Interactions between a nuclear transporter and a subset of nuclear pore complex proteins depend on Ran GTPase

RKIP localizes to the nucleus through a bipartite nuclear localization signal and interaction with importin α to regulate mitotic progression

Raf kinase inhibitor protein (RKIP) is a multifunctional modulator of intracellular signal transduction. Although most of its functions have been considered cytosolic, we show here that the localization of RKIP is primarily nuclear in both growing and quiescent Madin-Darby canine kidney epithelial cells and in Cal-51 and BT-20 human breast cancer cells. We have identified a putative bipartite nuclear localization signal (NLS) in RKIP that maps to the surface of the protein surrounding a known regulatory region. Like classical NLS sequences, the putative NLS of RKIP is rich in arginine and lysine residues. Deletion of and point mutations in the putative NLS lead to decreased nuclear localization. Point mutation of all the basic residues in the putative NLS of RKIP particularly strongly reduces nuclear localization. We found consistent results in reexpression experiments with wildtype or mutant RKIP in RKIP-silenced cells. A fusion construct of the putative NLS of RKIP alone to a heterologous reporter protein leads to nuclear localization of the fusion protein, demonstrating that this sequence alone is sufficient for import into the nucleus. We found that RKIP interacts with the nuclear transport factor importin α in BT-20 and MDA-MB-231 human breast cancer cells, suggesting importin-mediated active nuclear translocation. Taken together, these findings suggest that a bipartite NLS in RKIP interacts with importin α for active transport of RKIP into the nucleus and that this process may be involved in the regulation of mitotic progression. Evaluating the biological function of nuclear localization of RKIP, we found that the presence of the putative NLS is important for the role of RKIP in mitotic checkpoint regulation in MCF-7 human breast cancer cells.

Prevalence and functionality of intrinsic disorder in human FG-nucleoporins

The nuclear-cytoplasmic transport of biomolecules is assisted by the nuclear pores composed of evolutionarily conserved proteins termed nucleoporins (Nups). The central Nups, characterized by multiple FG-repeats, are highly dynamic and contain a high level of intrinsically disordered regions (IDPRs). FG-Nups bind several protein partners and play critical roles in molecular interactions and the regulation of cellular functions through their IDPRs. In the present study, we performed a multiparametric bioinformatics analysis to characterize the prevalence and functionality of IDPRs in human FG-Nups. These analyses revealed that the sequence of all FG-Nups contained >50% IDPRs (except Nup54 and Nup358). Nup98, Nup153, and POM121 were extremely disordered with ~80% IDPRs. The functional disorder-based binding regions in the FG-Nups were identified. The phase separation behavior of FG-Nups indicated that all FG-Nups have the potential to undergo liquid-to-liquid phase separation that could stabilize their liquid state. The inherent structural flexibility in FG-Nups is mechanistically and functionally advantageous. Since certain FG-Nups interact with disease-relevant protein aggregates, their complexes can be exploited for drug design. Furthermore, consideration of the FG-Nups from the intrinsic disorder perspective provides critical information that can guide future experimental studies to uncover novel pathways associated with diseases linked with protein misfolding and aggregation.

Importin-11 is Essential for Normal Embryonic Development in Mice

Importin-11 (Ipo11) is a novel member of the human importin family of transport receptors (karyopherins), which are known to mediate the nucleocytoplasmic transport of protein and RNA cargos. Despite its role in the transport of protein, we found that knockout of Ipo11 nuclear import factor affects normal embryonic development and govern embryo-lethal phenotypes in mice. In this study, we for the first time produced a mouse line containing null mutation in Ipo11 gene utilized by gene trapping. The Ipo11^-/- embryos showed an embryonic lethal phenotype. The Ipo11^-/- embryos showed a reduced size at embryonic day 10.5 (E10.5) when compared with Ipo11^+/+ or Ipo11^+/- embryos and died by E11.5. Whereas Ipo11^+/- mice were healthy and fertile, and there was no detectable changes in embryonic lethality and phenotype when reviewed. In the X-gal staining with the Ipo11^-/- or Ipo11^+/- embryos, strong X-gal staining positivity was detected systematically in the whole mount embryos at E10.5, although almost no X-gal positivity was detected at E9.5, indicating that the embryos die soon after the process of Ipo11 expression started. These results indicate that Ipo11 is essential for the normal embryonic development in mice.

The endocytic activity of the flagellar pocket in Trypanosoma brucei is regulated by an adjacent phosphatidylinositol phosphate kinase

Phosphoinositides are spatially restricted membrane signaling molecules. Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]--a phosphoinositide that is highly enriched in, and present throughout, the plasma membrane--has been implicated in endocytosis. Trypanosoma brucei has one of the highest known rates of endocytosis, a process it uses to evade the immune system. To determine whether phosphoinositides play a role in endocytosis in this organism, we have identified and characterized one of the enzymes that is responsible for generating PI(4,5)P2. Surprisingly, this phosphoinositide was found to be highly concentrated in the flagellar pocket, the only site of endocytosis and exocytosis in this organism. The enzyme (designated TbPIPKA, annotated as Tb927.10.1620) was present at the neck of the pocket, towards the anterior-end of the parasite. Depletion of TbPIPKA led to depletion of PI(4,5)P2 and enlargement of the pocket, the result of impaired endocytosis. Taken together, these data suggest that TbPIPKA and its product PI(4,5)P2 are important for endocytosis and, consequently, for homeostasis of the flagellar pocket.

Ultrastructural imaging of endocytic sites in Saccharomyces cerevisiae by transmission electron microscopy and immunolabeling

Defining the ultrastructure of endocytic sites and localization of endocytic proteins in Saccharomyces cerevisiae by immunoelectron microscopy is central in understanding the mechanisms of membrane deformation and scission during endocytosis. We show that an improved sample preparation protocol based on high-pressure freezing, freeze substitution, and low-temperature embedding allows us to maintain the cellular fine structure and to immunolabel green fluorescent protein-tagged endocytic proteins or actin in the same sections. Using this technique we analyzed the stepwise deformation of endocytic membranes and immunolocalized the endocytic proteins Abp1p, Sla1p, Rvs167p, and actin, and were able to draw a clear ultrastructural distinction between endocytic sites and eisosomes by immunolocalizing Pil1p. In addition to defining the geometry and the fine structure of budding yeast endocytic sites, we observed associated actin filaments forming a cage-like meshwork around the endocytic membrane.

