Catalysis of guanine nucleotide exchange of Ran by RCC1 and stimulation of hydrolysis of Ran-bound GTP by Ran-GAP1

Systematic pan-cancer analysis on the expression and role of regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene 12

Regulator of chromatin condensation 1 (RCC1) is the major guanine nucleotide exchange factor of RAN GTPase, which plays a key role in various biological processes such as cell cycle and DNA damage repair. Small nucleolar RNA host gene 3 (SNHG3) and small nucleolar RNA host gene12 are long-stranded non-coding RNAs (lncRNAs) and are located on chromatin very close to the sequence of Regulator of chromatin condensation 1. Many studies have shown that they are aberrantly expressed in tumor tissues and can affect the proliferation and viability of cancer cells. Although the effects of Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 on cellular activity have been reported, respectively, their overall analysis on the pan-cancer level has not been performed. Here, we performed a comprehensive analysis of Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 in 33 cancers through the Cancer Genome Atlas and Gene Expression Database. The results showed that Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 were highly expressed in a variety of tumor tissues compared to normal tissues. The expression of Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 in BRCA, LGG and LIHC was associated with TP53 mutations. In addition, Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 expression was closely associated with the prognosis of patients with multiple tumors. Immunocorrelation analysis indicated that Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 showed a correlation with multiple immune cell infiltration. The results of enrichment analysis suggested that Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 was involved in the regulation of cell cycle, apoptosis and other pathways. We found that these effects were mainly mediated by Regulator of chromatin condensation 1, while the trend of small nucleolar RNA host gene 3/small nucleolar RNA host gene12 regulation was also consistent with regulator of chromatin condensation 1. The important role played by Regulator of chromatin condensation 1 in tumor diseases was further corroborated by the study of adjacent lncRNAs.These findings provide new and comprehensive insights into the role of Regulator of chromatin condensation 1/small nucleolar RNA host gene 3/small nucleolar RNA host gene12 in tumor development and show their potential as clinical monitoring and therapy.

Clinical Manifestations and Pathogenesis of Acute Necrotizing Encephalopathy: The Interface Between Systemic Infection and Neurologic Injury

Acute necrotizing encephalopathy (ANE) is a devastating neurologic condition that can arise following a variety of systemic infections, including influenza and SARS-CoV-2. Affected individuals typically present with rapid changes in consciousness, focal neurological deficits, and seizures. Neuroimaging reveals symmetric, bilateral deep gray matter lesions, often involving the thalami, with evidence of necrosis and/or hemorrhage. The clinical and radiologic picture must be distinguished from direct infection of the central nervous system by some viruses, and from metabolic and mitochondrial disorders. Outcomes following ANE are poor overall and worse in those with brainstem involvement. Specific management is often directed toward modulating immune responses given the potential role of systemic inflammation and cytokine storm in potentiating neurologic injury in ANE, though benefits of such approaches remain unclear. The finding that many patients have mutations in the nucleoporin gene RANBP2, which encodes a multifunctional protein that plays a key role in nucleocytoplasmic transport, may allow for the development of disease models that provide insights into pathogenic mechanisms and novel therapeutic approaches.

Overexpression of a small GTP-binding protein Ran1 in Arabidopsis leads to promoted elongation growth and enhanced disease resistance against P. syringae DC3000

Plants resist infection through an innate immune response, which is usually associated with slowing of growth. The molecular mechanisms underlying the trade-off between plant growth and defense remain unclear. The present study reveals that growth/defense trade-offs mediated by gibberellin (GA) and salicylic acid (SA) signaling pathways are uncoupled during constitutive overexpression of transgenic AtRAN1 and AtRAN1_Q72L (active, GTP-locked form) Arabidopsis plants. It is well known that the small GTP-binding protein Ran (a Ras-related nuclear protein) functions in the nucleus-cytoplasmic transport of proteins. Although there is considerable evidence indicating that nuclear-cytoplasmic partitioning of specific proteins can participate in hormone signaling, the role of Ran-dependent nuclear transport in hormone signaling is not yet fully understood. In this report, we used a combination of genetic and molecular methods to reveal whether AtRAN1 is involved in both GA and SA signaling pathways. Constitutively overexpressed AtRAN1 promoted both elongation growth and the disease resistance response, whereas overexpression of AtRAN1_Q72L in the atran2atran3 double mutant background clearly inhibited elongation growth and the defense response. Furthermore, we found that AtRAN1 coordinated plant growth and defense by promoting the stability of the DELLA protein RGA in the nucleus and by modulating NPR1 nuclear localization. Interestingly, genetically modified rice (Oryza sativa) overexpressing AtRAN1 exhibited increased plant height and yield per plant. Altogether, the ability to achieve growth/defense trade-offs through AtRAN1 overexpression provides an approach to maximizing crop yield to meet rising global food demands.

