PML regulates p53 stability by sequestering Mdm2 to the nucleolus

The promyelocytic leukaemia (PML) tumour-suppressor protein potentiates p53 function by regulating post-translational modifications, such as CBP-dependent acetylation and Chk2-dependent phosphorylation, in the PML-Nuclear Body (NB). PML was recently shown to interact with the p53 ubiquitin-ligase Mdm2 (refs 4-6); however, the mechanism by which PML regulates Mdm2 remains unclear. Here, we show that PML enhances p53 stability by sequestering Mdm2 to the nucleolus. We found that after DNA damage, PML and Mdm2 accumulate in the nucleolus in an Arf-independent manner. In addition, we found that the nucleolar localization of PML is dependent on ATR activation and phosphorylation of PML by ATR. Notably, in Pml(-/-) cells, sequestration of Mdm2 to the nucleolus was impaired, as well as p53 stabilization and the induction of apoptosis. Furthermore, we demonstrate that PML physically associates with the nucleolar protein L11, and that L11 knockdown impairs the ability of PML to localize to nucleoli after DNA damage. These findings demonstrate an unexpected role of PML in the nucleolar network for tumour suppression.

HIF activation by pH-dependent nucleolar sequestration of VHL

Hypoxia and acidosis occur in a wide variety of physiological and pathological settings that include muscle stress, tumour development and ischaemic disorders. A central element in the adaptive response to cellular hypoxia is HIF (hypoxia-inducible factor), a transcription factor that activates an array of genes implicated in oxygen homeostasis, tumour vascularization and ischaemic preconditioning. HIF is activated by hypoxia, but undergoes degradation by the VHL (von Hippel-Lindau) tumour suppressor protein in the presence of oxygen. Here, we demonstrate that hypoxia induction or normoxic acidosis can neutralize the function of VHL by triggering its nucleolar sequestration, a regulatory mechanism of protein function that is observed rarely. VHL is confined to nucleoli until neutral pH conditions are reinstated. Nucleolar sequestration of VHL enables HIF to evade destruction in the presence of oxygen and activate its target genes. Our findings suggest that an increase in hydrogen ions elicits a transient and reversible loss of VHL function by promoting its nucleolar sequestration.

Ligand-regulated association of ErbB-4 to the transcriptional co-activator YAP65 controls transcription at the nuclear level

It has been proposed that ligand-dependent Regulated Intramembrane Proteolysis (RIP) of ErbB-4 receptors generates 80 kDa Intra-Cellular Domains (E4.ICDs) that relocate to the nuclear compartments where they implement the signaling abilities of the ErbB-4 receptors. The E4.ICD may directly regulate gene transcription or, in an alternative scenario, the tyrosine kinase activity of E4.ICDs may target proteins involved in transcriptional regulation upon its relocation into the nucleus. We have identified the transcriptional coactivator YAP65, here referred as YAP (Yes Associated Protein), as binding partner of ErbB-4 in a two hybrid screening in yeast. Interaction between YAP and ErbB-4 occurs via the WW domain of YAP and the PPPPY at positions 1297-1301 and the PPPAY at positions 1052-1056 of the amino acid sequence of the Cyt-1 isoform of ErbB-4. Stechiometry of binding is regulated by the ligand-dependent phosphorylation of Tyr 1056 in the PPPAYTPM module that function as "biochemical switch" to decrease the association of YAP to ErbB-4. In principle, this novel interaction highlights new mechanisms of signaling propagation from the ErbB-4 receptors, offering supporting evidences that the E4.ICDs forms released following ligand-receptor engagement may recruit YAP and relocate to the nucleus to implement or regulate transcription.

Interaction of apoptosis signal-regulating kinase 1 with isoforms of 14-3-3 proteins

