormone dependent and is localized in the second zinc finger of the DNA binding domain. The use of various inhibitors of energy synthesis in cells expressing permanently or transiently the wild-type receptor or a receptor mutated within the nuclear localization signals, demonstrated that the nuclear residency of the receptor reflects a dynamic situation: the receptor diffusing into the cytoplasm and being constantly and actively transported back into the nucleus. The existence of this nucleo-cytoplasmic shuttle mechanism was confirmed by receptor transfer from one nucleus to the other in heterokaryons. Preliminary evidence was obtained, using oestrogen receptor, that this phenomenon may be of general significance for steroid receptors.

The amino terminus of the adenovirus fiber protein encodes the nuclear localization signal

Using a recombinant vaccinia virus vector, the fiber protein from adenovirus serotype 2 has been expressed in human cells; the protein expressed was correctly assembled into trimers, glycosylated, and transported to the nucleus. Deletion of amino acids 2-5 (KRAR) resulted in accumulation of fiber in the cytoplasm; fusion of the sequence TKRVRL, found at the beginning of Ad7 fiber, to the N-terminus of this mutant restored correct targeting. Changing the charge of amino acids 91 and 92 within another potential targeting sequence (LKKTK to LEETK) had little effect on nuclear targeting. When fused to the N-terminus of beta-galactosidase and expressed in recombinant vaccinia virus, neither MKRARP nor MTKRVRL (from Ad2 and Ad7 fibers, respectively), were sufficient for efficient transport of the hybrid protein to the nucleus; on the other hand, fusions of either MKRARPSEDTF (from Ad2 fiber) or of MKRPRP (a known targeting sequence from the C-terminus of Ad2 E1A proteins) to beta-galactosidase were localized to the nucleus. These results suggest that sequences at the N-terminus of Ad2 and Ad7 fiber are required for correct nuclear targeting.

Brain-derived neurotrophic factor: subcellular compartmentalization and interneuronal transfer as visualized with anti-peptide antibodies

The recent cloning of a second member of the nerve growth factor family, brain-derived neurotrophic factor (BDNF), has prompted investigation into the cells that express this factor's mRNA and protein. In the present study, antibodies raised against unique peptide sequences within the porcine BDNF protein detect BDNF-like immunoreactivity in neurons in rat hippocampal and cortical areas consistent with the distribution of BDNF mRNA as detected with in situ hybridization. Within these neurons, BDNF-like immunoreactivity was observed in the cytoplasm, dendrites, and nuclei. In addition, BDNF immunoreactivity was observed in the cytoplasm of cholinergic neurons that do not express detectable levels of BDNF mRNA. Thus, anti-peptide antibodies can be used to detect this neurotrophic factor protein in cytoplasmic sites of synthesis and in areas of probable action. We propose that one form of the BDNF protein enters the nucleus and may directly influence transcription, while another fraction of the protein is transported out of the synthesizing cell and can be detected, after retrograde axonal transport, in cytoplasmic granules in the perikarya of cholinergic neurons. These basal forebrain cholinergic neurons project to regions enriched in BDNF-synthesizing cells and are known to be responsive to BDNF in vitro. Our data provide information regarding the cellular distribution of BDNF protein in vivo and suggest a dendro-axonic interneuronal transfer of BDNF as well as an additional, intracellular signaling pathway not previously thought to occur in postmitotic neurons in brain.

HIV-1 Rev expressed in recombinant Escherichia coli: purification, polymerization, and conformational properties

The high-level expression of HIV-1 Rev in Escherichia coli is described. Protein in crude bacterial extracts was dissociated from bound nucleic acid with urea. A simple purification and renaturation protocol, monitored by circular dichroism, is described which results in high yields of pure protein. The purified protein binds with high affinity to the Rev-responsive element mRNA and has nativelike spectroscopic properties. The protein exhibits concentration-dependent self-association as judged by analytical ultracentrifugation and gel filtration measurements. Purified Rev showed reversible heat-induced aggregation over the temperature range 0-30 degrees C. This hydrophobic-driven and nonspecific protein association was inhibited by low concentrations of sulfate ions. Rev solutions at greater than 80 micrograms/mL, incubated at 0-4 degrees C, slowly polymerized to form long hollow fibers of 20-nm diameter. Filament formation occurs at a lower protein concentration and more rapidly in the presence of Rev-responsive mRNA. The nucleic acid containing filaments are about 8 nm in diameter and up to 0.4 micron in length. On the basis of physical properties of the purified protein, we have suggested that in the nucleus of infected cells, Rev binding to the Rev-responsive region of env mRNA may be followed by helical polymerization of the protein which results in coating of the nucleic acid. Coated nucleic acid could be protected from splicing in the nucleus and exported to the cytoplasm.

Transport across the nuclear envelope: enigmas and explanations

The transport of molecules across the nuclear envelope plays a central role in the metabolism of the cell. Significant advances in three major areas highlight the limits of our current knowledge and point to the prospect of exciting future developments. Firstly, findings that ions and small proteins do not diffuse freely into the nucleus call into question the current views of nuclear envelope permeability. Secondly, indications that nuclear protein import can be regulated in conjunction with the cell cycle and development have been confirmed by the clear demonstration of regulated nuclear entry of specific transcription factors and morphogens. Thirdly, identification and characterisation of candidate receptor proteins indicates that the recognition of the nuclear targeting sequence occurs in the cytoplasm, suggesting that a different recognition event occurs at the nuclear pore.