Integrative and Comprehensive Pancancer Analysis of Regulator of Chromatin Condensation 1 (RCC1)

Regulator of Chromatin Condensation 1 (RCC1) is the only known guanine nucleotide exchange factor that acts on the Ras-like G protein Ran and plays a key role in cell cycle regulation. Although there is growing evidence to support the relationship between RCC1 and cancer, detailed pancancer analyses have not yet been performed. In this genome database study, based on The Cancer Genome Atlas, Genotype-Tissue Expression and Gene Expression Omnibus databases, the potential role of RCC1 in 33 tumors' entities was explored. The results show that RCC1 is highly expressed in most human malignant neoplasms in contrast to healthy tissues. RCC1 expression is closely related to the prognosis of a broad variety of tumor patients. Enrichment analysis showed that some tumor-related pathways such as "cell cycle" and "RNA transport" were involved in the functional mechanism of RCC1. In particular, the conducted analysis reveals the relation of RCC1 to multiple immune checkpoint genes and suggests that the regulation of RCC1 is closely related to tumor infiltration of cancer-associated fibroblasts and CD8⁺ T cells. Coherent data demonstrate the association of RCC1 with the tumor mutation burden and microsatellite instability in various tumors. These findings provide new insights into the role of RCC1 in oncogenesis and tumor immunology in various tumors and indicate its potential as marker for therapy prognosis and targeted treatment strategies.

The Multifaceted Roles of RCC1 in Tumorigenesis

RCC1 (regulator of chromosome condensation 1) is the only known guanine nucleotide exchange factor of Ran, a nuclear Ras-like G protein. RCC1 combines with chromatin and Ran to establish a concentration gradient of RanGTP, thereby participating in a series of cell physiological activities. In this review, we discuss the structure of RCC1 and describe how RCC1 affects the formation and function of the nuclear envelope, spindle formation, and nuclear transport. We mainly focus on the effect of RCC1 on the cell cycle during tumorigenesis and the recent research progress that has been made in relation to different tumor types.

RanBP1 Couples Nuclear Export and Golgi Regulation through LKB1 to Promote Cortical Neuron Polarity

Neuronal polarity in the developing cortex begins during the early stages of neural progenitor migration toward the cortical plate and culminates with the specification of the axon and dendrites. Here, we demonstrate that the Ran-dependent nucleocytoplasmic transport machinery is essential for the establishment of cortical neuron polarity. We found that Ran-binding protein 1 (RanBP1) regulates axon specification and dendritic arborization in cultured neurons in vitro and radial neural migration in vivo. During axonogenesis, RanBP1 regulates the cytoplasmic levels of the polarity protein LKB1/Par4, and this is dependent on the nuclear export machinery. Our results show that downstream of RanBP1, LKB1 function is mediated by the STK25-GM130 pathway, which promotes axonogenesis through Golgi regulation. Our results indicate that the nucleocytoplasmic transport machinery is a main regulator of neuron polarity, including radial migration, and that the regulated export of LKB1 through RanBP1 is a limiting step of axonogenesis.

Samp1, a RanGTP binding transmembrane protein in the inner nuclear membrane

Samp1 is a transmembrane protein of the inner nuclear membrane (INM), which interacts with the nuclear lamina and the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex in interphase and during mitosis, it localizes to the mitotic spindle. Samp1 was recently found to coprecipitate a protein complex containing Ran, a GTPase with fundamental regulatory functions both in interphase and in mitosis. To investigate the interaction between Samp1 and Ran in further detail, we have designed and expressed recombinant fusion proteins of the Chaetomium thermophilum homolog of Samp1 (Ct.Samp1) and human Ran. Pulldown experiments show that Samp1 binds directly to Ran and that Samp1 binds better to RanGTP compared to RanGDP. Samp1 also preferred RanGTP over RanGDP in living tsBN2 cells. We also show that the Ran binding domain is located between amino acids 75–135 in the nucleoplasmically exposed N-terminal tail of Samp1. This domain is unique for Samp1, without homology in any other proteins in fungi or metazoa. Samp1 is the first known transmembrane protein that binds to Ran and could provide a unique local binding site for RanGTP in the INM. Samp1 overexpression resulted in increased Ran concentrations in the nuclear periphery supporting this idea.