Apoptosis signal-regulating kinase 1 (ASK1) is a critical mediator of apoptotic signaling pathways initiated by a variety of death stimuli. Its activity is tightly controlled by various mechanisms such as covalent modification and protein-protein interaction. One of the proteins that control ASK1 function is 14-3-3zeta, a member of the 14-3-3 protein family. Here, we report that ASK1 is capable of binding to other isoforms of 14-3-3, suggesting that binding ASK1 is a general property of the 14-3-3 family. In support of this notion, mutational analysis revealed that the ASK1/14-3-3 interaction was mediated by the conserved amphipathic groove of 14-3-3 with some residue selectivity. Functionally, expression of various isoforms of 14-3-3 suppressed ASK1-induced apoptosis. To understand how 14-3-3 controls the ASK1 activity, we examined intracellular localization of ASK1 upon 14-3-3 co-expression. We found that 14-3-3 co-expression is correlated with the translocation of ASK1 from the cytoplasm to a perinuclear localization, likely the ER compartment. Consistent with this notion, ASK1(S967A), a 14-3-3 binding defective mutant of ASK, showed no change in intracellular distribution upon 14-3-3 co-expression. These data support a model that 14-3-3 proteins regulate the proapoptotic function of ASK1 in part by controlling its subcellular distribution.

Cell cycle-related variation in subcellular localization of eIF3e/INT6 in human fibroblasts

The Int-6 gene is a site of mouse mammary tumour virus (MMTV) integration in murine tumours and INT6 protein has been identified independently as a subunit (eIF3e) of the eukaryotic translation initiation factor eIF3. In addition, the protein can interact with two other multi-subunit complexes: the COP9 signalosome (CSN) and the proteasome. The role of INT6 in tumourigenesis is nonetheless currently unclear. Here, using immunofluorescence microscopy, we show that eIF3e/INT6 is localized in part to the nucleus, while other eIF3 components are cytoplasmic. Primary human fibroblasts, but not their transformed counterparts, showed reduced nuclear INT6 staining in some cells, and this reduction was maximal in early S phase. This variation in eIF3e/INT6 may indicate regulated shuttling between cellular compartments and would be consistent with the presence of a nuclear export signal as well as a nuclear localization signal in the protein sequence. Loss of regulation of eIF3e/INT6 redistribution may therefore be a significant feature of malignancy in human cells.

Cytoplasmic foci are sites of mRNA decay in human cells

Understanding gene expression control requires defining the molecular and cellular basis of mRNA turnover. We have previously shown that the human decapping factors hDcp2 and hDcp1a are concentrated in specific cytoplasmic structures. Here, we show that hCcr4, hDcp1b, hLsm, and rck/p54 proteins related to 5'-3' mRNA decay also localize to these structures, whereas DcpS, which is involved in cap nucleotide catabolism, is nuclear. Functional analysis using fluorescence resonance energy transfer revealed that hDcp1a and hDcp2 interact in vivo in these structures that were shown to differ from the previously described stress granules. Our data indicate that these new structures are dynamic, as they disappear when mRNA breakdown is abolished by treatment with inhibitors. Accumulation of poly(A)(+) RNA in these structures, after RNAi-mediated inactivation of the Xrn1 exonuclease, demonstrates that they represent active mRNA decay sites. The occurrence of 5'-3' mRNA decay in specific subcellular locations in human cells suggests that the cytoplasm of eukaryotic cells may be more organized than previously anticipated.

Nuclear DNA helicase II (RNA helicase A) binds to an F-actin containing shell that surrounds the nucleolus

Nuclear DNA helicase II (NDH II), alternatively named RNA helicase A (RHA), is an F-actin binding protein that is particularly enriched in the nucleolus of mouse cells. Here, we show that the nucleolar localization of NDH II of murine 3T3 cells depended on an ongoing rRNA synthesis. NDH II migrated out of the nucleolus after administration of 0.05 microg/ml actinomycin D, while nucleolin and the upstream binding factor (UBF) remained there. In S phase-arrested mouse cells, NDH II was frequently found at the nucleolar periphery, where it was accompanied by newly synthesized nucleolar RNA. Human NDH II was mainly distributed through the whole nucleoplasm and not enriched in the nucleoli. However, in the human breast carcinoma cell line MCF-7, NDH II was also found at the nucleolar periphery, together with the tumor suppressor protein p53. Both NDH II and p53 were apparently attached to the F-actin-based filamentous network that surrounded the nucleoli. Accordingly, this subnuclear structure was sensitive to F-actin depolymerizing agents. Depolymerization with gelsolin led to a striking accumulation of NDH II in the nucleoli of MCF-7 cells. This effect was abolished by RNase, which extensively released nucleolus-bound NDH II when added together with gelsolin. Taken together, these results support the idea that an actin-based filamentous network may anchor NDH II at the nucleolar periphery for pre-ribosomal RNA processing, ribosome assembly, and/or transport.