The zinc-binding region of IL-2 inducible T cell kinase (Itk) is required for interaction with Gα13 and activation of serum response factor

Tec family kinases play critical roles in the activation of immune cells. In particular, Itk is important for the activation of T cells via the T cell Receptor (TcR), however, molecules that cooperate with Itk to activate downstream targets remain little explored. Here we show that Itk interacts with the heterotrimeric G-protein α subunit Gα13 during TcR triggering. This interaction requires membrane localization of both partners, and is partially dependent on GDP- and GTP-bound states of Gα13. Furthermore, we find that Itk interacts with Gα13 via the zinc binding regions within its Tec homology domain. The interaction between Itk and Gα13 also results in tyrosine phosphorylation of Gα13, however this is not required for the interaction. Itk enhances Gα13 mediated activation of serum response factor (SRF) transcriptional activity dependent on its ability to interact with Gα13, but its kinase activity is not required to enhance SRF activity. These data reveal a new pathway regulated by Itk in cells, and suggest cross talk between Itk and G-protein signaling downstream of the TcR.

Controlling subcellular delivery to optimize therapeutic effect

This article focuses on drug targeting to specific cellular organelles for therapeutic purposes. Drugs can be delivered to all major organelles of the cell (cytosol, endosome/lysosome, nucleus, nucleolus, mitochondria, endoplasmic reticulum, Golgi apparatus, peroxisomes and proteasomes) where they exert specific effects in those particular subcellular compartments. Delivery can be achieved by chemical (e.g., polymeric) or biological (e.g., signal sequences) means. Unidirectional targeting to individual organelles has proven to be immensely successful for drug therapy. Newer technologies that accommodate multiple signals (e.g., protein switch and virus-like delivery systems) mimic nature and allow for a more sophisticated approach to drug delivery. Harnessing different methods of targeting multiple organelles in a cell will lead to better drug delivery and improvements in disease therapy.

Morphology of the trypanosome bilobe, a novel cytoskeletal structure

The trypanosome bilobe is a cytoskeletal structure of unclear function. To date, four proteins have been shown to localize stably to it: TbMORN1, TbLRRP1, TbCentrin2, and TbCentrin4. In this study, a combination of immunofluorescence microscopy and electron microscopy was used to explore the morphology of the bilobe and its relationship to other nearby cytoskeletal structures in the African trypanosome procyclic trypomastigote. The use of detergent/salt-extracted flagellum preparations was found to be an effective way of discerning features of the cytoskeletal ultrastructure that are normally obscured. TbMORN1 and TbCentrin4 together define a hairpin structure comprising an arm of TbCentrin4 and a fishhook of TbMORN1. The two arms flank a specialized microtubule quartet and the flagellum attachment zone filament, with TbMORN1 running alongside the former and TbCentrin4 alongside the latter. The hooked part of TbMORN1 sits atop the flagellar pocket collar marked by TbBILBO1. The TbMORN1 bilobe occasionally exhibits tendrillar extensions that seem to be connected to the basal and probasal bodies. The TbMORN1 molecules present on these tendrils undergo higher rates of turnover than those for molecules on the main bilobe structure. These observations have been integrated with previous detailed descriptions of the cytoskeletal elements in trypanosome cells.

Ribosome synthesis-unrelated functions of the preribosomal factor Rrp12 in cell cycle progression and the DNA damage response

Given the high metabolic cost required to generate ribosomes, it has been assumed that proteins involved in ribosome synthesis might establish functional cross talk with other intracellular processes to efficiently couple ribosome production and cell growth. However, such interconnections have remained elusive due to the difficulty in separating the intra- and extraribosomal roles of ribosome biogenesis factors. Using a yeast functional screen, I have discovered that Rrp12, a conserved protein involved in ribosome maturation and export, plays roles in the cell cycle and the DNA damage response. These results indicate that Rrp12 participates in a karyopherin Kap121-dependent import route that is crucial for nuclear sequestration of ribonucleotide reductase subunits and, thereby, ensures the proper kinetics of deoxyribonucleotide production during the cell cycle. Within this route, Rrp12 acts as a cofactor important for the full functionality of Kap121. This activity is mechanistically different from the known roles of Rrp12 in ribosome biogenesis. I propose that the functional duality of Rrp12 may couple the control of ribosome production to the regulation of other cellular processes during cell cycle progression.

Regulation of base excision repair: Ntg1 nuclear and mitochondrial dynamic localization in response to genotoxic stress

Numerous human pathologies result from unrepaired oxidative DNA damage. Base excision repair (BER) is responsible for the repair of oxidative DNA damage that occurs in both nuclei and mitochondria. Despite the importance of BER in maintaining genomic stability, knowledge concerning the regulation of this evolutionarily conserved repair pathway is almost nonexistent. The Saccharomyces cerevisiae BER protein, Ntg1, relocalizes to organelles containing elevated oxidative DNA damage, indicating a novel mechanism of regulation for BER. We propose that dynamic localization of BER proteins is modulated by constituents of stress response pathways. In an effort to mechanistically define these regulatory components, the elements necessary for nuclear and mitochondrial localization of Ntg1 were identified, including a bipartite classical nuclear localization signal, a mitochondrial matrix targeting sequence and the classical nuclear protein import machinery. Our results define a major regulatory system for BER which when compromised, confers a mutator phenotype and sensitizes cells to the cytotoxic effects of DNA damage.

Flexible gates: dynamic topologies and functions for FG nucleoporins in nucleocytoplasmic transport

The nuclear envelope is a physical barrier between the nucleus and cytoplasm and, as such, separates the mechanisms of transcription from translation. This compartmentalization of eukaryotic cells allows spatial regulation of gene expression; however, it also necessitates a mechanism for transport between the nucleus and cytoplasm. Macromolecular trafficking of protein and RNA occurs exclusively through nuclear pore complexes (NPCs), specialized channels spanning the nuclear envelope. A novel family of NPC proteins, the FG-nucleoporins (FG-Nups), coordinates and potentially regulates NPC translocation. The extensive repeats of phenylalanine-glycine (FG) in each FG-Nup directly bind to shuttling transport receptors moving through the NPC. In addition, FG-Nups are essential components of the nuclear permeability barrier. In this review, we discuss the structural features, cellular functions, and evolutionary conservation of the FG-Nups.

The bilobe structure of Trypanosoma brucei contains a MORN-repeat protein

The Golgi of the kinetoplastid parasite Trypanosoma brucei is closely apposed to a bilobe structure containing TbCentrin2 and TbCentrin4 in procyclic cells. However, both are additionally localized to the basal bodies. Here we report the characterization of a membrane occupation and recognition nexus (MORN)-repeat protein, TbMORN1, present at the bilobe but not at the basal body. The anterior part of the TbMORN1 structure partially overlapped with the flagellar attachment zone while the posterior part overlapped with the flagellar pocket. Depletion studies using RNAi showed that there was a modest growth inhibition in procyclic cells but lethality in bloodstream cells, showing that it is an essential protein in the bloodstream form of the organism. TbMORN1 appears to be a useful marker for the bilobe in T. brucei.