Extensive cargo identification reveals distinct biological roles of the 12 importin pathways

Vast numbers of proteins are transported into and out of the nuclei by approximately 20 species of importin-β family nucleocytoplasmic transport receptors. However, the significance of the multiple parallel transport pathways that the receptors constitute is poorly understood because only limited numbers of cargo proteins have been reported. Here, we identified cargo proteins specific to the 12 species of human import receptors with a high-throughput method that employs stable isotope labeling with amino acids in cell culture, an in vitro reconstituted transport system, and quantitative mass spectrometry. The identified cargoes illuminated the manner of cargo allocation to the receptors. The redundancies of the receptors vary widely depending on the cargo protein. Cargoes of the same receptor are functionally related to one another, and the predominant protein groups in the cargo cohorts differ among the receptors. Thus, the receptors are linked to distinct biological processes by the nature of their cargoes.

Importins and exportins as therapeutic targets in cancer

The nuclear transport proteins, importins and exportins (karyopherin-β proteins), may play an important role in cancer by transporting key mediators of oncogenesis across the nuclear membrane in cancer cells. During nucleocytoplasmic transport of tumor suppressor proteins and cell cycle regulators during the processing of these proteins, aberrant cellular growth signaling and inactivation of apoptosis can occur, both critical to growth and development of tumors. Karyopherin-β proteins bind to these cargo proteins and RanGTP for active transport across the nuclear membrane through the nuclear pore complex. Importins and exportins are overexpressed in multiple tumors including melanoma, pancreatic, breast, colon, gastric, prostate, esophageal, lung cancer, and lymphomas. Furthermore, some of the karyopherin-β proteins such as exportin-1 have been implicated in drug resistance in cancer. Importin and exportin inhibitors are being considered as therapeutic targets against cancer and have shown preclinical anticancer activity. Moreover, synergistic activity has been observed with various chemotherapeutic and targeted agents. However, clinical development of the exportin-1 inhibitor leptomycin B was stopped due to adverse events, including vomiting, anorexia, and dehydration. Selinexor, a selective nuclear export inhibitor, is being tested in multiple clinical trials both as a single agent and in combination with chemotherapy. Selinexor has demonstrated clinical activity in multiple cancers, especially acute myelogenous leukemia and multiple myeloma. The roles of other importin and exportin inhibitors still need to be investigated clinically. Targeting the key mediators of nucleocytoplasmic transport in cancer cells represents a novel strategy in cancer intervention with the potential to significantly affect outcomes.

A Homogenous Bioluminescent System for Measuring GTPase, GTPase Activating Protein, and Guanine Nucleotide Exchange Factor Activities

GTPases play a major role in various cellular functions such as cell signaling, cell proliferation, cell differentiation, cytoskeleton modulation, and cell motility. Deregulation or mutation of these proteins has considerable consequences resulting in multiple pathological conditions. Targeting GTPases and its regulators has been challenging due to paucity of convenient assays. In this study, we describe a homogenous bioluminescent assay for monitoring the activities of GTPase and its immediate regulators: GTPase activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). Since Mg²⁺ plays a critical role in influencing the affinity of GTPases with guanosine triphosphate/guanosine diphosphate (GTP/GDP) and the process of nucleotide exchange, manipulating Mg²⁺ concentrations in the GTPase reaction buffer allows continuous progression of the GTPase cycle and faster hydrolysis of GTP. The assay relies on enzymatic conversion of GTP that remains after the GTPase reaction to ATP and detection of the generated ATP using the luciferin/luciferase combination. The GTPase/GAP/GEF-Glo assay system enables monitoring of GTPase, GAP-stimulated GTPase, GAP, and GEF activities. The system can also be used to analyze these proteins when expressed in cells as fusion proteins by performing the assay in a pulldown format. The assays showed minimal false hits upon testing for compound interference using the library of pharmacologically active compounds and its robustness was demonstrated by a high Z′-factor of 0.93 and CV of 2.2%. The assay system has a high dynamic range, formatted in a convenient add–mix–read, and applicable to high-throughput screening.