An LQT mutant minK alters KvLQT1 trafficking

Cardiac I(Ks), the slowly activated delayed-rectifier K(+) current, is produced by the protein complex composed of alpha- and beta-subunits: KvLQT1 and minK. Mutations of genes encoding KvLQT1 and minK are responsible for the hereditary long QT syndrome (loci LQT1 and LQT5, respectively). MinK-L51H fails to traffic to the cell surface, thereby failing to produce effective I(Ks). We examined the effects that minK-L51H and an endoplasmic reticulum (ER)-targeted minK (minK-ER) exerted over the electrophysiology and biosynthesis of coexpressed KvLQT1. Both minK-L51H and minK-ER were sequestered primarily in the ER as confirmed by lack of plasma membrane expression. Glycosylation and immunofluorescence patterns of minK-L51H were qualitatively different for minK-ER, suggesting differences in trafficking. Cotransfection with the minK mutants resulted in reduced surface expression of KvLQT1 as assayed by whole cell voltage clamp and immunofluorescence. MinK-L51H reduced current amplitude by 91% compared with wild-type (WT) minK/KvLQT1, and the residual current was identical to KvLQT1 without minK. The phenotype of minK-L51H on I(Ks) was not dominant because coexpressed WT minK rescued the current and surface expression. Collectively, our data suggest that ER quality control prevents minK-L51H/KvLQT1 complexes from trafficking to the plasma membrane, resulting in decreased I(Ks). This is the first demonstration that a minK LQT mutation is capable of conferring trafficking defects onto its associated alpha-subunit.

APPL Proteins Link Rab5 to Nuclear Signal Transduction via an Endosomal Compartment

Signals generated in response to extracellular stimuli at the plasma membrane are transmitted through cytoplasmic transduction cascades to the nucleus. We report the identification of a pathway directly linking the small GTPase Rab5, a key regulator of endocytosis, to signal transduction and mitogenesis. This pathway operates via APPL1 and APPL2, two Rab5 effectors, which reside on a subpopulation of endosomes. In response to extracellular stimuli such as EGF and oxidative stress, APPL1 translocates from the membranes to the nucleus where it interacts with the nucleosome remodeling and histone deacetylase multiprotein complex NuRD/MeCP1, an established regulator of chromatin structure and gene expression. Both APPL1 and APPL2 are essential for cell proliferation and their function requires Rab5 binding. Our findings identify an endosomal compartment bearing Rab5 and APPL proteins as an intermediate in signaling between the plasma membrane and the nucleus.

Function, intracellular localization and the importance in salt tolerance of a vacuolar Na(+)/H(+) antiporter from rice

We examined the function and intracellular localization of the product of the Na(+)/H(+) antiporter gene (OsNHX1) cloned from rice (Oryza sativa). OsNHX1 has the ability to suppress Na(+), Li(+) and hygromycin sensitivity of yeast nhx1 mutants and sensitivity to a high K(+) concentration, a novel phenotype of the nhx1 mutants. Analysis using rice cells expressing a fusion protein of OsNHX1 and green fluorescent protein and Western blot analysis using antibodies specific for OsNHX1 confirmed the localization of OsNHX1 on the tonoplasts. These results indicate that the OsNHX1 gene encodes a vacuolar (Na(+), K(+))/H(+) antiporter. Treatment with high concentrations of NaCl and KCl increased the transcript levels of OsNHX1 in rice roots and shoots. In addition, overexpression of OsNHX1 improved the salt tolerance of transgenic rice cells and plants. These results suggest that OsNHX1 on the tonoplasts plays important roles in the compartmentation of Na(+) and K(+) highly accumulated in the cytoplasm into the vacuoles, and the amount of the antiporter is one of the most important factors determining salt tolerance in rice.