Ultrastructural study of Golgi duplication in Trypanosoma brucei

Golgi duplication in the protozoan parasite Trypanosoma brucei has been tracked using serial thin section three-dimensional reconstructions of transmission electron micrographs. The old Golgi maintains a constant size (approximately 0.060 microm(3)) throughout the cell cycle. A morphologically identifiable new Golgi appears at approximately 0.20 of the cell cycle (defined by the size of the nucleus and lasting about 9 h) and grows from approximately 0.018 microm(3) until it is the same size as the old Golgi (by approximately 0.55 of the cell cycle). Morphologically identifiable late Golgi appear at approximately 0.58 of the cell cycle, but their volume ( approximately 0.036 microm(3)) did not change significantly. Cryoimmunoelectron microscopy was used to identify candidates for the earliest new Golgi structures, and these comprised clusters of vesicles containing Golgi reassembly stacking protein (GRASP) near an endoplasmic reticulum exit site. These results, combined with earlier fluorescence data, suggest that the new Golgi begins functioning before cisternal stacks are formed.

Splice variants of the human ZC3H14 gene generate multiple isoforms of a zinc finger polyadenosine RNA binding protein

The human ZC3H14 gene encodes an evolutionarily conserved Cys(3)His zinc finger protein that binds specifically to polyadenosine RNA and is thus postulated to modulate post-transcriptional gene expression. Expressed sequence tag (EST) data predicts multiple splice variants of both human and mouse ZC3H14. Analysis of ZC3H14 expression in both human cell lines and mouse tissues confirms the presence of multiple alternatively spliced transcripts. Although all of these transcripts encode protein isoforms that contain the conserved C-terminal zinc finger domain, suggesting that they could all bind to polyadenosine RNA, they differ in other functionally important domains. Most of the alternative transcripts encode closely related proteins (termed isoforms 1, 2, 3, and 3 short) that differ primarily in the inclusion of three small exons, 9, 10, and 11, resulting in predicted protein isoforms ranging from 82 to 64 kDa. Each of these closely related isoforms contains predicted classical nuclear localization signals (cNLS) within exons 7 and 11. Consistent with the presence of these putative nuclear targeting signals, these ZC3H14 isoforms are all localized to the nucleus. In contrast, an additional transcript encodes a smaller protein (34 kDa) with an alternative first exon (isoform 4). Consistent with the absence of the predicted cNLS motifs located in exons 7 and 11, ZC3H14 isoform 4 is localized to the cytoplasm. Both EST data and experimental data suggest that this variant is enriched in testes and brain. Using an antibody that detects endogenous ZC3H14 isoforms 1-3 reveals localization of these isoforms to nuclear speckles. These speckles co-localize with the splicing factor, SC35, suggesting a role for nuclear ZC3H14 in mRNA processing. Taken together, these results demonstrate that multiple transcripts encoding several ZC3H14 isoforms exist in vivo. Both nuclear and cytoplasmic ZC3H14 isoforms could have distinct effects on gene expression mediated by the common Cys(3)His zinc finger polyadenosine RNA binding domain.

Artificial nanopores that mimic the transport selectivity of the nuclear pore complex

Nuclear pore complexes (NPCs) act as effective and robust gateways between the nucleus and the cytoplasm, selecting for the passage of particular macromolecules across the nuclear envelope. NPCs comprise an elaborate scaffold that defines a approximately 30 nm diameter passageway connecting the nucleus and the cytoplasm. This scaffold anchors proteins termed 'phenylalanine-glycine' (FG)-nucleoporins, the natively disordered domains of which line the passageway and extend into its lumen. Passive diffusion through this lined passageway is hindered in a size-dependent manner. However, transport factors and their cargo-bound complexes overcome this restriction by transient binding to the FG-nucleoporins. To test whether a simple passageway and a lining of transport-factor-binding FG-nucleoporins are sufficient for selective transport, we designed a functionalized membrane that incorporates just these two elements. Here we demonstrate that this membrane functions as a nanoselective filter, efficiently passing transport factors and transport-factor-cargo complexes that specifically bind FG-nucleoporins, while significantly inhibiting the passage of proteins that do not. This inhibition is greatly enhanced when transport factor is present. Determinants of selectivity include the passageway diameter, the length of the nanopore region coated with FG-nucleoporins, the binding strength to FG-nucleoporins, and the antagonistic effect of transport factors on the passage of proteins that do not specifically bind FG-nucleoporins. We show that this artificial system faithfully reproduces key features of trafficking through the NPC, including transport-factor-mediated cargo import.

Dynamic compartmentalization of base excision repair proteins in response to nuclear and mitochondrial oxidative stress

DNAs harbored in both nuclei and mitochondria of eukaryotic cells are subject to continuous oxidative damage resulting from normal metabolic activities or environmental insults. Oxidative DNA damage is primarily reversed by the base excision repair (BER) pathway, initiated by N-glycosylase apurinic/apyrimidinic (AP) lyase proteins. To execute an appropriate repair response, BER components must be distributed to accommodate levels of genotoxic stress that may vary considerably between nuclei and mitochondria, depending on the growth state and stress environment of the cell. Numerous examples exist where cells respond to signals, resulting in relocalization of proteins involved in key biological transactions. To address whether such dynamic localization contributes to efficient organelle-specific DNA repair, we determined the intracellular localization of the Saccharomyces cerevisiae N-glycosylase/AP lyases, Ntg1 and Ntg2, in response to nuclear and mitochondrial oxidative stress. Fluorescence microscopy revealed that Ntg1 is differentially localized to nuclei and mitochondria, likely in response to the oxidative DNA damage status of the organelle. Sumoylation is associated with targeting of Ntg1 to nuclei containing oxidative DNA damage. These studies demonstrate that trafficking of DNA repair proteins to organelles containing high levels of oxidative DNA damage may be a central point for regulating BER in response to oxidative stress.

Differential palmitoylation of the endosomal SNAREs syntaxin 7 and syntaxin 8

Palmitoylation is a posttranslational modification that regulates protein trafficking and stability. In this study we investigated whether the endosomal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNARE) proteins syntaxin 7 and syntaxin 8 are modified with palmitate. Using metabolic labeling and site-directed mutagenesis, we show that human syntaxins 7 and 8 are modified with palmitate through a thioester linkage. Palmitoylation is dependent upon cysteine 239 of human syntaxin 7 and cysteine 214 of syntaxin 8, residues that are located on the cytoplasmic face of the transmembrane domain (TMD). Palmitoylation of syntaxin 8 is minimally affected by the Golgi-disturbing agent brefeldin A (BFA), whereas BFA dramatically inhibits palmitoylation of syntaxin7. The differential effect of BFA suggests that palmitoylation of syntaxins 7 and 8 occurs in distinct subcellular compartments. Palmitoylation does not affect the rate of protein turnover of syntaxins 7 and 8 nor does it influence the steady-state localization of syntaxin 8 in late endosomes. Syntaxin 7 actively cycles between endosomes and the plasma membrane. Palmitoylation-defective syntaxin 7 is selectively retained on the plasma membrane, suggesting that palmitoylation is important for intercompartmental transport of syntaxin 7.

