Translation and the cytoskeleton: a mechanism for targeted protein synthesis

A kinesin adapter directly mediates dendritic mRNA localization during neural development in mice

Motor protein-based active transport is essential for mRNA localization and local translation in animal cells, yet how mRNA granules interact with motor proteins remains poorly understood. Using an unbiased yeast two-hybrid screen for interactions between murine RNA-binding proteins (RBPs) and motor proteins, here we identified protein interaction with APP tail-1 (PAT1) as a potential direct adapter between zipcode-binding protein 1 (ZBP1, a β-actin RBP) and the kinesin-I motor complex. The amino acid sequence of mouse PAT1 is similar to that of the kinesin light chain (KLC), and we found that PAT1 binds to KLC directly. Studying PAT1 in mouse primary hippocampal neuronal cultures from both sexes and using structured illumination microscopic imaging of these neurons, we observed that brain-derived neurotrophic factor (BDNF) enhances co-localization of dendritic ZBP1 and PAT1 within granules that also contain kinesin-I. PAT1 is essential for BDNF-stimulated neuronal growth cone development and dendritic protrusion formation, and we noted that ZBP1 and PAT1 co-locate along with β-actin mRNA in actively transported granules in living neurons. Acute disruption of the PAT1-ZBP1 interaction in neurons with PAT1 siRNA or a dominant-negative ZBP1 construct diminished localization of β-actin mRNA but not of Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα) mRNA in dendrites. The aberrant β-actin mRNA localization resulted in abnormal dendritic protrusions and growth cone dynamics. These results suggest a critical role for PAT1 in BDNF-induced β-actin mRNA transport during postnatal development and reveal a new molecular mechanism for mRNA localization in vertebrates.

Triiodothyronine differentially modulates the LH and FSH synthesis and secretion in male rats

Hypothyroidism and thyrotoxicosis produce adverse effects in male reproduction by unknown mechanisms. We investigated whether triiodothyronine (T3) modulates luteinizing hormone (LH) and follicle stimulating hormone (FSH) synthesis/secretion, by inducing different thyroid states. In hypothyroidism, the content of Lhb and Fshb mRNAs was increased, while their association to ribosomes and the protein content were reduced and the serum LH and FSH concentrations were augmented and decreased, respectively. Thyrotoxicosis reduced Lhb mRNA and LH serum concentration, and increased Lhb mRNA translational rate. The Fshb mRNA content and its association to ribosomes were also increased, whereas FSH serum concentrations were comparable to euthyroid levels. Acute T3 treatment decreased the total content of Lhb and Fshb mRNAs, and increased their association to ribosomes, as well as the LHB and FSHB contents in secretory granules. This study shows that T3 acts on gonadotrophs, resulting in direct effects on LH and FSH synthesis/secretion of male rats, suggesting that some reproductive disorders observed in men may be associated with thyroid hormone imbalances.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

Transcriptome-wide regulation of pre-mRNA splicing and mRNA localization by muscleblind proteins

The muscleblind-like (Mbnl) family of RNA-binding proteins plays important roles in muscle and eye development and in myotonic dystrophy (DM), in which expanded CUG or CCUG repeats functionally deplete Mbnl proteins. We identified transcriptome-wide functional and biophysical targets of Mbnl proteins in brain, heart, muscle, and myoblasts by using RNA-seq and CLIP-seq approaches. This analysis identified several hundred splicing events whose regulation depended on Mbnl function in a pattern indicating functional interchangeability between Mbnl1 and Mbnl2. A nucleotide resolution RNA map associated repression or activation of exon splicing with Mbnl binding near either 3' splice site or near the downstream 5' splice site, respectively. Transcriptomic analysis of subcellular compartments uncovered a global role for Mbnls in regulating localization of mRNAs in both mouse and Drosophila cells, and Mbnl-dependent translation and protein secretion were observed for a subset of mRNAs with Mbnl-dependent localization. These findings hold several new implications for DM pathogenesis.

Histone deacetylase 6 associates with ribosomes and regulates de novo protein translation during arsenite stress

Histone deacetylase 6 (HDAC6) is known as a cytoplasmic enzyme that regulates cell migration, cell adhesion, and degradation of misfolded proteins by deacetylating substrates such as α-tubulin and Hsp90. When HaCaT keratinocytes were exposed to 1-200μM sodium arsenite, we observed perinuclear localization of HDAC6 within 30 min. Although the overall level of HDAC6 protein did not change, sodium arsenite caused an increase of HDAC6 in ribosomal fractions. Separation of ribosomal subunits versus intact ribosomes or polysomes indicated that HDAC6 was mainly detected in 40/43S fractions containing the small ribosomal subunit in untreated cells but was associated with 40/43S and 60/80S ribosomal fractions in arsenite-treated cells. Immunocytochemistry studies revealed that arsenite caused colocalization of HDAC6 with the ribosomal large and small subunit protein L36a and S6. Both L36a and S6 were detected in the immunocomplex of HDAC6 isolated from arsenite-treated cells. The observed physical interaction of HDAC6 with ribosomes pointed to a role of HDAC6 in stress-induced protein translation. Among arsenite stress-induced proteins, de novo Nrf2 protein translation was inhibited by Tubastatin A. These data demonstrate that HDAC6 was recruited to ribosomes, physically interacted with ribosomal proteins, and regulated de novo protein translation in keratinocytes responding to arsenite stress.

Interactions between laminin receptor and the cytoskeleton during translation and cell motility

Human laminin receptor acts as both a component of the 40S ribosomal subunit to mediate cellular translation and as a cell surface receptor that interacts with components of the extracellular matrix. Due to its role as the cell surface receptor for several viruses and its overexpression in several types of cancer, laminin receptor is a pathologically significant protein. Previous studies have determined that ribosomes are associated with components of the cytoskeleton, however the specific ribosomal component(s) responsible has not been determined. Our studies show that laminin receptor binds directly to tubulin. Through the use of siRNA and cytoskeletal inhibitors we demonstrate that laminin receptor acts as a tethering protein, holding the ribosome to tubulin, which is integral to cellular translation. Our studies also show that laminin receptor is capable of binding directly to actin. Through the use of siRNA and cytoskeletal inhibitors we have shown that this laminin receptor-actin interaction is critical for cell migration. These data indicate that interactions between laminin receptor and the cytoskeleton are vital in mediating two processes that are intimately linked to cancer, cellular translation and migration.

Pelota interacts with HAX1, EIF3G and SRPX and the resulting protein complexes are associated with the actin cytoskeleton

Background

Pelota (PELO) is an evolutionary conserved protein, which has been reported to be involved in the regulation of cell proliferation and stem cell self-renewal. Recent studies revealed the essential role of PELO in the No-Go mRNA decay, by which mRNA with translational stall are endonucleotically cleaved and degraded. Further, PELO-deficient mice die early during gastrulation due to defects in cell proliferation and/or differentiation.