Perinuclear localization of Na-K-Cl-cotransporter protein after human cytomegalovirus infection

We (41) previously reported that Na-K-Cl-cotransporter (NKCC) function and microsomal protein expression are both dramatically reduced late in human cytomegalovirus (HCMV) infection of a human fibroblast cell line (MRC-5). We now report DNA microarray data showing that no significant HCMV-dependent NKCC gene repression can be detected 30 h postexposure (PE) to the virus. Consequently, we used plasma membrane biotinylation and subsequent subcellular fractionation in combination with semiquantitative immunoblotting and confocal microscopy to investigate the possibility that intracellular redistribution of the NKCC protein after HCMV infection could be a cause of the HCMV-induced loss of NKCC ion transport function. Our results show that the lifetime of plasmalemmal NKCC protein in quiescent, uninfected MRC-5 cells is approximately 48 h, and <20% of the total expressed NKCC protein are in the plasma membrane. The remainder (approximately 80%) was detected as diffusely distributed, small punctate structures in the cytoplasm. Following HCMV infection: 1) NKCC protein expression in the plasmalemma was sharply reduced (approximately 75%) within 24 h PE and thereafter continued to slowly decrease; 2) total cellular NKCC protein content remained unchanged or slightly increased during the course of the viral infection; and 3) HCMV infection caused NKCC protein to accumulate in the perinuclear region late in the HCMV infection (72 h PE). Thus our results imply that, in the process of productive HCMV infection, NKCC protein continues to be synthesized, but, instead of being delivered to the plasma membrane, it is clustered in a large, detergent-soluble perinuclear structure.

Expression of basic fibroblast growth factor isoforms in postmitotic sympathetic neurons: synthesis, intracellular localization and involvement in karyokinesis

Low and high molecular weight isoforms of the mitogen and multifunctional cytokine basic fibroblast growth factor (FGF-2) are up-regulated in neurons and glial cells in response to peripheral nerve lesion. While synthesis, regulation and functions of FGF-2 in non-neuronal cells are well established, the significance of neuronal FGF-2 remains to be investigated in the peripheral nervous system. Therefore, the expression, intracellular localization and possible effects of FGF-2 isoforms were analyzed in primary sympathetic neurons derived from the rat superior cervical ganglion. FGF-2 is detected in the nucleus and in perinuclear Golgi fields of early postnatal neurons which also express mRNA and protein for the FGF receptor type 1. Biolistic transfection of plasmids encoding FGF-2 isoforms fused to fluorescent proteins demonstrates nuclear targeting of 18 kDa FGF-2 and 23 kDa FGF-2 with prominent accumulation in the nucleolus of neurons. Neither overexpression nor treatment with FGF-2 isoforms promotes survival of sympathetic neurons deprived of nerve growth factor; however, neuronal transfection of the high molecular weight FGF-2 isoform in dissociated and slice cultures results in a bi- or multinuclear phenotype. The present study provides evidence for neuronal synthesis and targeting of FGF-2 to the nucleus and Golgi apparatus supporting a dual role of FGF-2 in the nucleus and secretory pathway of sympathetic neurons.

Dynamic control of Rad51 recombinase by self-association and interaction with BRCA2

Here, we visualize GFP-Rad51 fusion proteins in the nucleus of living cells to demonstrate the dynamic compartmentalization of Rad51 by self-association or by binding to BRCA2. Mutants of Rad51 that fail to oligomerize and/or to bind BRCA2 distinguish three fractions of Rad51 within the nucleoplasm: a relatively mobile fraction, an immobile oligomerized fraction, and an immobile BRCA2-bound fraction. Strikingly, inhibition of replication by hydroxyurea reduces the immobile fraction of nucleoplasmic Rad51. This effect is specific to Rad51 mutants that retain the capacity to bind BRCA2, indicating that the BRCA2-bound fraction is selectively mobilized. We propose that arrested replication triggers a switch between dual functions of BRCA2 in sequestering or mobilizing a small fraction of nucleoplasmic Rad51 and suggest a mechanism for the dynamic control of protein complexes that participate in homologous recombination.

The protein network of HIV budding

HIV release requires TSG101, a cellular factor that sorts proteins into vesicles that bud into multivesicular bodies (MVB). To test whether other proteins involved in MVB biogenesis (the class E proteins) also participate in HIV release, we identified 22 candidate human class E proteins. These proteins were connected into a coherent network by 43 different protein-protein interactions, with AIP1 playing a key role in linking complexes that act early (TSG101/ESCRT-I) and late (CHMP4/ESCRT-III) in the pathway. AIP1 also binds the HIV-1 p6(Gag) and EIAV p9(Gag) proteins, indicating that it can function directly in virus budding. Human class E proteins were found in HIV-1 particles, and dominant-negative mutants of late-acting human class E proteins arrested HIV-1 budding through plasmal and endosomal membranes. These studies define a protein network required for human MVB biogenesis and indicate that the entire network participates in the release of HIV and probably many other viruses.