Centrin4 coordinates cell and nuclear division in T. brucei

Centrins are Ca2+-binding proteins that have been implicated in a number of biological processes, including organelle duplication, mRNA export, DNA repair and signal transduction. In the protozoan parasite Trypanosoma brucei we have previously described TbCentrin2, which is present on a bi-lobed structure, and involved in the duplication and segregation of the Golgi complex. Recently, another centrin, TbCentrin4, was also found at the bi-lobe and has been implicated in organelle segregation and cytokinesis. We now show that cytokinesis is not inhibited, but that a dysregulation of nuclear and cell division leads to the production of zoids – daughter siblings that contain all organelles except the nucleus. Our results, therefore, suggest that TbCentrin4 is involved in processes that coordinate karyokinesis and cytokinesis.

The Ty1 integrase protein can exploit the classical nuclear protein import machinery for entry into the nucleus

Like its retroviral relatives, the long terminal repeat retrotransposon Ty1 in the yeast Saccharomyces cerevisiae must traverse a permanently intact nuclear membrane for successful transposition and replication. For retrotransposition to occur, at least a subset of Ty1 proteins, including the Ty1 integrase, must enter the nucleus. Nuclear localization of integrase is dependent upon a C-terminal nuclear targeting sequence. However, the nuclear import machinery that recognizes this nuclear targeting signal has not been defined. We investigated the mechanism by which Ty1 integrase gains access to nuclear DNA as a model for how other retroelements, including retroviruses like HIV, may utilize cellular nuclear transport machinery to import their essential nuclear proteins. We show that Ty1 retrotransposition is significantly impaired in yeast mutants that alter the classical nuclear protein import pathway, including the Ran-GTPase, and the dimeric import receptor, importin-alpha/beta. Although Ty1 proteins are made and processed in these mutant cells, our studies reveal that an integrase reporter is not properly targeted to the nucleus in cells carrying mutations in the classical nuclear import machinery. Furthermore, we demonstrate that integrase coimmunoprecipitates with the importin-alpha transport receptor and directly binds to importin-alpha. Taken together, these data suggest Ty1 integrase can employ the classical nuclear protein transport machinery to enter the nucleus.

TbG63, a golgin involved in Golgi architecture in Trypanosoma brucei

Golgins are coiled-coil proteins that have been implicated in the structure and function of the Golgi complex. Here, we identify and characterize a trypanosomal golgin, TbG63, showing that it has a C-terminal membrane anchor and an N-terminus that projects into the cytoplasm. TbG63 in procyclic parasites is localized to the Golgi and interacts with the active, GTP-form of TbRab1A. Overexpression of TbG63 has dramatic effects on Golgi architecture -- effects that require the N-terminus -- whereas depletion has little, if any, effect on the growth rate. By contrast, in the bloodstream form of the parasite, depletion of TbG63 slows growth, although it has no obvious effect on the transport of a variant surface glycoprotein (VSG) or on Golgi structure. TbG63 might be a useful tool to study the structure and functioning of the Golgi complex.

Cse1p-binding dynamics reveal a binding pattern for FG-repeat nucleoporins on transport receptors

Nuclear pore proteins with phenylalanine-glycine repeats are vital to the functional transport of molecules across the nuclear pore complex. The current study investigates the binding of these FG-nucleoporins to the Cse1p:Kap60p:RanGTP nuclear export complex. Fourteen binding spots for FG-nucleoporin peptides are revealed on the surface of Cse1p, and 5 are revealed on the Kap60p surface. Taken together, and along with binding data for two other transport receptors, the data suggest that the ability to bind FG-nucleoporins by itself is not enough to ensure viable nuclear transport. Rather, it is proposed that the density of binding spots on the transport receptor surface is key in determining transport viability. The number of binding spots on the transport receptor surface should be large enough to ensure multiple, simultaneous FG-repeat binding, and their arrangement should be close enough to ensure multiple binding from the same FG-nucleoporin.

Analysis of a predicted nuclear localization signal: implications for the intracellular localization and function of the Saccharomyces cerevisiae RNA-binding protein Scp160

Gene expression is controlled by RNA-binding proteins that modulate the synthesis, processing, transport and stability of various classes of RNA. Some RNA-binding proteins shuttle between the nucleus and cytoplasm and are thought to bind to RNA transcripts in the nucleus and remain bound during translocation to the cytoplasm. One RNA-binding protein that has been hypothesized to function in this manner is the Saccharomyces cerevisiae Scp160 protein. Although the steady-state localization of Scp160 is cytoplasmic, previous studies have identified putative nuclear localization (NLS) and nuclear export (NES) signals. The goal of this study was to test the hypothesis that Scp160 is a nucleocytoplasmic shuttling protein. We exploited a variety of yeast export mutants to capture any potential nuclear accumulation of Scp160 and found no evidence that Scp160 enters the nucleus. These localization studies were complemented by a mutational analysis of the predicted NLS. Results indicate that key basic residues within the predicted NLS of Scp160 can be altered without severely affecting Scp160 function. This finding has important implications for understanding the function of Scp160, which is likely limited to the cytoplasm. Additionally, our results provide strong evidence that the presence of a predicted nuclear localization signal within the sequence of a protein should not lead to the assumption that the protein enters the nucleus in the absence of additional experimental evidence.

Simple kinetic relationships and nonspecific competition govern nuclear import rates in vivo

Many cargoes destined for nuclear import carry nuclear localization signals that are recognized by karyopherins (Kaps). We present methods to quantitate import rates and measure Kap and cargo concentrations in single yeast cells in vivo, providing new insights into import kinetics. By systematically manipulating the amounts, types, and affinities of Kaps and cargos, we show that import rates in vivo are simply governed by the concentrations of Kaps and their cargo and the affinity between them. These rates fit to a straightforward pump–leak model for the import process. Unexpectedly, we deduced that the main limiting factor for import is the poor ability of Kaps and cargos to find each other in the cytoplasm in a background of overwhelming nonspecific competition, rather than other more obvious candidates such as the nuclear pore complex and Ran. It is likely that most of every import round is taken up by Kaps and nuclear localization signals sampling other cytoplasmic proteins as they locate each other in the cytoplasm.

Association of nuclear pore FG-repeat domains to NTF2 import and export complexes

Transport into and out of the nucleus is regulated by the nuclear pore complex. Vital to this regulation are nuclear pore proteins with FG sequence repeats, which have been shown to be crucial for cell viability and which interact with nuclear transport receptors. Here we use molecular dynamics simulations to investigate the binding of FG-repeat peptides to the surface of NTF2, the Ran importer. The simulations, covering over 254 ns, agree with previous X-ray, mutational, NMR, and computational data in identifying four binding spots. They also serve to provide an all-atom view of binding at each spot, whereas FG-repeat binding has been only directly observed at a single spot. Furthermore, the simulations identify two novel binding spots in addition to the four others. All six binding spots broadly form a stripe across the surface of NTF2. The resulting regularity and proximity of binding spots on the surface may be necessary for identification of the transport receptor by the FG-repeats in the nuclear pore complex and for the successful transit of NTF2 through the pore.