Results

We show here that PELO is associated with actin microfilaments of mammalian cells. Overexpression of human PELO in Hep2G cells had prominent effect on cell growth, cytoskeleton organization and cell spreading. To find proteins interacting with PELO, full-length human PELO cDNA was used as a bait in a yeast two-hybrid screening assay. Partial sequences of HAX1, EIF3G and SRPX protein were identified as PELO-interacting partners from the screening. The interactions between PELO and HAX1, EIF3G and SRPX were confirmed in vitro by GST pull-down assays and in vivo by co-immunoprecipitation. Furthermore, the PELO interaction domain was mapped to residues 268-385 containing the c-terminal and acidic tail domain. By bimolecular fluorescence complementation assay (BiFC), we found that protein complexes resulting from the interactions between PELO and either HAX1, EIF3G or SRPX were mainly localized to cytoskeletal filaments.

Conclusion

We could show that PELO is subcellularly localized at the actin cytoskeleton, interacts with HAX1, EIF3G and SRPX proteins and that this interaction occurs at the cytoskeleton. Binding of PELO to cytoskeleton-associated proteins may facilitate PELO to detect and degrade aberrant mRNAs, at which the ribosome is stalled during translation.

The 5' cap of tobacco mosaic virus (TMV) is required for virion attachment to the actin/endoplasmic reticulum network during early infection

Almost nothing is known of the earliest stages of plant virus infections. To address this, we microinjected Cy3 (UTP)-labelled tobacco mosaic virus (TMV) into living tobacco trichome cells. The Cy3-virions were infectious, and the viral genome trafficked from cell to cell. However, neither the fluorescent vRNA pool nor the co-injected green fluorescent protein (GFP) left the injected trichome, indicating that the synthesis of (unlabelled) progeny viral (v)RNA is required to initiate cell-to-cell movement, and that virus movement is not accompanied by passive plasmodesmatal gating. Cy3-vRNA formed granules that became anchored to the motile cortical actin/endoplasmic reticulum (ER) network within minutes of injection. Granule movement on actin/ER was arrested by actin inhibitors indicating actin-dependent RNA movement. The 5' methylguanosine cap was shown to be required for vRNA anchoring to the actin/ER. TMV vRNA lacking the 5' cap failed to form granules and was degraded in the cytoplasm. Removal of the 3' untranslated region or replicase both inhibited replication but did not prevent granule formation and movement. Dual-labelled TMV virions in which the vRNA and the coat protein were highlighted with different fluorophores showed that both fluorescent signals were initially located on the same ER-bound granules, indicating that TMV virions may become attached to the ER prior to uncoating of the viral genome.

Micropatterning of single endothelial cell shape reveals a tight coupling between nuclear volume in G1 and proliferation

Shape-dependent local differentials in cell proliferation are considered to be a major driving mechanism of structuring processes in vivo, such as embryogenesis, wound healing, and angiogenesis. However, the specific biophysical signaling by which changes in cell shape contribute to cell cycle regulation remains poorly understood. Here, we describe our study of the roles of nuclear volume and cytoskeletal mechanics in mediating shape control of proliferation in single endothelial cells. Micropatterned adhesive islands were used to independently control cell spreading and elongation. We show that, irrespective of elongation, nuclear volume and apparent chromatin decondensation of cells in G1 systematically increased with cell spreading and highly correlated with DNA synthesis (percent of cells in the S phase). In contrast, cell elongation dramatically affected the organization of the actin cytoskeleton, markedly reduced both cytoskeletal stiffness (measured dorsally with atomic force microscopy) and contractility (measured ventrally with traction microscopy), and increased mechanical anisotropy, without affecting either DNA synthesis or nuclear volume. Our results reveal that the nuclear volume in G1 is predictive of the proliferative status of single endothelial cells within a population, whereas cell stiffness and contractility are not. These findings show that the effects of cell mechanics in shape control of proliferation are far more complex than a linear or straightforward relationship. Our data are consistent with a mechanism by which spreading of cells in G1 partially enhances proliferation by inducing nuclear swelling and decreasing chromatin condensation, thereby rendering DNA more accessible to the replication machinery.

Elongation factor 1alpha binds to the region of the metallothionein-1 mRNA implicated in perinuclear localization--importance of an internal stem-loop

In eukaryotic cells, mRNA localization can provide local protein synthesis. Metallothionein-1 (MT-1) mRNA is associated with the perinuclear cytoskeleton, and this is essential for subsequent nuclear import of the protein. The present study defines the cis-acting localization signal and a trans-acting binding protein. Gel retardation and UV cross-linking assays using MT-1 3'UTR transcripts and CHO cell extracts revealed formation of a complex containing a approximately 50-kDa protein. Only localization-positive mutant transcripts competed for binding of this protein. Using an RNA affinity technique, Western blotting, mass spectrometry, and a supershift assay, the protein was identified as Elongation factor 1alpha (eEF1alpha). Mutation and deletion analysis showed that two regions, nucleotides 21-36 and 66-76, were required for both binding and localization. RNA-folding prediction combined with chemical and enzymatic probing experiments suggest that these regions are in juxtaposition within a stem/internal loop structure. Mutations that are predicted to alter this structure abrogate protein binding. Our hypothesis is that the cis-acting signal in MT-1 3'UTR is formed by this stem/internal loop, that it binds eEF1alpha, and that eEF1alpha-cytoskeleton interactions play a role in perinuclear mRNA localization.

Rapid depletion of mutant eukaryotic initiation factor 5A at restrictive temperature reveals connections to actin cytoskeleton and cell cycle progression

Eukaryotic initiation factor 5A (eIF5A) is the only protein in nature that contains hypusine, an unusual amino acid derived from the modification of lysine by spermidine. Two genes, TIF51A and TIF51B, encode eIF5A in the yeast Saccharomyces cerevisiae. In an effort to understand the structure-function relationship of eIF5A, we have generated yeast mutants by introducing plasmid-borne tif51A into a double null strain where both TIF51A and TIF51B have been disrupted. One of the mutants, tsL102A strain (tif51A L102A tif51aDelta tif51bDelta) exhibits a strong temperature-sensitive growth phenotype. At the restrictive temperature, tsL102A strain also exhibits a cell shape change, a lack of volume change in response to temperature increase and becomes more sensitive to ethanol, a hallmark of defects in the PKC/WSC cell wall integrity pathway. In addition, a striking change in actin dynamics and a complete cell cycle arrest at G1 phase occur in tsL102A cells at restrictive temperature. The temperature-sensitivity of tsL102A strain is due to a rapid loss of mutant eIF5A with the half-life reduced from 6 h at permissive temperature to 20 min at restrictive temperature. Phenylmethyl sulfonylfluoride (PMSF), an irreversible inhibitor of serine protease, inhibited the degradation of mutant eIF5A and suppressed the temperature-sensitive growth arrest. Sorbitol, an osmotic stabilizer that complement defects in PKC/WSC pathways, stabilizes the mutant eIF5A and suppresses all the observed temperature-sensitive phenotypes.