Identification of cargo proteins specific for importin-β with importin-α applying a stable isotope labeling by amino acids in cell culture (SILAC)-based in vitro transport system

The human importin (Imp)-β family consists of 21 nucleocytoplasmic transport carrier proteins, which transport thousands of proteins (cargoes) across the nuclear envelope through nuclear pores in specific directions. To understand the nucleocytoplasmic transport in a physiological context, the specificity of cargoes for their cognate carriers should be determined; however, only a limited number of nuclear proteins have been linked to specific carriers. To address this biological question, we recently developed a novel method to identify carrier-specific cargoes. This method includes the following three steps: (i) the cells are labeled by stable isotope labeling by amino acids in cell culture (SILAC); (ii) the labeled cells are permeabilized, and proteins in the unlabeled cell extracts are transported into the nuclei of the permeabilized cells by a particular carrier; and (iii) the proteins in the nuclei are quantitatively identified by LC-MS/MS. The effectiveness of this method was demonstrated by the identification of transportin (Trn)-specific cargoes. Here, we applied this method to identify cargo proteins specific for Imp-β, which is a predominant carrier that exclusively utilizes Imp-α as an adapter for cargo binding. We identified candidate cargoes, which included previously reported and potentially novel Imp-β cargoes. In in vitro binding assays, most of the candidate cargoes bound to Imp-β in one of three binding modes: directly, via Imp-α, or via other cargoes. Thus, our method is effective for identifying a variety of Imp-β cargoes. The identified Imp-β and Trn cargoes were compared, ensuring the carrier specificity of the method and illustrating the complexity of these transport pathways.

Identification of cargo proteins specific for the nucleocytoplasmic transport carrier transportin by combination of an in vitro transport system and stable isotope labeling by amino acids in cell culture (SILAC)-based quantitative proteomics

The human importin-β family consists of 21 nucleocytoplasmic transport carrier proteins that carry proteins and RNAs across the nuclear envelope through nuclear pores in specific directions. These transport carriers are responsible for the nucleocytoplasmic transport of thousands of proteins, but the cargo allocation of each carrier, which is necessary information if one wishes to understand the physiological context of transport, is poorly characterized. To address this issue, we developed a high-throughput method to identify the cargoes of transport carriers by applying stable isotope labeling by amino acids in cell culture to construct an in vitro transport system. Our method can be outlined in three steps. (1) Cells are cultured in a medium containing a stable isotope. (2) The cell membranes of the labeled cells are permeabilized, and proteins extracted from unlabeled cells are transported into the nuclei of the permeabilized cells. In this step, the reaction system is first depleted of all importin-β family carriers and then supplemented with a particular importin-β family carrier of interest. (3) Proteins in the nuclei are extracted and analyzed quantitatively via LC-MS/MS. As an important test case, we used this method to identify cargo proteins of transportin, a representative member of the importin-β family. As expected, the identified candidate cargo proteins included previously reported transportin cargoes as well as new potential cargoes, which we corroborated via in vitro binding assays. The identified cargoes are predominately RNA-interacting proteins, affirming that cargoes allotted to the same carrier share functional characteristics. Finally, we found that the transportin cargoes possessed at least two classes of signal sequences: the well characterized PY-nuclear localization signals specific for transportin, and Lys/Arg-rich segments capable of binding to both transportin and importin-β. Thus, our method will be useful for linking a carrier to features shared among its cargoes and to specific nuclear localization signals.