The NH2-terminal extension of high molecular weight bFGF is a nuclear targeting signal

Basic fibroblast growth factor (bFGF) is a member of the heparin-binding growth factor (HBGF) family that includes at least seven species. These proteins are potent regulators of a number of cellular processes, including cell division and angiogenesis. Multiple forms of bFGF exist differing only in the length of their NH2-terminal extensions. These species of bFGF also have unique subcellular distributions. The smallest form (18 kD) occurs predominantly in the cytosol, while the higher molecular weight forms (22, 22.5, 24 kD) are associated with the nucleus and ribosomes. Here we report that the nuclear localization of the higher molecular weight forms of bFGF derives specifically from the amino acid sequences within the NH2-terminal extension. This has been demonstrated by constructing a chimeric protein containing the NH2-terminal extension of the highest molecular weight form of bFGF fused to beta-galactosidase (beta-gal). After transfection in a transient expression system, the chimeric protein accumulated in the nuclei of transfected cells, while the wild-type beta-gal was found predominantly in the cytoplasm.

Recovery of mitogenic activity of a growth factor mutant with a nuclear translocation sequence

Heparin-binding growth factor-1 (HBGF-1) is an angiogenic polypeptide mitogen for mesoderm- and neuroectoderm-derived cells in vitro and remains biologically active after truncation of the amino-terminal domain (HBGF-1 alpha) of the HBGF-1 beta precursor. Polymerase chain reaction mutagenesis and prokaryotic expression systems were used to prepare a mutant of HBGF-1 alpha lacking a putative nuclear translocation sequence (amino acid residues 21 to 27; HBGF-1U). Although HBGF-1U retains its ability to bind to heparin, HBGF-1U fails to induce DNA synthesis and cell proliferation at concentrations sufficient to induce intracellular receptor-mediated tyrosine phosphorylation and c-fos expression. Attachment of the nuclear translocation sequence from yeast histone 2B at the amino terminus of HBGF-1U yields a chimeric polypeptide (HBGF-1U2) with mitogenic activity in vitro and indicates that nuclear translocation is important for this biological response.

Human DNA ligase I cDNA: cloning and functional expression in Saccharomyces cerevisiae

Human cDNA clones encoding the major DNA ligase activity in proliferating cells, DNA ligase I, were isolated by two independent methods. In one approach, a human cDNA library was screened by hybridization with oligonucleotides deduced from partial amino acid sequence of purified bovine DNA ligase I. In an alternative approach, a human cDNA library was screened for functional expression of a polypeptide able to complement a cdc9 temperature-sensitive DNA ligase mutant of Saccharomyces cerevisiae. The sequence of an apparently full-length cDNA encodes a 102-kDa protein, indistinguishable in size from authentic human DNA ligase I. The deduced amino acid sequence of the human DNA ligase I cDNA is 40% homologous to the smaller DNA ligases of S. cerevisiae and Schizosaccharomyces pombe, homology being confined to the carboxyl-terminal regions of the respective proteins. Hybridization between the cloned sequences and mRNA and genomic DNA indicates that the human enzyme is transcribed from a single-copy gene on chromosome 19.

Isolation and sequence analysis of CDC43, a gene involved in the control of cell polarity in Saccharomyces cerevisiae

The Saccharomyces cerevisiae CDC43 gene product is involved in establishing cell polarity during the cell-division cycle. When grown at restrictive temperatures, temperature-sensitive cdc43 mutants are unable to form buds and display delocalized cell-surface deposition [Adams et al., J. Cell Biol. (1990) in press]. We have isolated a cdc43-complementing plasmid from a yeast genomic-DNA library and localized the CDC43 gene, by subcloning and transposon-mutagenesis experiments, to a 1.2-kb region of DNA that contained only one significant ATG-initiated open reading frame of 213 codons. The putative CDC43 gene product contains a possible nuclear-localization signal sequence, a cysteine-rich domain and a histidine-rich domain, and a region that is similar in structure to alpha-helix-turn-alpha-helix structural domains present in some prokaryotic and eukaryotic DNA-binding proteins.

Signal transduction by steroid hormones: nuclear localization is differentially regulated in estrogen and glucocorticoid receptors

The glucocorticoid receptor accumulates in nuclei only in the presence of bound hormone, whereas the estrogen receptor has been reported to be constitutively nuclear. To investigate this distinction, we compared the nuclear localization domains of the two receptors and the capacity of their respective hormone-binding regions to regulate nuclear localization activity. As with the glucocorticoid receptor, we showed that the human estrogen receptor contained a nuclear localization signal between the DNA-binding and hormone-binding regions (amino acids 256-303); however, in contrast to the glucocorticoid receptor, the estrogen receptor lacked a second nuclear localization domain within the hormone-binding region. Moreover, the hormone-binding domain of the unliganded estrogen receptor failed to regulate nuclear localization signals, although it efficiently regulated other receptor functions. We conclude that the two receptors employ a common mechanism for signal transduction involving a novel "inactivation" function, but that they differ in their control of nuclear localization. Thus, despite the strong relatedness of the estrogen and glucocorticoid receptors in structure and activity, certain differences in their properties could have important functional implications.