Replication of mitochondrial DNA occurs throughout the mitochondria of cultured human cells

Replication of mitochondrial DNA (mtDNA) is dependent on nuclear-encoded factors. It has been proposed that this reliance may exert spatial restrictions on the sites of mtDNA replication within the cytoplasm, as a previous study only detected mtDNA synthesis in perinuclear mitochondria. We have studied mtDNA replication in situ in a variety of human cell cultures labeled with 5-bromo-2'-deoxyuridine. In contrast to what has been reported, mtDNA synthesis was detected at multiple sites throughout the mitochondrial network following short pulses with bromodeoxyuridine. Although no bromodeoxyuridine incorporation was observed in anuclear platelets, incorporation into mtDNA of fibroblasts that had been enucleated 2 h prior to labeling was readily detectable. Blotting experiments indicated that the bromodeoxyuridine incorporation into mtDNA observed in situ represents replication of the entire mtDNA molecule. The studies also showed that replication of mtDNA occurred at any stage of the cell cycle in proliferating cells and continued in postmitotic cells, although at a lower level. These results demonstrate that mtDNA replication is not restricted to mitochondria in the proximity of the nucleus and imply that all components of the replication machinery are available at sufficient levels throughout the mitochondrial network to permit mtDNA replication throughout the cytoplasm.

NUFIP1 (nuclear FMRP interacting protein 1) is a nucleocytoplasmic shuttling protein associated with active synaptoneurosomes

Fragile X syndrome, the most common cause of inherited mental retardation, is caused by the absence of FMRP (Fragile X Mental Retardation Protein). FMRP is an RNA binding protein reported to be involved in translational control, notably at postsynaptic sites of protein synthesis as a part of a multiprotein/mRNA complex. One of the FMRP interactors, NUFIP1, is an RNA binding protein with an expression profile matching that of FMRP. We now show that in the nucleus NUFIP1 is localized in the nuclear matrix in RNA-containing structures lying in the proximity of, but not overlapping with, sites of nascent RNA. NUFIP1 is also present in the cytoplasm, where it is associated with ribosomes, similarly to FMRP. In neurons NUFIP1 can be detected in functional synaptoneurosomes, colocalizing with ribosomes. Consistent with its subcellular localization in both nucleus and cytoplasm, we show that NUFIP1 contains a functional CRM1-dependent nuclear export signal and is able to shuttle between these two cellular compartments. These findings suggest the involvement of NUFIP1 in the export and localization of mRNA and, in association with FMRP, in the regulation of local protein synthesis near synapses.

Nuclear and territorial topography of chromosome telomeres in human lymphocytes

Nuclear and territorial positioning of p- and q-telomeres and centromeres of chromosomes 3, 8, 9, 13, and 19 were studied by repeated fluorescence in situ hybridization, high-resolution cytometry, and three-dimensional image analysis in human blood lymphocytes before and after stimulation. Telomeres were found on the opposite side of the territories as compared with the centromeres for all chromosome territories investigated. Mutual distances between telomeres of submetacentric chromosomes were very short, usually shorter than centromere-to-telomere distances, which means that the chromosome territory is nonrandomly folded. Telomeres are, on average, much nearer to the center of the cell nucleus than centromeres; q-telomeres were found, on average, more centrally localized as compared with p-telomeres. Consequently, we directly showed that chromosome territories in the cell nucleus are (1) polar and (2) partially oriented in cell nuclei. The distributions of genetic elements relative to chromosome territories (territorial distributions) can be either narrower or broader than their nuclear distributions, which reflects the degree of adhesion of an element to the territory or to the nucleus. We found no tethering of heterologous telomeres of chromosomes 8, 9, and 19. In contrast, both pairs of homologous telomeres of chromosome 19 (but not in other chromosomes) are tethered (associated) very frequently.