An eleven nucleotide section of the 3'-untranslated region is required for perinuclear localization of rat metallothionein-1 mRNA

Localization of mRNAs provides a novel mechanism for synthesis of proteins close to their site of function. MT1 (metallothionein-1) is a small, metal-binding protein that is largely cytoplasmic but which can be found in the nucleus. The localization of rat MT1 requires the perinuclear localization of its mRNA by a mechanism dependent on the 3'-UTR (3'-untranslated region). The present study investigates the nature of this mRNA localization signal using Chinese-hamster ovary cells transfected with gene constructs in which either MT1 or the globin coding region is linked to different sequences from the MT1 3'-UTR. Deletion, mutagenesis and antisense oligonucleotide approaches indicate that nt 45-76 of the 3'-UTR, in particular nt 66-76, are required for the localization of either MT1 mRNA or chimaeric transcripts in which a beta-globin coding region is linked to sequences from the MT1 3'-UTR. This section of the 3'-UTR contains a CACC repeat. Two mutations that are predicted to alter the secondary structure of this region also impair localization. Our hypothesis is that the perinuclear localization signal in MT1 mRNA is formed by a combination of the CACC repeat and its structural context.

Differential translation and fragile X syndrome

Fragile X syndrome (FXS) is caused by the transcriptional silencing of the Fmr1 gene, which encodes a protein (FMRP) that can act as a translational suppressor in dendrites, and is characterized by a preponderance of abnormally long, thin and tortuous dendritic spines. According to a current theory of FXS, the loss of FMRP expression leads to an exaggeration of translation responses linked to group I metabotropic glutamate receptors. Such responses are involved in the consolidation of a form of long-term depression that is enhanced in Fmr1 knockout mice and in the elongation of dendritic spines, resembling synaptic phenotypes over-represented in fragile X brain. These observations place fragile X research at the heart of a long-standing issue in neuroscience. The consolidation of memory, and several distinct forms of synaptic plasticity considered to be substrates of memory, requires mRNA translation and is associated with changes in spine morphology. A recent convergence of research on FXS and on the involvement of translation in various forms of synaptic plasticity has been very informative on this issue and on mechanisms underlying FXS. Evidence suggests a general relationship in which the receptors that induce distinct forms of efficacy change differentially regulate translation to produce unique spine shapes involved in their consolidation. We discuss several potential mechanisms for differential translation and the notion that FXS represents an exaggeration of one 'channel' in a set of translation-dependent consolidation responses.

Isolation and identification of a protein binding to the localization element of Metallothionein-1 mRNA

mRNA localization provides a mechanism for localized protein synthesis. mRNAs encoding certain proteins, including c-MYC, c-FOS, MT-1 (Metallothionein-1) and vimentin, are localized around the nuclei of mammalian cells and are associated with the cytoskeleton. Targeting of these mRNAs to the perinuclear cytoplasm is mediated by elements within their 3'-UTRs (3'-untranslated regions), but many of the trans-acting proteins remain unidentified. UV cross-linking assays using radiolabelled transcripts indicated that a protein of approx. 50 kDa (from the Chinese-hamster ovary cell extracts) bound to the MT-1 3'-UTR sequence. Competition experiments using unlabelled mutant 3'-UTR RNAs revealed that the binding of this protein is specific to localization-positive mutants. Isolation of a 50 kDa protein was achieved by an RNA affinity-based method in which biotinylated MT-1 3'-UTR RNA was anchored to paramagnetic beads. Bound proteins were eluted and analysed by SDS/PAGE. The 50 kDa protein was extracted from the gel, subjected to trypsin digestion and identified by matrix-assisted laser-desorption/ionization-time-of-flight mass spectrometry as eukaryote elongation factor 1alpha.

Annexin A2 binds to the localization signal in the 3' untranslated region of c-myc mRNA

Messenger RNA trafficking, which provides a mechanism for local protein synthesis, is dependent on cis-acting sequences in the 3' untranslated regions (3'UTRs) of the mRNAs concerned acting together with trans-acting proteins. The C-MYC transcription factor is a proto-oncogene product involved in cell proliferation, differentiation and apoptosis. Localization of c-myc mRNA to the perinuclear cytoplasm and its association with the cytoskeleton is determined by a signal in the 3'UTR. Here we show the specific binding of a trans-acting factor to the perinuclear localization element in the 3'UTR of c-myc mRNA and identify this protein as annexin A2. Gel retardation and UV cross-linking experiments showed that proteins in fibroblast extracts formed complexes with the region of c-myc 3'UTR implicated in localization; a protein of approximately 36 kDa exhibited specific, Ca(2+)-dependent binding. Binding was reduced by introduction of a mutation that abrogates localization. Using RNA-affinity columns followed by gel electrophoresis and mass spectrometry this protein was identified as annexin A2. The RNA-protein complex formed by cell extracts was further retarded by anti-(annexin A2). Purified annexin A2 bound to the same region of the c-myc 3'UTR but binding was reduced by introduction of a mutation, as with cell extracts. It is proposed that binding of annexin A2 to the localization signal in the c-myc mRNA leads to association with the cytoskeleton and perinuclear localization. The data indicate a novel functional role for the RNA-binding properties of annexin A2 in perinuclear localization of mRNA and the association with the cytoskeleton.

The microtubule-associated protein tumor overexpressed gene binds to the RNA trafficking protein heterogeneous nuclear ribonucleoprotein A2

In neural cells, such as oligodendrocytes and neurons, transport of certain RNAs along microtubules is mediated by the cis-acting heterogeneous nuclear ribonucleoprotein A2 response element (A2RE) trafficking element and the cognate trans-acting heterogeneous nuclear ribonucleoprotein (hnRNP) A2 trafficking factor. Using a yeast two-hybrid screen, we have identified a microtubule-associated protein, tumor overexpressed gene (TOG)2, as an hnRNP A2 binding partner. The C-terminal third of TOG2 is sufficient for hnRNP A2 binding. TOG2, the large protein isoform of TOG, is the only isoform detected in oligodendrocytes in culture. TOG coimmunoprecipitates with hnRNP A2 present in the cytoskeleton (CSK) fraction of neural cells, and both coprecipitate with microtubule stabilized pellets. Staining with anti-TOG reveals puncta that are localized in proximity to microtubules, often at the plus ends. TOG is colocalized with hnRNP A2 and A2RE-mRNA in trafficking granules that remain associated with CSK-insoluble tissue. These data suggest that TOG mediates the association of hnRNP A2-positive granules with microtubules during transport and/or localization.