The unfolded protein response transducer Ire1p contains a nuclear localization sequence recognized by multiple beta importins

The Ire1p transmembrane receptor kinase/endonuclease transduces the unfolded protein response (UPR) from the endoplasmic reticulum (ER) to the nucleus in Saccharomyces cerevisiae. In this study, we analyzed the capacity of a highly basic sequence in the linker region of Ire1p to function as a nuclear localization sequence (NLS) both in vivo and in vitro. This 18-residue sequence is capable of targeting green fluorescent protein to the nucleus of yeast cells in a process requiring proteins involved in the Ran GTPase cycle that facilitates nuclear import. Mutagenic analysis and importin binding studies demonstrate that the Ire1p linker region contains overlapping potential NLSs: at least one classical NLS (within sequences 642KKKRKR647 and/or 653KKGR656) that is recognized by yeast importin alpha (Kap60p) and a novel betaNLS (646KRGSRGGKKGRK657) that is recognized by several yeast importin beta homologues. Kinetic binding data suggest that binding to importin beta proteins would predominate in vivo. The UPR, and in particular ER stress-induced HAC1 mRNA splicing, is inhibited by point mutations in the Ire1p NLS that inhibit nuclear localization and also requires functional RanGAP and Ran GEF proteins. The NLS-dependent nuclear localization of Ire1p would thus seem to be central to its role in UPR signaling.

Controlling protein compartmentalization to overcome disease

Over the past decade, considerable progress has been made to improve our understanding of the intracellular transport of proteins. Mechanisms of nuclear import and export involving classical receptors have been studied. Signal sequences required for directing a protein molecule to a specific cellular compartment have been defined. Knowledge of subcellular trafficking of proteins has also increased our understanding of diseases caused due to mislocalization of proteins. A specific protein on deviating from its native cellular compartment may result in disease due to loss of its normal functioning and aberrant activity in the "wrong" compartment. Mislocalization of proteins results in diseases that range from metabolic disorders to cancer. In this review we discuss some of the diseases caused due to mislocalization. We further focus on application of nucleocytoplasmic transport to drug delivery. Various rationales to treat diseases by exploiting intracellular transport machinery have been proposed. Although the pathways for intracellular movement of proteins have been defined, these have not been adequately utilized for management of diseases involving mislocalized proteins. This review stresses the need for designing drug delivery systems utilizing these mechanisms as this area is least exploited but offers great potential.

The conserved ATPase Get3/Arr4 modulates the activity of membrane-associated proteins in Saccharomyces cerevisiae

The regulation of cellular membrane dynamics is crucial for maintaining proper cell growth and division. The Cdc48-Npl4-Ufd1 complex is required for several regulated membrane-associated processes as part of the ubiquitin-proteasome system, including ER-associated degradation and the control of lipid composition in yeast. In this study we report the results of a genetic screen in Saccharomyces cerevisiae for extragenic suppressors of a temperature-sensitive npl4 allele and the subsequent analysis of one suppressor, GET3/ARR4. The GET3 gene encodes an ATPase with homology to the regulatory component of the bacterial arsenic pump. Mutants of GET3 rescue several phenotypes of the npl4 mutant and transcription of GET3 is coregulated with the proteasome, illustrating a functional relationship between GET3 and NPL4 in the ubiquitin-proteasome system. We have further found that Get3 biochemically interacts with the trans-membrane domain proteins Get1/Mdm39 and Get2/Rmd7 and that Deltaget3 is able to suppress phenotypes of get1 and get2 mutants, including sporulation defects. In combination, our characterization of GET3 genetic and biochemical interactions with NPL4, GET1, and GET2 implicates Get3 in multiple membrane-dependent pathways.

Nuclear Localization Signal Receptor Affinity Correlates with in Vivo Localization in Saccharomyces cerevisiae

Nuclear localization signals (NLSs) target proteins into the nucleus through mediating interactions with nuclear import receptors. Here, we perform a quantitative analysis of the correlation between NLS receptor affinity and the steady-state distribution of NLS-bearing cargo proteins between the cytoplasm and the nucleus of live yeast, which reflects the relative import rates of various NLS sequences. We find that there is a complicated, but monotonic quantitative relationship between the affinity of an NLS for the import receptor, importin alpha, and the steady-state accumulation of the cargo in the nucleus. This analysis takes into consideration the impact of protein size. In addition, the hypothetical upper limit to an NLS affinity for the receptors is explored through genetic approaches. Overall, our results indicate that there is a correlation between the binding affinity of an NLS cargo for the NLS receptor, importin alpha, and the import rate for this cargo. This correlation, however, is not maintained for cargoes that bind to the NLS receptor with very weak or very strong affinity.

Oncogenic NRAS rapidly and efficiently induces CMML- and AML-like diseases in mice

Activating mutations in RAS, predominantly NRAS, are common in myeloid malignancies. Previous studies in animal models have shown that oncogenic NRAS is unable to induce myeloid malignancies effectively, and it was suggested that oncogenic NRAS might only act as a secondary mutation in leukemogenesis. In this study, we examined the leukemogenicity of NRAS using an improved mouse bone marrow transduction and transplantation model. We found that oncogenic NRAS rapidly and efficiently induced chronic myelomonocytic leukemia (CMML)- or acute myeloid leukemia (AML)- like disease in mice, indicating that mutated NRAS can function as an initiating oncogene in the induction of myeloid malignancies. In addition to CMML and AML, we found that NRAS induced mastocytosis in mice. This result indicates that activation of the RAS pathway also plays an important role in the pathogenesis of mastocytosis. The mouse model for NRAS leukemogenesis established here provides a system for further studying the molecular mechanisms in the pathogenesis of myeloid malignancies and for testing relevant therapies.