Ran and calcineurin can participate collaboratively in the regulation of spermatogenesis in scallop

Calcineurin is a calcium/calmodulin-dependent protein phosphatase that plays important roles in the transduction of calcium signals in a variety of tissues. In addition, calcineurin has been implicated in the process of spermatogenesis. A novel calcineurin-binding protein, CaNBP75, has been identified in scallop testis. The C-terminal region of CaNBP75 is homologous to the C-terminal region of RanBP3, a Ran-binding domain-containing protein. A small G protein Ran has been involved in spermiogenesis by virtue of the fact that its localization in spermatids changes during spermiogenesis. The current study was performed to investigate the functions of Ran and CaNBP75 in the regulation of calcineurin in testis to further understand the basic functions of calcineurin during spermatogenesis. First, cloning and sequencing of a scallop Ran cDNA isolated from testis revealed that scallop Ran is well-conserved at the amino acid level. Secondly, direct binding of Ran to CaNBP75 was demonstrated in an in vitro pull-down assay. Thirdly, analysis of the tissue distribution of Ran, CaNBP75, and calcineurin showed that these proteins are abundantly expressed in testis. Fourthly, comparison of the expression profiles of Ran and CaNBP75 with that of calcineurin in scallop testis during the maturation cycle revealed that Ran and CaNBP75 mRNA levels increase during meiosis and spermiogenesis, similar to calcineurin. Finally, co-immunoprecipitation analysis suggests that Ran, CaNBP75, and calcineurin interact in scallop testis during maturation. These results suggest that Ran, CaNBP75, and calcineurin may act in a coordinated manner to regulate spermatogenesis.

Nuclear import by karyopherin-βs: recognition and inhibition

Proteins in the karyopherin-β family mediate the majority of macromolecular transport between the nucleus and the cytoplasm. Eleven of the 19 known human karyopherin-βs and 10 of the 14S. cerevisiae karyopherin-βs mediate nuclear import through recognition of nuclear localization signals or NLSs in their cargos. This receptor-mediated process is essential to cellular viability as proteins are translated in the cytoplasm but many have functional roles in the nucleus. Many known karyopherin-β-cargo interactions were discovered through studies of the individual cargos rather than the karyopherins, and this information is thus widely scattered in the literature. We consolidate information about cargos that are directly recognized by import-karyopherin-βs and review common characteristics or lack thereof among cargos of different import pathways. Knowledge of karyopherin-β-cargo interactions is also critical for the development of nuclear import inhibitors and the understanding of their mechanisms of inhibition. This article is part of a Special Issue entitled: Regulation of Signaling and Cellular Fate through Modulation of Nuclear Protein Import.

Ran on tracks – cytoplasmic roles for a nuclear regulator

The GTPase Ran is best known for its crucial roles in the regulation of nucleocytoplasmic transport in interphase cells and in the organization of the spindle apparatus during mitosis. A flurry of recent reports has now implicated Ran in diverse cytoplasmic events, including trafficking of an ephrin receptor homolog in nematode oocytes, control of neurite outgrowth in Drosophila and mammalian neurons, and retrograde signaling in nerve axons after injury. Striking findings suggest that the guanine-nucleotide state of Ran can be regulated by local translation of the Ran-binding protein RanBP1 in axons, and that an additional Ran-binding protein, RanBP10, can act as a microtubule-binding cytoplasmic guanine-nucleotide exchange factor for Ran (RanGEF) in megakaryocytes. Thus, the Ran GTPase system can act as a spatial regulator of importin-dependent transport and signaling in distal cytoplasm, and as a regulator of cytoskeletal dynamics at sites that are distant from the nucleus.

Apoptotic histone modification inhibits nuclear transport by regulating RCC1

A number of signalling pathways have been identified that regulate apoptosis, but the mechanism that initiates apoptosis remains incompletely understood. We have found that the nuclear RanGTP level is diminished during the early stages of apoptosis, which correlates with immobilization of RCC1 on the chromosomes. Furthermore, the expression of phosphomimetic histone H2B or caspase-activated Mst1 immobilizes RCC1 and causes reduction of nuclear RanGTP levels, which leads to inactivation of the nuclear transport machinery. As a consequence, nuclear localization signal (NLS)-containing proteins, including NF-kappaB-p65, remain bound to importins alpha and beta in the cytoplasm. Knocking down Mst1 allows resumption of nuclear transport and the nuclear entry of NF-kappaB-p65, which have important roles in rescuing cells from apoptosis. Therefore, we propose that RCC1 reads the histone code created by caspase-activated Mst1 to initiate apoptosis by reducing the level of RanGTP in the nucleus.