3′-Untranslated regions are important in mRNA localization and translation: lessons from selenium and metallothionein

There is increasing evidence that 3′-UTRs (3′-untranslated regions) of mRNAs contain regulatory elements that have important roles in post-transcriptional control of gene expression. For example, 3′-UTRs are important in determining mRNA localization and directing selenocysteine insertion during selenoprotein synthesis. Metallothionein mRNA is localized around the nucleus and associated with the cytoskeleton; this is determined by the 3′-UTR. Deletion and mutagenesis studies are defining the nature of the signal. Incorrect mRNA localization prevents subsequent nuclear localization of metallothionein protein and affects its function. Selenium (Se) is incorporated as selenocysteine into approx. 30 mammalian proteins by a mechanism that requires a specific structure within the 3′-UTR of the corresponding mRNAs. When Se supply is low the effect on selenoprotein expression is not uniform but shows differential effects that are tissue- and protein-specific; there is a ‘prioritization’ of selenoprotein synthesis that is partly influenced by the 3′-UTRs of the different mRNAs. Single-nucleotide polymorphisms in the gene regions corresponding to 3′-UTRs could potentially influence gene regulation. We have discovered a common polymorphism in a part of the glutathione peroxidase 4 gene which corresponds to the 3′-UTR, and our recent results suggest that this single-nucleotide polymorphism has functional and physiological effects, as well as altered frequency in disease.

Expression of fragments of translation initiation factor eIF4GI reveals a nuclear localisation signal within the N-terminal apoptotic cleavage fragment N-FAG

The eukaryotic initiation factor eIF4GI plays a central role in the assembly of a competent initiation complex at the 5′ end of an mRNA. Five isoforms of eIF4G exist in cells, arising from alternative translation initiation. During picornaviral infection or apoptosis, eIF4GI is cleaved proteolytically to yield distinct fragments. Using HeLa cells, we have examined the fate of these proteins in the cell. We have found that while endogenous eIF4GI is predominantly cytoplasmic, a population can also be visualised in the nucleus. Furthermore, eIF4GI is localised primarily at the nuclear periphery in the vicinity of eIF4E and PABP1. Transient transfection of HeLa cells with different myc-tagged isoforms of eIF4GI did not result in any obvious differences in their localisation. However, expression of discrete fragments of eIF4GI corresponding to those generated after apoptosis or picornaviral infection generated a distinctive, but intricate localisation pattern. Our work shows that the N-terminal apoptotic cleavage fragment N-FAG contains a sequence of basic amino acids that can act as a nuclear localisation signal. In addition, the presence or absence of the sequence flanking and including the eIF4E binding site (residues 533-682) confers a distinct cellular distribution pattern for the central domain of eIF4GI.

Translationally distinct populations of NMDA receptor subunit NR1 mRNA in the developing rat brain

The translational activity of the NMDA subunit 1 (NR1) mRNA was examined in the developing rat brain by sucrose gradient fractionation. One translationally-active pool of NR1 mRNA was associated with large polyribosomes (polysomes) over the entire developmental period examined. A second NR1 mRNA pool, approximately half of the NR1 mRNA at post-natal day 4, sedimented only within the two to three ribosome range, indicating that it was translationally blocked during early brain development despite active translation of mRNAs coding for the NR2 subunits of the receptor. At post-natal day 4, both NR1 mRNA pools were distributed throughout the brain and contained similar profiles of NR1 mRNA splice variants, except that NR1-3 appeared to be present only in the translationally-blocked NR1 pool. After post-natal day 8, the translationally-blocked NR1 mRNA pool became progressively active within a background of globally-decreasing brain translational activity.

eEF1A isoforms change in abundance and actin-binding activity during maize endosperm development

Eukaryotic elongation factor 1A (eEF1A) appears to be a multifunctional protein because several biochemical activities have been described for this protein, in addition to its role in protein synthesis. In maize (Zea mays) endosperm, the synthesis of eEF1A is increased in o2 (opaque2) mutants, and its concentration is highly correlated with the protein-bound lysine content. To understand the basis of this relationship, we purified eEF1A isoforms from developing endosperm and investigated their accumulation and their functional and structural properties. Formation of three isoforms appears to be developmentally regulated and independent of the o2 mutation, although one isoform predominated in one high lysine o2 inbred. The purified proteins differ in their ability to bind F-actin in vitro, suggesting that they are functionally distinct. However, they share similar aminoacyl-tRNA-binding activities. Tandem mass spectrometry revealed that each isoform is composed of the four same gene products, which are modified posttranslationally by methylation and phosphorylation. The chemical differences that account for their different actin-binding activities could not be determined.

Overproduction of a conserved domain of fission yeast and mammalian translation initiation factor eIF4G causes aberrant cell morphology and results in disruption of the localization of F-actin and the organization of microtubules

BACKGROUND

The recruitment of mRNA for translation involves the assembly at the 5'cap of a complex of three initiation factors: the cap binding protein eIF4E, the ATP-dependent RNA helicase eIF4A and the scaffold protein eIF4G. eIF4G mediates the binding of this mRNA-protein complex to the 43S ribosomal preinitiation complex. There is growing recognition that the components of the translational apparatus interact functionally with cytoskeletal components. Here we report specific effects of the over-expression of human and fission yeast eIF4G domains on cell morphology in Schizosaccharomyces pombe.

RESULTS

A single gene encoding fission yeast eIF4G was identified and demonstrated to be essential. We have over-expressed fragments corresponding to the conserved functional domains of eIF4G. At expression levels that did not disrupt rates of overall translation or protein accumulation, a fragment of S. pombe eIF4G, 4G-NOB, corresponding to the minimal region of human eIF4G required to support cap-independent mRNA recruitment, was found to impair cell proliferation in fission yeast. This resulted from defects in cytokinesis, and was associated with the disruption of both microtubules and actin microfilaments. The over-expressed fragment was itself localized to the cell ends, the nuclear periphery and the septum.

CONCLUSIONS

This is the first demonstration of a link between a translation initiation factor and mechanisms controlling cell morphology. The data suggest a direct or indirect interaction between the functional domains of eIF4G and cellular structures involved in cytokinesis.