Staying in aerobic shape: how the structural integrity of mitochondria and mitochondrial DNA is maintained

The structure and integrity of the mitochondrial compartment are features essential for it to function efficiently. The maintenance of mitochondrial structure in cells ranging from yeast to humans has been shown to require both ongoing fission and fusion. Recent characterization of many of the molecular components that direct mitochondrial fission and fusion events have led to a more complete understanding of how these processes take place. Further, mitochondrial fragmentation observed when cells undergo apoptosis requires mitochondrial fission, underlying the importance of mitochondrial dynamics in cellular homeostasis. Mitochondrial structure also impacts mitochondrial DNA inheritance. Recent studies suggest that faithful transmission of mitochondrial DNA to daughter cells might require a mitochondrial membrane tethering apparatus.

Specific protein targeting during cell differentiation: polarized localization of Fus1p during mating depends on Chs5p in Saccharomyces cerevisiae

In budding yeast, chs5 mutants are defective in chitin synthesis and cell fusion during mating. Chs5p is a late-Golgi protein required for the polarized transport of the chitin synthase Chs3p to the membrane. Here we show that Chs5p is also essential for the polarized targeting of Fus1p, but not of other cell fusion proteins, to the membrane during mating.

Identification of sequence determinants of human nuclear dUTPase isoform localization

dUTP nucleotidohydrolase (dUTPase) catalyzes the hydrolysis of dUTP to dUMP and pyrophosphate and is the central regulator of cellular dUTP pools. Nuclear (DUT-N) and mitochondrial (DUT-M) isoforms of the protein have been identified in humans and arise from the same gene by the alternative use of 5' exons. Recently, it has been shown that these isoforms are aberrantly expressed in some cancers and overexpression of dUTPase in the nucleus is associated with resistance to chemotherapeutic agents that target thymidylate biosynthesis. In this study, we have examined the signals necessary for dUTPase isoform localization using green fluorescent protein fusion constructs. We report that the N-terminal 23 amino acids of DUT-N are required but not sufficient for complete nuclear localization. Within this region, we identified a small cluster of basic residues (K(14)R(15)R(17)) that resemble a classic monopartite nuclear localization signal (NLS). Mutation of these residues completely abolishes nuclear localization. In addition, phosphorylation of Ser11 near the putative NLS has no affect on DUT-N nuclear localization. Through deletion analysis we show improved sorting of DUT-N to the nucleus when most of the protein sequence is present. Therefore, we conclude that DUT-N may contain a complex NLS that is located throughout the entire protein.

Phenotypic differences between peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) mutations associated with Charcot-Marie-Tooth-related diseases

Mutations in the genes for peripheral myelin protein-22 (PMP22) and myelin protein zero (P0) cause human hereditary neuropathies with varying clinical and pathological phenotypes. In this study, we examine the effects of representative disease-causing mutations on the subcellular distribution of their corresponding PMP22- and P0-enhanced green fluorescent protein (EGFP) fusion proteins. In transiently transfected HeLa and 293 cells, we find that wild-type P0-EGFP and PMP22-EGFP are efficiently synthesized and transported through the secretory pathway to the plasma membrane. The P0-EGFP and PMP22-EGFP mutants can be classified into several groups: those that are transported to the plasma membrane as in the majority of P0 mutants; those that are retained in the endoplasmic reticulum as in the majority of PMP22 mutants; and those that are a mixture of the two. In addition, several of these disease-causing mutations are associated with the development of abnormal intracellular cytoplasmic structures that we have previously identified as either intracellular myelin figures or aggresomes. Our studies indicate that different types of PMP22 and P0 mutations are associated with specific intracellular chaperone proteins, including calnexin and BiP, and that these associations can be altered by glycosylation. These findings indicate that the various P0 and PMP22 mutants may exert their pathogenic effects in different subcellular compartments and by different mechanisms in the mammalian cell.

Molecular evidence for the nuclear localization of FADD

The Fas-associated death domain (FADD) adaptor protein FADD/Mort-1 is recruited by several members of the tumor necrosis factor receptor (TNFR) superfamily during cell death activated via death receptors. Since most studies have focused on the interaction of FADD with plasma membrane proteins, FADD's subcellular location is thought to be confined to the cytoplasm. In this report, we show for the first time that FADD is present in both the cytoplasm and the nucleus of cells, and that its nuclear localization relies on strong nuclear localization and nuclear export signals (NLS and NES, respectively) that reside in the death-effector domain (DED) of the protein. Specifically, we found that a conserved basic KRK35 sequence of the human protein is necessary for FADD's nuclear localization, since disruption of this motif leads to the confinement of FADD in the cytoplasm. Furthermore, we show that the leucine-rich motif LTELKFLCL28 in the DED is necessary for FADD's nuclear export. Functionally, mutation of the NES of FADD and its seclusion in the nucleus reduces the cell death-inducing efficacy of FADD reconstituted in FADD-deficient T cells.