Nuclear export receptor Xpo1/Crm1 is physically and functionally linked to the spindle pole body in budding yeast

The spindle pole body (SPB) represents the microtubule organizing center in the budding yeast Saccharomyces cerevisiae. It is a highly structured organelle embedded in the nuclear membrane, which is required to anchor microtubules on both sides of the nuclear envelope. The protein Spc72, a component of the SPB, is located at the cytoplasmic face of this organelle and serves as a receptor for the gamma-tubulin complex. In this paper we show that it is also a binding partner of the nuclear export receptor Xpo1/Crm1. Xpo1 binds its cargoes in a Ran-dependent fashion via a short leucine-rich nuclear export signal (NES). We show that binding of Spc72 to Xpo1 depends on Ran-GTP and a functional NES in Spc72. Mutations in this NES have severe consequences for mitotic spindle morphology in vivo. This is also the case for xpo1 mutants, which show a reduction in cytoplasmic microtubules. In addition, we find a subpopulation of Xpo1 localized at the SPB. Based on these data, we propose a functional link between Xpo1 and the SPB and discuss a role for this exportin in spindle biogenesis in budding yeast.

Intracellular bacterial growth is controlled by a kinase network around PKB/AKT1

With the emergence of multidrug resistant (MDR) bacteria, it is imperative to develop new intervention strategies. Current antibiotics typically target pathogen rather than host-specific biochemical pathways. Here we have developed kinase inhibitors that prevent intracellular growth of unrelated pathogens such as Salmonella typhimurium and Mycobacterium tuberculosis. An RNA interference screen of the human kinome using automated microscopy revealed several host kinases capable of inhibiting intracellular growth of S. typhimurium. The kinases identified clustered in one network around AKT1 (also known as PKB). Inhibitors of AKT1 prevent intracellular growth of various bacteria including MDR-M. tuberculosis. AKT1 is activated by the S. typhimurium effector SopB, which promotes intracellular survival by controlling actin dynamics through PAK4, and phagosome-lysosome fusion through the AS160 (also known as TBC1D4)-RAB14 pathway. AKT1 inhibitors counteract the bacterial manipulation of host signalling processes, thus controlling intracellular growth of bacteria. By using a reciprocal chemical genetics approach, we identified kinase inhibitors with antibiotic properties and their host targets, and we determined host signalling networks that are activated by intracellular bacteria for survival.

Significant proportions of nuclear transport proteins with reduced intracellular mobilities resolved by fluorescence correlation spectroscopy

Nuclear transport requires freely diffusing nuclear transport proteins to facilitate movement of cargo molecules through the nuclear pore. We analyzed dynamic properties of importin alpha, importin beta, Ran and NTF2 in nucleus, cytoplasm and at the nuclear pore of neuroblastoma cells using fluorescence correlation spectroscopy. Mobile components were quantified by global fitting of autocorrelation data from multiple cells. Immobile components were quantified by analysis of photobleaching kinetics. Wild-type Ran was compared to various mutant Ran proteins to identify components representing GTP or GDP forms of Ran. Untreated cells were compared to cells treated with nocodazole or latrunculin to identify components associated with cytoskeletal elements. The results indicate that freely diffusing importin alpha, importin beta, Ran and NTF2 are in dynamic equilibrium with larger pools associated with immobile binding partners such as microtubules in the cytoplasm. These findings suggest that formation of freely diffusing nuclear transport intermediates is in competition with binding to immobile partners. Variation in concentrations of freely diffusing nuclear transport intermediates among cells indicates that the nuclear transport system is sufficiently robust to function over a wide range of conditions.

Purification of RanGDP, RanGTP, and RanGMPPNP by ion exchange chromatography

Ran is a small GTPase that cycles between a guanosine diphosphate (GDP)-bound form (RanGDP) and a guanosine triphosphate (GTP)-bound form (RanGTP) and plays important roles in nuclear transport and mitosis. For studies of Ran function and its interactions with partner proteins, pure RanGDP and RanGTP complexes are critical. Ran complexed with the nonhydrolyzable GTP analog, GMPPNP (RanGMPPNP), is used instead of RanGTP when inhibition of hydrolysis is required. In this study, we demonstrate that the binding of Ran to a UNO Q ion exchange column is remarkably sensitive to small shifts in MgCl(2) concentration, and we use this property to purify recombinant RanGTP, RanGMPPNP, and RanGDP complexes. At 10 mM MgCl(2), Ran was found predominantly in the flow-through and, thus, was separated from the vast majority of bacterial proteins. After reducing the concentration of MgCl(2) to 5 mM, further purification of RanGTP, RanGMPPNP, and RanGDP was achieved by loading onto ion exchange columns and elution with an NaCl gradient. Purity of the resulting preparations was confirmed by releasing the bound nucleotide and checking it against a known nucleotide by high-performance liquid chromatography (HPLC). To further confirm the purity and function of the Ran preparations, appropriate protein-binding, enzymatic, and nuclear import assays were carried out. These methods should facilitate studies of cellular processes involving Ran by providing pure functional Ran-nucleotide complexes.