The 3' untranslated region of human vimentin mRNA interacts with protein complexes containing eEF-1gamma and HAX-1

Previously, we have shown that the vimentin 3' untranslated region (3'UTR) contains a highly conserved region, which is sufficient for the perinuclear localization of a reporter mRNA. This region was shown to specifically bind protein(s) by band shift analyses. UV-cross-linking studies suggest these proteins are 46- and 35-kDa in mass. Here, we have used this sequence as 'bait' to isolate RNA binding proteins using the yeast three-hybrid method. This technique relies on a functional assay detecting bona fide RNA-protein interaction in vivo. Three cDNA isolates, HAX-1, eEF-1gamma and hRIP, code for proteins of a size consistent with in vitro cross- linking studies. In all cases, recombinant proteins were capable of binding RNA in vitro. Although hRIP is thought to be a general mRNA binding protein, this represents an unreported activity for eEF-1gamma and HAX-1. Moreover, HAX-1 binding appears to be specific to vimentin's 3'UTR. Both in vivo synthesized eEF-1gamma and HAX-1 proteins were 'pulled out' of HeLa whole cell extracts by binding to a RNA affinity column comprised of vimentin's 3'UTR. Moreover, size-fractionation of extracts results in the separation of large complexes containing either eEF-1gamma or HAX-1. Thus, in addition to their known functions, both eEF-1gamma and HAX-1 are RNA binding proteins, which suggests new roles in mRNA translation and/or perinuclear localization.

Molecular interaction between human tumor marker protein p150, the largest subunit of eIF3, and intermediate filament protein K7

The human tumor marker protein p150 was identified as the largest subunit of eukaryotic translation initiation factor 3 (eIF3) (also known as p170/p180). Its expression level is not only upregulated in many transformed cell lines, but also in several human cancers including breast, cervical, esophageal, and stomach carcinomas. The function of p150 in cancer and initiation of translation are not well understood. Using the yeast two-hybrid genetic screen, we found that a portion of p150 interacts with hPrt1, another subunit of eIF3, and cytokeratin 7, an intermediate filament protein. The interactions between p150 and hPrt1, and between p150 and cytokeratin 7 were verified both in vivo and in vitro. The interaction site for hPrt1 was mapped to the carboxyl half of the coiled-coil region of the p150 protein between amino acids 664-835. The expression of hPrt1 was clearly upregulated in cancer tissue, similarly to that of p150. By contrast, no substantial difference in the expression level of cytokeratin 7 was observed between cancer and normal breast tissue, suggesting that cytokeratin 7 expression is not co-regulated with p150. Taken together, our studies suggest a new role for p150 in translation initiation, possibly by acting as an adapter molecule between the translation initiation apparatus and the cytoskeleton structure in the cell.

Rpg1p/Tif32p, a subunit of translation initiation factor 3, interacts with actin-associated protein Sla2p

The yeast two-hybrid system was used to screen for proteins that interact in vivo with Saccharomyces cerevisiae Rpg1p/Tif32p, the large subunit of the translation initiation factor 3 core complex (eIF3). Eight positive clones encoding portions of the SLA2/END4/MOP2 gene were isolated. They overlapped in the region of amino acids 318-550. Subsequent deletion analysis of Sla2p showed that amino acids 318-373 were essential for the two-hybrid protein-protein interaction. The N-terminal part of Rpg1p (aa 1-615) was essential and sufficient for the Rpg1p-Sla2p interaction. A coimmunoprecipitation assay provided additional evidence for the physical interaction of Rpg1p/Tif32p with Sla2p in vivo. Using immunofluorescence microscopy, Rpg1p and Sla2p proteins were colocalized at the patch associated with the tip of emerging bud. Considering the essential role of Rpg1p as the large subunit of the eIF3 core complex and the association of Sla2p with the actin cytoskeleton, a putative role of the Rpg1p-Sla2p interaction in localized translation is discussed.

Shape-dependent control of cell growth, differentiation, and apoptosis: switching between attractors in cell regulatory networks

Development of characteristic tissue patterns requires that individual cells be switched locally between different phenotypes or "fates;" while one cell may proliferate, its neighbors may differentiate or die. Recent studies have revealed that local switching between these different gene programs is controlled through interplay between soluble growth factors, insoluble extracellular matrix molecules, and mechanical forces which produce cell shape distortion. Although the precise molecular basis remains unknown, shape-dependent control of cell growth and function appears to be mediated by tension-dependent changes in the actin cytoskeleton. However, the question remains: how can a generalized physical stimulus, such as cell distortion, activate the same set of genes and signaling proteins that are triggered by molecules which bind to specific cell surface receptors. In this article, we use computer simulations based on dynamic Boolean networks to show that the different cell fates that a particular cell can exhibit may represent a preprogrammed set of common end programs or "attractors" which self-organize within the cell's regulatory networks. In this type of dynamic network model of information processing, generalized stimuli (e.g., mechanical forces) and specific molecular cues elicit signals which follow different trajectories, but eventually converge onto one of a small set of common end programs (growth, quiescence, differentiation, apoptosis, etc.). In other words, if cells use this type of information processing system, then control of cell function would involve selection of preexisting (latent) behavioral modes of the cell, rather than instruction by specific binding molecules. Importantly, the results of the computer simulation closely mimic experimental data obtained with living endothelial cells. The major implication of this finding is that current methods used for analysis of cell function that rely on characterization of linear signaling pathways or clusters of genes with common activity profiles may overlook the most critical features of cellular information processing which normally determine how signal specificity is established and maintained in living cells.

Protein 4.1R binding to eIF3-p44 suggests an interaction between the cytoskeletal network and the translation apparatus

Erythroid protein 4.1 (4.1R) is an 80-kd cytoskeletal protein that stabilizes the membrane-skeletal network structure underlying the lipid bilayer. Using the carboxyl terminal domain (22/24 kd) of 4.1R as bait in a yeast 2-hybrid screen, we isolated cDNA clones encoding a polypeptide of eIF3-p44, which represents a subunit of a eukaryotic translation initiation factor 3 (eIF3) complex. The eIF3 complex consists of at least 10 subunits that play an essential role in the pathway of protein translation initiation. Northern blot analysis revealed that eIF3-p44 (approximately 1.35 kb) is constitutively expressed in many tissues. The essential sequence for this interaction was mapped to the carboxyl-terminus of 4.1R (residues 525-622) and a region (residues 54-321) of eIF3-p44. The direct association between 4.1R and eIF3-p44 was further confirmed by in vitro binding assays and coimmunoprecipitation studies. To characterize the functions of eIF3-p44, we depleted eIF3-p44 from rabbit reticulocyte lysates either by anti-eIF3-p44 antibody or by GST/4.1R-80 fusion protein. Our results show that the eIF3-p44 depleted cell-free translation system was unable to synthesize proteins efficiently. The direct association between 4.1R and elF3-p44 suggests that 4.1R may act as an anchor protein that links the cytoskeleton network to the translation apparatus.