Copper induces cytoplasmic retention of fission yeast transcription factor cuf1

Copper homeostasis within the cell is established and preserved by different mechanisms. Changes in gene expression constitute a way of maintaining this homeostasis. In Schizosaccharomyces pombe, the Cuf1 transcription factor is critical for the activation of copper transport gene expression under conditions of copper starvation. However, in the presence of elevated intracellular levels of copper, the mechanism of Cuf1 inactivation to turn off gene expression remains unclear. In this study, we provide evidence that inactivation of copper transport gene expression by Cuf1 is achieved through a copper-dependent, cytosolic retention of Cuf1. We identify a minimal nuclear localization sequence (NLS) between amino acids 11 to 53 within the Cuf1 N terminus. Deletion of this region and specific mutation of the Lys13, Arg16, Arg19, Lys24, Arg28, Lys45, Arg47, Arg50, and Arg53 residues to alanine within this putative NLS is sufficient to abrogate nuclear targeting of Cuf1. Under conditions of copper starvation, Cuf1 resides in the nucleus. However, in the presence of excess copper as well as silver ions, Cuf1 is sequestered in the cytoplasm, a process which requires the putative copper binding motif, 328Cys-X-Cys-X3-Cys-X-Cys-X2-Cys-X2-His342 (designated C-rich), within the C-terminal region of Cuf1. Deletion of this region and mutation of the Cys residues within the C-rich motif result in constitutive nuclear localization of Cuf1. By coexpressing the Cuf1 N terminus with its C terminus in trans and by using a two-hybrid assay, we show that these domains physically interact with each other in a copper-dependent manner. We propose a model wherein copper induces conformational changes in Cuf1 that promote a physical interaction between the Cuf1 N terminus and the C-rich motif in the C terminus that masks the NLS. Cuf1 is thereby sequestered in the cytosol under conditions of copper excess, thereby extinguishing copper transport gene expression.

Ordered assembly of the duplicating Golgi in Trypanosoma brucei

The new Golgi in the protozoan parasite Trypanosoma brucei grows near to the old and adjacent to the growing new endoplasmic reticulum exit site. Growth is now shown to be at least a two-stage process, in which a representative matrix marker (GRASP) and enzyme (GntB) are delivered to the site of assembly, followed approximately 10 min later by a COPI component (epsilon-COP) and a trans-Golgi network (TGN) marker (GRIP70). A secretory cargo marker (signal sequence-YFP) appeared early near the new endoplasmic reticulum exit site but did not enter the Golgi until the second stage. Together these data suggest that structural and enzymatic components of the new Golgi stack are laid down first, followed by those needed to move and sort the cargo passing through it.

Highwire function at the Drosophila neuromuscular junction: spatial, structural, and temporal requirements

Highwire is a huge, evolutionarily conserved protein that is required to restrain synaptic growth and promote synaptic transmission at the Drosophila neuromuscular junction. Current models of highwire function suggest that it may act as a ubiquitin ligase to regulate synaptic development. However, it is not known in which cells highwire functions, whether its putative ligase domain is required for function, or whether highwire regulates the synapse during development or alternatively sets cell fate in the embryo. We performed a series of transgenic rescue experiments to test the spatial, structural, and temporal requirements for highwire function. We find that presynaptic activity of highwire is both necessary and sufficient to regulate both synapse morphology and physiology. The Highwire RING domain, which is postulated to function as an E3 ubiquitin ligase, is required for highwire function. In addition, highwire acts throughout larval development to regulate synaptic morphology and function. Finally, we show that the morphological and physiological phenotypes of highwire mutants have different dosage and temporal requirements for highwire, demonstrating that highwire may independently regulate the molecular pathways controlling synaptic growth and function.

Saccharomyces cerevisiae Npc2p is a functionally conserved homologue of the human Niemann-Pick disease type C 2 protein, hNPC2

Niemann-Pick Disease Type C (NP-C) is a fatal neurodegenerative disease, which is biochemically distinguished by the lysosomal accumulation of exogenously derived cholesterol. Mutation of either the hNPC1 or hNPC2 gene is causative for NP-C. We report the identification of the yeast homologue of human NPC2, Saccharomyces cerevisiae Npc2p. We demonstrate that scNpc2p is evolutionarily related to the mammalian NPC2 family of proteins. We also show, through colocalization, subcellular fractionation, and secretion analyses, that yeast Npc2p is treated similarly to human NPC2 when expressed in mammalian cells. Importantly, we show that yeast Npc2p can efficiently revert the unesterified cholesterol and GM1 accumulation seen in hNPC2-/- patient fibroblasts demonstrating that it is a functional homologue of human NPC2. The present study reveals that the fundamental process of NPC2-mediated lipid transport has been maintained throughout evolution.

The vacuolar DHHC-CRD protein Pfa3p is a protein acyltransferase for Vac8p

Palmitoylation of the vacuolar membrane protein Vac8p is essential for vacuole fusion in yeast (Veit, M., R. Laage, L. Dietrich, L. Wang, and C. Ungermann. 2001. EMBO J. 20:3145–3155; Wang, Y.X., E.J. Kauffman, J.E. Duex, and L.S. Weisman. 2001. J. Biol. Chem. 276:35133–35140). Proteins that contain an Asp-His-His-Cys (DHHC)–cysteine rich domain (CRD) are emerging as a family of protein acyltransferases, and are therefore candidates for mediators of Vac8p palmitoylation. Here we demonstrate that the DHHC-CRD proteins Pfa3p (protein fatty acyltransferase 3, encoded by YNL326c) and Swf1p are important for vacuole fusion. Cells lacking Pfa3p had fragmented vacuoles when stressed, and cells lacking both Pfa3p and Swf1p had fragmented vacuoles under normal growth conditions. Pfa3p promoted Vac8p membrane association and palmitoylation in vivo and partially purified Pfa3p palmitoylated Vac8p in vitro, establishing Vac8p as a substrate for palmitoylation by Pfa3p. Vac8p is the first N-myristoylated, palmitoylated protein identified as a substrate for a DHHC-CRD protein.

Binding Dynamics of Isolated Nucleoporin Repeat Regions to Importin-β

The nuclear pore complex, through the interaction of its proteins with transport receptors, controls the transport of large molecules into and out of the cell's nucleus. There is ample evidence for proteins with FG sequence repeats playing an essential role in this control. Previous studies have elucidated binding spots for FG sequence repeats on the surface of the transport receptor importin-beta by X-ray crystallography and mutational studies. Molecular dynamics simulations have been performed to characterize the interaction of FG sequence repeats with the transport receptor. Observed binding spots have been verified and novel sites discovered, suggesting that importin-beta features many more binding spots than suspected so far. The observed binding spots are in accord with several models of nucleocytoplasmic transport, and the large number of binding spots on importin-beta may be necessary for the pore complex to distinguish between importin-beta and inert proteins, and to allow for its passage through the pore.

The homologous putative GTPases Grn1p from fission yeast and the human GNL3L are required for growth and play a role in processing of nucleolar pre-rRNA

Grn1p from fission yeast and GNL3L from human cells, two putative GTPases from the novel HSR1_MMR1 GTP-binding protein subfamily with circularly permuted G-motifs play a critical role in maintaining normal cell growth. Deletion of Grn1 resulted in a severe growth defect, a marked reduction in mature rRNA species with a concomitant accumulation of the 35S pre-rRNA transcript, and failure to export the ribosomal protein Rpl25a from the nucleolus. Deleting any of the Grn1p G-domain motifs resulted in a null phenotype and nuclear/nucleolar localization consistent with the lack of nucleolar export of preribosomes accompanied by a distortion of nucleolar structure. Heterologous expression of GNL3L in a Deltagrn1 mutant restored processing of 35S pre-rRNA, nuclear export of Rpl25a and cell growth to wild-type levels. Genetic complementation in yeast and siRNA knockdown in HeLa cells confirmed the homologous proteins Grn1p and GNL3L are required for growth. Failure of two similar HSR1_MMR1 putative nucleolar GTPases, Nucleostemin (NS), or the dose-dependent response of breast tumor autoantigen NGP-1, to rescue deltagrn1 implied the highly specific roles of Grn1p or GNL3L in nucleolar events. Our analysis uncovers an important role for Grn1p/GNL3L within this unique group of nucleolar GTPases.