Identification and characterization of a functional nuclear localization signal in the HIV-1 integrase interactor LEDGF/p75

Human lens epithelium-derived growth factor (LEDGF)/p75 protein forms a specific nuclear complex with human immunodeficiency virus type 1 (HIV-1) integrase and is essential for nuclear localization and chromosomal association of the viral protein. We now studied nuclear import of LEDGF/p75 in live and semipermeabilized cells. We show that nuclear import of LEDGF/p75 is GTP-, Ran-, importin-alpha/beta-, and energy-dependent and that the protein competes with the canonical SV40 large T antigen nuclear localization signal (NLS) for nuclear import receptors. We identified the NLS of LEDGF/p75 through deletion analysis and site-directed mutagenesis. The LEDGF/p75 NLS, 148GRKRKAEKQ156, belongs to the canonical SV40-like family. Fusion of this short peptide to the amino terminus of Escherichia coli beta-galactosidase rendered the fusion protein nuclear, confirming that the LEDGF/p75 NLS is transferable. Moreover, a single amino acid change in the NLS was sufficient to exclude the mutant LEDGF/p75 protein from the nucleus and abolish nuclear import of HIV-1 integrase.

CRM1 and Ran are present but a NES-CRM1-RanGTP complex is not required in Balbiani ring mRNP particles from the gene to the cytoplasm

Messenger RNA is formed from precursors known as pre-mRNA. These precursors associate with proteins to form pre-mRNA-protein (pre-mRNP) complexes. Processing machines cap, splice and polyadenylate the pre-mRNP and in this way build the mRNP. These processing machines also affect the export of the mRNP complexes from the nucleus to the cytoplasm. Export to the cytoplasm takes place through a structure in the nuclear membrane called the nuclear pore complex (NPC). Export involves adapter proteins in the mRNP and receptor proteins that bind to the adapter proteins and to components of the NPC. We show that the export receptor chromosomal region maintenance protein 1 (CRM1), belonging to a family of proteins known as importin-beta-like proteins, binds to gene-specific Balbiani ring (BR) pre-mRNP while transcription takes place. We also show that the GTPase known as Ran binds to BR pre-mRNP, and that it binds mainly in the interchromatin. However, we also show using leptomycin B treatment that a NES-CRM1-RanGTP complex is not essential for export, even though both CRM1 and Ran accompany the BR mRNP through the NPC. Our results therefore suggest that several export receptors associate with BR mRNP and that these receptors have redundant functions in the nuclear export of BR mRNP.

A novel angiotensin II type 1 receptor-associated protein induces cellular hypertrophy in rat vascular smooth muscle and renal proximal tubular cells

Angiotensin II stimulates cellular hypertrophy in cultured vascular smooth muscle and renal proximal tubular cells. This effect is believed to be one of earliest morphological changes of heart and renal failure. However, the precise molecular mechanism involved in angiotensin II-induced hypertrophy is poorly understood. In the present study we report the isolation of a novel angiotensin II type 1 receptor-associated protein. It encodes a 531-amino acid protein. Its mRNA is detected in all human tissues examined but highly expressed in the human kidney, pancreas, heart, and human embryonic kidney cells as well as rat vascular smooth muscle and renal proximal tubular cells. Protein synthesis and relative cell size analyzed by flow cytometry studies indicate that overexpression of the novel angiotensin II type 1 receptor-associated protein induces cellular hypertrophy in cultured rat vascular smooth muscle and renal proximal tubular cells. In contrast, the hypertrophic effects was reversed in renal proximal tubular cell lines expressing the novel gene in the antisense orientation and its dominant negative mutant, which lacks the last 101 amino acids in its carboxyl-terminal tail. The hypertrophic effects are at least in part mediated via protein kinase B activation or cyclin-dependent kinase inhibitor, p27(kip1) protein expression level in vascular smooth muscle, and renal proximal tubular cells. Moreover, angiotensin II could not stimulate cellular hypertrophy in renal proximal tubular cells expressing the novel gene in the antisense orientation and its mutant. These findings may provide new molecular mechanisms to understand hypertrophic agents such as angiotensin II-induced cellular hypertrophy.