Protein 4.1R binding to eIF3-p44 suggests an interaction between the cytoskeletal network and the translation apparatus

Erythroid protein 4.1 (4.1R) is an 80-kd cytoskeletal protein that stabilizes the membrane-skeletal network structure underlying the lipid bilayer. Using the carboxyl terminal domain (22/24 kd) of 4.1R as bait in a yeast 2-hybrid screen, we isolated cDNA clones encoding a polypeptide of eIF3-p44, which represents a subunit of a eukaryotic translation initiation factor 3 (eIF3) complex. The eIF3 complex consists of at least 10 subunits that play an essential role in the pathway of protein translation initiation. Northern blot analysis revealed that eIF3-p44 (approximately 1.35 kb) is constitutively expressed in many tissues. The essential sequence for this interaction was mapped to the carboxyl-terminus of 4.1R (residues 525-622) and a region (residues 54-321) of eIF3-p44. The direct association between 4.1R and eIF3-p44 was further confirmed by in vitro binding assays and coimmunoprecipitation studies. To characterize the functions of eIF3-p44, we depleted eIF3-p44 from rabbit reticulocyte lysates either by anti-eIF3-p44 antibody or by GST/4.1R-80 fusion protein. Our results show that the eIF3-p44 depleted cell-free translation system was unable to synthesize proteins efficiently. The direct association between 4.1R and elF3-p44 suggests that 4.1R may act as an anchor protein that links the cytoskeleton network to the translation apparatus.

Nuclear import of metallothionein requires its mRNA to be associated with the perinuclear cytoskeleton

The influence of mRNA localization on metallothionein-1 protein distribution was studied by immunocytochemistry. We used Chinese hamster ovary cells that had been transfected with either a native metallothionein-1 gene construct or metallothionein-1 5'-untranslated region and coding sequences linked to the 3'-untranslated region from glutathione peroxidase. The change in the 3'-untranslated region caused the delocalization of the mRNA with a loss of the perinuclear localization and association with the cytoskeleton. Clones were selected which expressed similar levels of metallothionein-1 protein, as assessed by radioimmunoassay. The results showed that loss of metallothionein-1 mRNA localization was associated with a loss of metallothionein-1 protein localization, most notably with a lack of metallothionein-1 protein in the nucleus of synchronized cells which were beginning to synthesize DNA. This indicates that the association of metallothionein-1 mRNA with the cytoskeleton around the nucleus is essential for efficient shuttling of the protein into the nucleus during the G(1) to S phase transition. This is the first demonstration of a physiological role for perinuclear mRNA localization and we propose that such localization may be important for a wide range of nuclear proteins, including those that shuttle between nucleus and cytoplasm in a cell cycle dependent manner.

Messenger RNA-binding properties of nonpolysomal ribonucleoproteins from heat-stressed tomato cells

Most cells experiencing heat stress reprogram their translational machinery to favor the synthesis of heat-stress proteins. Translation of other transcripts is almost completely repressed, but most untranslated messengers are not degraded. In contrast to yeast, Drosophila melanogaster, and HeLa cells, plant cells store repressed messengers in cytoplasmic nonpolysomal ribonucleoproteins (RNPs). To follow the fate of untranslated transcripts, we studied protein composition, mRNA content, and RNA-binding properties of nonpolysomal RNPs from heat-stressed tomato (Lycopersicon peruvianum) cells. Contrary to the selective interaction in vivo, RNPs isolated from tomato cells bound both stress-induced and repressed messengers, suggesting that the selection mechanism resides elsewhere. This binding was independent of a cap or a poly(A) tail. The possible role of proteasomes and heat-stress granules (HSGs) in mRNA storage is a topic of debate. We found in vitro messenger-RNA-binding activity in messenger RNP fractions free of C2-subunit-containing proteasomes and HSGs. In addition, mRNAs introduced into tobacco (Nicotiana plumbaginifolia) protoplasts were found in the cytoplasm but were not associated with HSGs.

Regulatory signals in messenger RNA: determinants of nutrient–gene interaction and metabolic compartmentation

Nutrition has marked influences on gene expression and an understanding of the interaction between nutrients and gene expression is important in order to provide a basis for determining the nutritional requirements on an individual basis. The effects of nutrition can be exerted at many stages between transcription of the genetic sequence and production of a functional protein. This review focuses on the role of post-transcriptional control, particularly mRNA stability, translation and localization, in the interactions of nutrients with gene expression. The effects of both macronutrients and micronutrients on regulation of gene expression by post-transcriptional mechanisms are presented and the post-transcriptional regulation of specific genes of nutritional relevance (glucose transporters, transferrin, selenoenzymes, metallothionein, lipoproteins) is described in detail. The function of the regulatory signals in the untranslated regions of the mRNA is highlighted in relation to control of mRNA stability, translation and localization and the importance of these mRNA regions to regulation by nutrients is illustrated by reference to specific examples. The localization of mRNA by signals in the untranslated regions and its function in the spatial organization of protein synthesis is described; the potential of such mechanisms to play a key part in nutrient channelling and metabolic compartmentation is discussed. It is concluded that nutrients can influence gene expression through control of the regulatory signals in these untranslated regions and that the post-transcriptional regulation of gene expression by these mechanisms may influence nutritional requirements. It is emphasized that in studies of nutritional control of gene expression it is important not to focus only on regulation through gene promoters but also to consider the possibility of post-transcriptional control.

Cloning of a beta1 tubulin cDNA from an insect endocrine gland: developmental and hormone-induced changes in mRNA expression

A rapid increase in ecdysteroid hormone synthesis results when the insect prothoracic gland is stimulated with prothoracicotropic hormone (PTTH), a brain neuropeptide hormone. PTTH also stimulates the specific synthesis of several proteins, one of which is a beta tubulin. To further understand the possible roles of beta tubulin in the prothoracic gland, beta tubulin cDNA clones were isolated from a tobacco hornworm (Manduca sexta) gland cDNA library. Sequence analysis indicated that these clones were assignable to the beta1 tubulin isoform. Gland beta1 tubulin mRNA levels during the last larval instar and early pupal-adult development exhibited peaks that coincided with peaks in ecdysteroid synthesis. Manipulations of the glands hormonal milieu showed that beta1 tubulin mRNA levels respond to 20 hydroxyecdysone and PTTH. The data also support our earlier proposal that the prothoracic gland beta1 tubulin gene is ubiquitously expressed but exhibits tissue- and developmental-specific regulation of transcription and translation.