A Yeast Model System for Functional Analysis of the Niemann-Pick Type C Protein 1 Homolog, Ncr1p

Niemann-Pick disease type C (NP-C) is a progressive, ultimately fatal, autosomal recessive neurodegenerative disorder. The major biochemical hallmark of the disease is the endocytic accumulation of low-density lipoprotein-derived cholesterol. The majority of NP-C patients have mutations in the Niemann-Pick type C1 gene, NPC1. This study focuses on the Saccharomyces cerevisiae homolog of the human NPC1 protein encoded by the NCR1 gene. Ncr1p localizes to the vacuole, the yeast equivalent to the mammalian endosome-lysosome system. Here, we identify the first phenotype caused by deletion of NCR1 from the yeast genome, resistance to the ether lipid drug, edelfosine. Our results indicate that edelfosine has a cytotoxic, rather than cytostatic, effect on wildtype yeast cells. We exploit the edelfosine resistance phenotype to assess the function of yeast Ncr1 proteins carrying amino acid changes corresponding to human NPC1 patient mutations. We find that one of these amino acid changes severely compromises Ncr1p function as assessed using the edelfosine resistance assay. These findings establish S. cerevisiae as a model system that can be exploited to analyze the molecular consequences of patient mutations in NPC1 and provide the basis for future genetic studies using yeast.

DHHC9 and GCP16 constitute a human protein fatty acyltransferase with specificity for H- and N-Ras

Covalent lipid modifications mediate the membrane attachment and biological activity of Ras proteins. All Ras isoforms are farnesylated and carboxyl-methylated at the terminal cysteine; H-Ras and N-Ras are further modified by palmitoylation. Yeast Ras is palmitoylated by the DHHC cysteine-rich domain-containing protein Erf2 in a complex with Erf4. Here we report that H- and N-Ras are palmitoylated by a human protein palmitoyltransferase encoded by the ZDHHC9 and GCP16 genes. DHHC9 is an integral membrane protein that contains a DHHC cysteine-rich domain. GCP16 encodes a Golgi-localized membrane protein that has limited sequence similarity to yeast Erf4. DHHC9 and GCP16 co-distribute in the Golgi apparatus, a location consistent with the site of mammalian Ras palmitoylation in vivo. Like yeast Erf2.Erf4, DHHC9 and GCP16 form a protein complex, and DHHC9 requires GCP16 for protein fatty acyltransferase activity and protein stability. Purified DHHC9.GCP16 exhibits substrate specificity, palmitoylating H- and N-Ras but not myristoylated G (alphai1) or GAP-43, proteins with N-terminal palmitoylation motifs. Hence, DHHC9.GCP16 displays the properties of a functional human ortholog of the yeast Ras palmitoyltransferase.

Sec17p and HOPS, in distinct SNARE complexes, mediate SNARE complex disruption or assembly for fusion

SNARE functions during membrane docking and fusion are regulated by Sec1/Munc18 (SM) chaperones and Rab/Ypt GTPase effectors. These functions for yeast vacuole fusion are combined in the six-subunit HOPS complex. HOPS facilitates Ypt7p nucleotide exchange, is a Ypt7p effector, and contains an SM protein. We have dissected the associations and requirements for HOPS, Ypt7p, and Sec17/18p during SNARE complex assembly. Vacuole SNARE complexes bind either Sec17p or the HOPS complex, but not both. Sec17p and its co-chaperone Sec18p disassemble SNARE complexes. Ypt7p regulates the reassembly of unpaired SNAREs with each other and with HOPS, forming HOPS.SNARE complexes prior to fusion. After HOPS.SNARE assembly, lipid rearrangements are still required for vacuole content mixing. Thus, Sec17p and HOPS have mutually exclusive interactions with vacuole SNAREs to mediate disruption of SNARE complexes or their assembly for docking and fusion. Sec17p may displace HOPS from SNAREs to permit subsequent rounds of fusion.

Biochemical and genetic characterization of Yra1p in budding yeast

Yra1p and its vertebrate homologues bind to the mRNA export factor Mex67p/TAP and are thought to play a role in mRNA export in vivo. To further characterize Yra1p, we used immunoaffinity chromatography to purify endogenous Yra1p complexes. These experiments demonstrated that two importin beta homologues (Kap123p and Pse1p) and the poly A tail-binding proteins Pab1p and Nab2p associate with Yra1p. The other major proteins that associate with Yra1p include proteins involved in mRNA and rRNA processing and the Yra1p-related protein Yra2p. Additional biochemical and genetic experiments suggest a close functional relationship between Yra1p and Yra2p. We generated a temperature-sensitive allele of YRA1 and used it to demonstrate that cells which lack the function of both Yra1p and Yra2p are able to exit a G0 arrest and go through several rounds of cell division before arresting. We also identified high-copy suppressors of the yra1-2 temperature-sensitive growth defect. These include SUB2, a splicing factor important in mRNA export, ULP1, a nuclear cysteine protease localized to the nuclear pore and involved in Smt3p/SUMO processing, and YRA2. Taken together, these results suggest that Yra1p has roles in diverse RNA processing events in addition to a role in mRNA export.

HIV-1 incorporates and proteolytically processes human NDR1 and NDR2 serine-threonine kinases

Mammalian genomes encode two related serine-threonine kinases, nuclear Dbf2 related (NDR)1 and NDR2, which are homologous to the Saccharomyces cerevisiae Dbf2 kinase. Recently, a yeast genetic screen implicated the Dbf2 kinase in Ty1 retrotransposition. Since several virion-incorporated kinases regulate the infectivity of human immunodeficiency virus type 1 (HIV-1), we speculated that the human NDR1 and NDR2 kinases might play a role in the HIV-1 life cycle. Here we show that the NDR1 and NDR2 kinases were incorporated into HIV-1 particles. Furthermore, NDR1 and NDR2 were cleaved by the HIV-1 protease (PR), both within virions and within producer cells. Truncation at the PR cleavage site altered NDR2 subcellular localization and inhibited NDR1 and NDR2 enzymatic activity. These studies identify two new virion-associated host cell enzymes and suggest a novel mechanism by which HIV-1 alters the intracellular environment of human cells.