Fission yeast meu14+ is required for proper nuclear division and accurate forespore membrane formation during meiosis II

Using a meiosis-specific subtracted cDNA library of Schizosaccharomyces pombe, we identified meu14+ as a gene whose expression is upregulated during meiosis. Transcription of meu14+ is induced abruptly after the cell enters meiosis. Its transcription is dependent on the meiosis-specific transcription factor Mei4. In meu14Delta cells, the segregation and modification of the SPBs (spindle pole bodies) and microtubule elongation during meiosis II were aberrant. Meiotic meu14Delta cells consequently produced a high frequency of abnormal tetranucleate cells harboring aberrant forespore membranes and failed to produce asci. In wild-type cells harboring the integrated meu14+-gfp fusion gene, Meu14-GFP first appeared inside the nuclear region at prophase II, after which it accumulated beside the two SPBs at metaphase II. Thereafter, it formed two ring-shaped structures that surrounded the nucleus at early anaphase II. At post-anaphase II, it disappeared. Meu14-GFP appears to localize at the border of the forespore membrane that later develops into spore walls at the end of sporulation. This was confirmed by coexpressing Spo3-HA, a component of the forespore membrane, with Meu14-GFP. Taken together, we conclude that meu14+ is crucial in meiosis in that it participates in both the nuclear division during meiosis II and the accurate formation of the forespore membrane.

RA175, which is the mouse ortholog of TSLC1, a tumor suppressor gene in human lung cancer, is a cell adhesion molecule

RA175, a new immunoglobulin superfamily member, is preferentially expressed during differentiation of P19 embryonic carcinoma (EC) cells induced by retinoic acid. In the present study, we isolated mouse RA175 cDNA in its entirety and showed that RA175 is the mouse ortholog of TSLC1, a tumor suppressor gene in human lung cancer. RA175/TSLC1 was localized in the adherent region of human lung squamous carcinoma cells and in the differentiated P19 EC cells. RA175/TSLC1 showed homophilic trans-interaction activity in a Ca(2+)-independent manner. RA175/TSLC1 was preferentially expressed in the polarized cells lining the lumen of developing mouse lung epithelium. This suggests that RA175/TSLC1 is a cell adhesion molecule that is acting as a tumor suppressor gene in the metastasis of lung tumors. RA175/TSLC1 may be necessary for cells to remain tightly associated in the epithelium, thereby suppressing metastasis.

Membrane targeting of protein tyrosine phosphatase PTPL1 through its FERM domain via binding to phosphatidylinositol 4,5-biphosphate

PTPL1 is the largest known cytoplasmic protein tyrosine phosphatase (PTP) containing a FERM (four point-1, ezrin, radixin and moesin) domain. Enzyme localization and PTP-substrate specificity are thought to play crucial roles in the regulation of PTP activity, which determines their functions. Here we report that PTPL1 is predominantly localized at the apical face of plasma membrane enriched in dorsal microvilli when expressed in HeLa cells. By comparing localization of the full-length enzyme with its FERM domain or FERM-deleted PTPL1 construct, we first concluded that PTPL1-FERM domain is necessary and sufficient to address the wild-type enzyme at the membrane. Two potential phosphatidylinositol 4,5-biphosphate [PtdIns(4,5)P2]-binding motifs were identified within the PTPL1-FERM sequence. We further showed that mutation of both sites altered PTPL1 localization similarly to FERM domain deletion, and impaired its subcellular distribution as confirmed biochemically by cell-fractionation experiments. Using protein-lipid overlays, we demonstrated an interaction of the FERM domain of PTPL1 with PtdIns(4,5)P2, which was lost after mutation of potential PtdIns(4,5)P2-binding motifs. Moreover, neomycin, which masks PtdIns(4,5)P2 polar heads, was shown to decrease by 50% the association of PTPL1 with the cytoskeletal fraction. These results identify the crucial role of the FERM domain in PTPL1 intracellular targeting and demonstrate that localization of PTPL1 is regulated by phosphoinositide metabolism.