Assembly of tropomyosin isoforms into the cytoskeleton of avian muscle cells

Tropomyosin (TM) is a component of microfilaments of most eukaryotic cells. In striated muscle, TM helps confer calcium sensitivity to the actin-myosin interaction. TM is a fibrillar, self-associating protein that binds to the extended actin filament system. We hypothesized that these structural features would permit TM to undergo assembly into the cytoskeleton during translation, or cotranslational assembly. Pulse-chase experiments with [35S]methionine and pulse experiments with [3H]puromycin followed by extraction and immunoprecipitation of TM were performed to examine the mechanism of assembly of TM into the cytoskeleton in cultured avian muscle cells. Pulse-chase experiments provide kinetic evidence for cotranslational assembly of TM in skeletal and cardiac muscle. Demonstration of a large majority of completed TM on purified skeletal muscle microfilaments after a short labeling period confirms that these kinetic data are not related to trapping of TM within the actin network of the cytoskeleton. Nascent TM peptides are demonstrated on the cytoskeleton of muscle cells after a short metabolic pulse followed by puromycin treatment to release nascent peptides from ribosomes or after labeling with [3H]puromycin. Nascent chain localization to the cytoskeleton independent of ribosomal attachment further confirms the high degree of cotranslational assembly of this protein. The extent of cotranslational assembly is similar before and after the formation of significant myofibril in myotubes, suggesting that cotranslational assembly of TM is active during contractile apparatus assembly in muscle differentiation. This is the first report where assembly mechanism has been predicted to be cotranslational based upon structural features of a cytoskeletal protein.

Actin depolymerization affects stress-induced translational activity of potato tuber tissue

Changes in polymerized actin during stress conditions were correlated with potato (Solanum tuberosum L.) tuber protein synthesis. Fluorescence microscopy and immunoblot analyses indicated that filamentous actin was nearly undetectable in mature, quiescent aerobic tubers. Mechanical wounding of postharvest tubers resulted in a localized increase of polymerized actin, and microfilament bundles were visible in cells of the wounded periderm within 12 h after wounding. During this same period translational activity increased 8-fold. By contrast, low-oxygen stress caused rapid reduction of polymerized actin coincident with acute inhibition of protein synthesis. Treatment of aerobic tubers with cytochalasin D, an agent that disrupts actin filaments, reduced wound-induced protein synthesis in vivo. This effect was not observed when colchicine, an agent that depolymerizes microtubules, was used. Neither of these drugs had a significant effect in vitro on run-off translation of isolated polysomes. However, cytochalasin D did reduce translational competence in vitro of a crude cellular fraction containing both polysomes and cytoskeletal elements. These results demonstrate the dependence of wound-induced protein synthesis on the integrity of microfilaments and suggest that the dynamics of the actin cytoskeleton may affect translational activity during stress conditions.

Evidence for a Cytoskeleton-Associated Binding Site Involved in Prolamine mRNA Localization to the Protein Bodies in Rice Endosperm Tissue

Previous studies have demonstrated that the mRNAs encoding the prolamine and glutelin storage proteins are localized to morphologically distinct membranes of the endoplasmic reticulum (ER) complex in developing rice (Oryza sativa L.) endosperm cells. To gain insight about this mRNA localization process, we investigated the association of prolamine polysomes on the ER that delimit the prolamine protein bodies (PBs). The bulk of the prolamine polysomes were resistant to extraction by 1% Triton X-100 either alone or together with puromycin, which suggests that these translation complexes are anchored to the PB surface through a second binding site in addition to the well-characterized ribosome-binding site of the ER-localized protein translocation complex. Suppression of translation initiation shows that these polysomes are bound through the mRNA, as shown by the simultaneous increase in the amounts of ribosome-free prolamine mRNAs and decrease in prolamine polysome content associated with the membrane-stripped PB fraction. The prolamine polysome-binding activity is likely to be associated with the cytoskeleton, based on the association of actin and tubulin with the prolamine polysomes and PBs after sucrose-density centrifugation.

Separate elements in the 3' untranslated region of the mouse protamine 1 mRNA regulate translational repression and activation during murine spermatogenesis

The mouse protamine mRNAs, Prm-1 and Prm-2, are translationally repressed for several days during male germ cell differentiation. The translational delay of mouse Prm-1 mRNA has previously been shown to be dependent upon cis-acting elements that reside in the last 62 nucleotides of the Prm-1 3' untranslated region (3' UTR). We have previously identified a 48/50-kDa protein that binds the 3' UTRs of both Prm-1 and Prm-2 mRNAs in a sequence-specific manner, is present in cytoplasmic fractions of postmeiotic round spermatids where the protamine mRNAs are translationally silent, and is markedly reduced in elongated spermatids where the protamine mRNAs become activated for translation. Surprisingly, the binding site for this activity maps to a region of the Prm-1 3' UTR not contained within the functional 62 nucleotides described above. In this report we show that the binding site for the 48/50-kDa protein can also delay translation of a reporter RNA in vivo, suggesting that the 48/50-kDa protein can repress the translation of Prm-1 mRNA during murine spermatogenesis. This observation proves that two separate regions of the Prm-1 3' UTR are sufficient to repress Prm-1 translation. In addition, immunocytochemistry and polysome analysis have revealed that this transgenic reporter mRNA fails to undergo proper translational activation. These results suggest that an additional region of the Prm-1 3' UTR is required for proper translational activation and that Prm-1 translational repression elements can be separated from those involved in translational activation.

Ribosomal protein L9 is the product of GRC5, a homolog of the putative tumor suppressor QM in S. cerevisiae

Genes encoding members of the highly conserved QM family have been identified in eukaryotic organisms from yeast to man. Results of previous studies have suggested roles for QM in control of cell growth and proliferation, perhaps as a tumor suppressor, and in energy metabolism. We identified recessive lethal alleles of the Saccharomyces cerevisiae QM homolog GRC5 that increased GCN4 expression when present in multiple copies. These alleles encode truncated forms of the yeast QM protein Grc5p. Using a functional epitope-tagged GRC5 allele, we localized Grc5p to a 60S fraction that contained the large ribosomal subunit. Two-dimensional gel analysis of highly purified yeast ribosomes indicated that Grc5p corresponds to 60S ribosomal protein L9. This identification is consistent with the predicted physical characteristics of eukaryotic QM proteins, the highly biased codon usage of GRC5, and the presence of putative Rap1p-binding sites in the 5' sequences of the yeast GRC5 gene.