The FG-repeat asymmetry of the nuclear pore complex is dispensable for bulk nucleocytoplasmic transport in vivo

Nucleocytoplasmic transport occurs through gigantic proteinaceous channels called nuclear pore complexes (NPCs). Translocation through the NPC is exquisitely selective and is mediated by interactions between soluble transport carriers and insoluble NPC proteins that contain phenylalanine-glycine (FG) repeats. Although most FG nucleoporins (Nups) are organized symmetrically about the planar axis of the nuclear envelope, very few localize exclusively to one side of the NPC. We constructed Saccharomyces cerevisiae mutants with asymmetric FG repeats either deleted or swapped to generate NPCs with inverted FG asymmetry. The mutant Nups localize properly within the NPC and exhibit exchanged binding specificity for the export factor Xpo1. Surprisingly, we were unable to detect any defects in the Kap95, Kap121, Xpo1, or mRNA transport pathways in cells expressing the mutant FG Nups. These findings suggest that the biased distribution of FG repeats is not required for major nucleocytoplasmic trafficking events across the NPC.

Diacylglycerol and its formation by phospholipase C regulate Rab- and SNARE-dependent yeast vacuole fusion

Although diacylglycerol (DAG) can trigger liposome fusion, biological membrane fusion requires Rab and SNARE proteins. We have investigated whether DAG and phosphoinositide-specific phospholipase C (PLC) have a role in the Rab- and SNARE-dependent homo-typic vacuole fusion in Saccharomyces cerevisiae. Vacuole fusion was blocked when DAG was sequestered by a recombinant C1b domain. DAG underwent ATP-dependent turnover during vacuole fusion, but was replenished by the hydrolysis of phosphatidylinositol 4,5-bisphosphate to DAG by PLC. The PLC inhibitors 3-nitrocoumarin and U73122 blocked vacuole fusion in vitro, whereas their inactive homologues did not. Plc1p is the only known PLC in yeast. Yeast cells lacking the PLC1 gene have many small vacuoles, indicating defects in protein trafficking to the vacuole or vacuole fusion, and purified Plc1p stimulates vacuole fusion. Docking-dependent Ca(2+) efflux is absent in plc1Delta vacuoles and was restored only upon the addition of both Plc1p and the Vam7p SNARE. However, vacuoles purified from plc1Delta strains still retain PLC activity and significant 3-nitrocoumarin- and U73122-sensitive fusion, suggesting that there is another PLC in S. cerevisiae with an important role in vacuole fusion.

Golgi duplication in Trypanosoma brucei

Duplication of the single Golgi apparatus in the protozoan parasite Trypanosoma brucei has been followed by tagging a putative Golgi enzyme and a matrix protein with variants of GFP. Video microscopy shows that the new Golgi appears de novo, near to the old Golgi, about two hours into the cell cycle and grows over a two-hour period until it is the same size as the old Golgi. Duplication of the endoplasmic reticulum (ER) export site follows exactly the same time course. Photobleaching experiments show that the new Golgi is not the exclusive product of the new ER export site. Rather, it is supplied, at least in part, by material directly from the old Golgi. Pharmacological experiments show that the site of the new Golgi and ER export is determined by the location of the new basal body.

Rrs1p, a ribosomal protein L11-binding protein, is required for nuclear export of the 60S pre-ribosomal subunit in Saccharomyces cerevisiae

Rrs1p is a ribosomal protein L11-binding protein in Saccharomyces cerevisiae. We have obtained temperature-sensitive rrs1 mutants by random PCR mutagenesis. [(3)H]Methionine pulse-chase analysis reveals that the rrs1 mutations cause a defect in maturation of 25S rRNA. Ribosomal protein L25-enhanced green fluorescent protein, a reporter of the 60S ribosomal subunit, concentrates in the nucleus with enrichment in the nucleolus when the rrs1 mutants are shifted to the restrictive temperature. These results suggest that Rrs1p stays on the pre-60S particle from the early stage to very late stage of the large-subunit maturation and is required for export of 60S subunits from the nucleolus to the cytoplasm.

Ebp2p, the yeast homolog of Epstein-Barr virus nuclear antigen 1-binding protein 2, interacts with factors of both the 60 S and the 40 s ribosomal subunit assembly

Ebp2p, the yeast homolog of human Epstein-Barr virus nuclear antigen 1-binding protein 2, is essential for biogenesis of the 60 S ribosomal subunit. Two-hybrid screening exhibited that, in addition to factors necessary for assembly of the 60 S subunit, Ebp2p interacts with Rps16p, ribosomal protein S16, and the 40 S ribosomal subunit assembly factor, Utp11p, as well as Yil019w, the function of which was previously uncharacterized. Depletion of Yil019w resulted in reduction in levels of both of 18 S rRNA and 40 S ribosomal subunit without affecting levels of 25 S rRNA and 60 S ribosomal subunits. 35 S pre-rRNA and aberrant 23 S RNA accumulated, indicating that pre-rRNA processing at sites A(0)-A(2) is inhibited when Yil019w is depleted. Each combination from Yil019w, Utp11p, and Rps16p showed two-hybrid interaction.

The p21-activated Protein Kinase-related Kinase Cla4 Is a Coincidence Detector of Signaling by Cdc42 and Phosphatidylinositol 4-Phosphate

Signal transduction pathways that co-regulate a given biological process often are organized into networks by molecules that act as coincidence detectors. Phosphoinositides and the Rho-type GTPase Cdc42 regulate overlapping processes in all eukaryotic cells. However, the coincidence detectors that link these pathways into networks remain unknown. Here we show that the p21-activated protein kinase-related kinase Cla4 of yeast integrates signaling by Cdc42 and phosphatidylinositol 4-phosphate (PI4P). We found that the Cla4 pleckstrin homology (PH) domain binds in vitro to several phosphoinositide species. To determine which phosphoinositides regulate Cla4 in vivo, we analyzed phosphatidylinositol kinase mutants (stt4, mss4, and pik1). This indicated that the plasma membrane pool of PI4P, but not phosphatidylinositol 4,5-bisphosphate or the Golgi pool of PI4P, is required for localization of Cla4 to sites of polarized growth. A combination of the Cdc42-binding and PH domains of Cla4 was necessary and sufficient for localization to sites of polarized growth. Point mutations affecting either domain impaired the ability of Cla4 to regulate cell morphogenesis and the mitotic exit network (localization of Lte1). Therefore, Cla4 must retain the ability to bind both Cdc42 and phosphoinositides, the hallmark of a coincidence detector. PI4P may recruit Cla4 to the plasma membrane where Cdc42 activates its kinase activity and refines its localization to cortical sites of polarized growth. In mammalian cells, the myotonic dystrophy-related Cdc42-binding kinase possesses p21-binding and PH domains, suggesting that this kinase may be a coincidence detector of signaling by Cdc42 and phosphoinositides.