Influence of drug exposure parameters on the activity of paclitaxel in multicellular spheroids

Paclitaxel is a chemotherapeutic drug which has clinical activity against several solid tumours including ovarian and metastatic breast cancers. Despite extensive preclinical evaluation in several experimental models, no studies have determined the effect of taxol on multicellular spheroids, a model which closely mimics the microregions of solid tumours. MCF-7 human breast carcinoma spheroids were significantly less sensitive than monolayers with IC50 values of 14.33 +/- 4.51 microM and 0.15 +/- 0.09 microM, respectively, following a 1 h drug exposure. Similarly, DLD-1 human colon carcinoma spheroids were also more resistant (IC50 = 33.0 +/- 8.89 microM) than monolayers (IC50 = 0.36 +/- 0.14 microM) following a 1 h drug exposure. Paclitaxel was unable to penetrate DLD-1 multicell layers (22 microns in thickness), suggesting that suboptimal drug exposures to paclitaxel occur in cells which reside some distance away from the surface of the spheroid. In the case of DLD-1 spheroids, extending the exposure time to 24 h whilst maintaining the same overall concentration x time (C x T) drug exposure parameters, resulted in greater cell kill (C x T required to kill 50% of cells = 13.67 +/- 3.21 microM/h) compared with 1 h drug exposures (C x T required to kill 50% of cells = 33.00 +/- 8.89 microM/h). Similar results were obtained with MCF-7 spheroids. In monolayers cultures, dose-response curves contained a marked plateau phase (a characteristic feature of cell cycle phase specific drug) and in the case of MCF-7 cells, cell kill was proportional to T as opposed to C x T. These results support the use of prolonged infusions of paclitaxel in the clinic, as extending the duration of drug exposure not only allows more cells to enter sensitive phases of the cell cycle, but would also allow paclitaxel more time to penetrate into avascular regions of solid tumours. It is likely that paclitaxel will only be effective against cells which reside close to tumour blood vessels and combination therapy with bioreductive drugs (such as tirapazamine) may produce synergistic effects in vivo.

HIP-I: a huntingtin interacting protein isolated by the yeast two-hybrid system

We report the discovery of the huntingtin interacting protein I (HIP-I) which binds specifically to the N-terminus of human huntingtin, both in the two-hybrid screen and in in vitro binding experiments. For the interaction in vivo, a protein region downstream of the polyglutamine stretch in huntingtin is essential. The HIP1 cDNA isolated by the two-hybrid screen encodes a 55 kDa fragment of a novel protein. Using an affinity-purified polyclonal antibody raised against recombinant HIP-I, a protein of 116 kDa was detected in brain extracts by Western blot analysis. The predicted amino acid sequence of the HIP-I fragment exhibits significant similarity to cytoskeleton proteins, suggesting that HIP-I and huntingtin play a functional role in the cell filament networks. The HIP1 gene is ubiquitously expressed in different brain regions at low level. HIP-I is enriched in human brain but can also be detected in other human tissues as well as in mouse brain. HIP-I and huntingtin behave almost identically during subcellular fractionation and both proteins are enriched in the membrane containing fractions.

Hepatic protein synthesis in the sheep: effect of intake as monitored by use of stable-isotope-labelled glycine, leucine and phenylalanine

Rates of protein synthesis for the liver, plasma albumin and total plasma protein were quantified in sheep either offered a supra-maintenance intake or fasted for 3 d. The technique of continuous infusion over a 12 h period was employed with the simultaneous infusion of [1-13C]glycine, [1-13C]leucine and [2H5]phenylalanine. Blood and plasma samples were removed at timed intervals from the hepatic portal and hepatic veins plus the aorta. Enrichments of the free amino acids (AA) were determined in all blood and plasma samples as was the protein-bound AA in an apolipoprotein B100 extract. Protein-bound phenylalanine enrichments were also measured in albumin and total protein from plasma plus samples from liver biopsies. The apolipoprotein B100 enrichments agreed well with those of the free AA in hepatic (and hepatic portal) plasma but were lower than for arterial free AA and greater than liver homogenate free AA. This adds support to the concept that export proteins may preferentially use AA directly from extracellular sources. Intake had no significant effect on constitutive liver protein synthesis and the values agreed well with those obtained by other isotopic approaches. There were, however, significant declines, based on hepatic venous free phenylalanine enrichment, at the lower intake in both the fractional (3.4 v. 4.7% per d; P = 0.024) and absolute (2.4 v. 4.2 g/d; P = 0.011) synthesis rates of albumin, which matched the estimated decrease in total plasma albumin content (52 v. 67 g, P < 0.01). In contrast, there was a smaller reduction in total plasma protein mass (145 v. 151 g, P = 0.035) with no observed significant difference in kinetic parameters. Albumin synthesis was calculated to account for a maximum of 17% of total liver protein synthesis in the fed condition and this may fall to 8% during moderate fasts.

Cytoskeleton reorganization induces the urokinase-type plasminogen activator gene via the Ras/extracellular signal-regulated kinase (ERK) signaling pathway

Urokinase-type plasminogen activator (uPA) expression is induced upon cytoskeletal reorganization (CSR) by a mechanism independent of protein kinase C and cAMP protein kinase in nontransformed renal epithelial (LLC-PK1) cells. This CSR-dependent uPA gene activation is mediated by an AP-1-recognizing element located 2 kilobases upstream of the transcription initiation site. The phosphorylation of c-Jun, a component of AP-1, is induced by CSR, which seems to increase both the activity and stability of c-Jun (Lee, J. S., von der Ahe, D., Kiefer, B., and Nagamine, Y. (1993) Nucleic Acids Res. 21, 3365-3372). It has been shown that c-Jun is phosphorylated by members of the mitogen-activated protein kinase family, i.e. ERKs and JNKs. ERKs are activated through a growth factor-coupled Ras/Raf-dependent signaling pathway, while JNKs are activated through a stress-induced signaling pathway. Although CSR induces both ERK-2 and JNK activity, JNK does not seem to be involved in the uPA gene induction because UV irradiation, which activates JNK as efficiently as CSR, does not activate the uPA promoter. Further analysis showed the involvement of SOS, Ras, and Raf-1 in the pathway induced by CSR. Our results suggest that cells sense changes in cell morphology using the cytoskeleton as a sensor and respond by activating the ERK-involving signaling pathway from within the cell.

Immunolocalization of neurophysin in cytokeratin-positive luteal cells of cows

We have recently detected a subgroup of cytokeratin (CK)-positive luteal cells in the bovine corpus luteum of the early and mid-luteal phase, but not in that of pregnancy. Since, according to the literature, neurophysin (NP)-positive luteal cells behave comparably, simple immunohistochemistry and double labeling were used to identify in serial sections whether the presence of NP co-incided with that of CK. The numbers of CK-positive cells and NP-positive luteal cells were comparable throughout the estrous cycle, decreasing from early to late luteal phase. While few CK-positive cells were found in the former thecal layer during the early luteal phase, many CK-positive cells appeared in the former granulosal layer. NP-positive cells were only detected in the former granulosal layer. During the mid-luteal phase, the CK-positive cells consisted of small and large luteal cells, but only large NP-positive cells were found. Roughly 80% of the large CK-positive cells contained NP, whereas CK was lacking in more than 50% of the NP-positive cells. The corpora lutea of pregnancy contained neither CK-positive nor NP-positive cells. The significance of the simultaneous occurrence of CK and NP remains to be elucidated.