Optical single transporter recording: transport kinetics in microarrays of membrane patches

Optical single transporter recording (OSTR) is an emerging technique for the fluorescence microscopic measurement of transport kinetics in membrane patches. Membranes are attached to transparent microarrays of cylindrical test compartments (TCs) approximately 0.1-100 mum in diameter and approximately 10-100 mum in depth. Transport across membrane patches that may contain single transporters or transporter populations is recorded by confocal microscopy. By these means transport of proteins through single nuclear pore complexes has been recorded at rates of <1 translocation/s. In addition to the high sensitivity in terms of measurable transport rates OSTR features unprecedented spatial selectivity and parallel processing. This article reviews the conceptual basis of OSTR and its realization. Applications to nuclear transport are summarized. The further development of OSTR is discussed and its extension to a diversity of transporters, including translocases and ATP-binding cassette (ABC) pumps, projected.

A mechanism of coupling RCC1 mobility to RanGTP production on the chromatin in vivo

The RanGTP gradient across the interphase nuclear envelope and on the condensed mitotic chromosomes is essential for many cellular processes, including nucleocytoplasmic transport and spindle assembly. Although the chromosome-associated enzyme RCC1 is responsible for RanGTP production, the mechanism of generating and maintaining the RanGTP gradient in vivo remains unknown. Here, we report that regulator of chromosome condensation (RCC1) rapidly associates and dissociates with both interphase and mitotic chromosomes in living cells, and that this mobility is regulated during the cell cycle. Our kinetic modeling suggests that RCC1 couples its catalytic activity to chromosome binding to generate a RanGTP gradient. Indeed, we have demonstrated experimentally that the interaction of RCC1 with the chromatin is coupled to the nucleotide exchange on Ran in vivo. The coupling is due to the stable binding of the binary complex of RCC1-Ran to chromatin. Successful nucleotide exchange dissociates the binary complex, permitting the release of RCC1 and RanGTP from the chromatin and the production of RanGTP on the chromatin surface.

The strategy for coupling the RanGTP gradient to nuclear protein export

The Ran GTPase plays critical roles in both providing energy for and determining the directionality of nucleocytoplasmic transport. The mechanism that couples the RanGTP gradient to nuclear protein export will determine the rate of and limits to accumulation of export cargoes in the cytoplasm, but is presently unknown. We reasoned that plausible coupling mechanisms could be distinguished by comparing the rates of reverse motion of export cargoes through the nuclear pore complex (NPC) with the predictions of a mathematical model. Measurement of reverse export rates in Xenopus oocytes revealed that nuclear export signals can facilitate RanGTP-dependent cargo movement into the nucleus against the RanGTP gradient at rates comparable to export rates. Although export cargoes with high affinity for their receptor are exported faster than those with low affinity, their reverse transport is also greater. The ratio of the rates of reverse and forward export of a cargo is proportional to its rate of diffusion through the NPC, i.e., to the ability of the cargo to penetrate the NPC permeability barrier. The data substantiate a diffusional mechanism of coupling and suggest the existence of a high concentration of RanGTP-receptor complexes within the NPC that decreases sharply at the cytoplasmic boundary of the NPC permeability barrier.

A synthetic HIV-1 Rev inhibitor interfering with the CRM1-mediated nuclear export

The HIV-1 Rev protein is an essential regulator of the HIV-1 mRNA expression that promotes the export of unspliced and partially spliced mRNA. The export receptor for the leucine-rich nuclear export signal (NES) of Rev has recently been recognized as CRM1. We identified a low molecular weight compound PKF050-638 as an inhibitor of HIV-1 Rev. This drug inhibits in a dose-dependent fashion Rev-dependent mRNA expression in a cellular assay for Rev function. We show that PKF050-638 is an inhibitor of the CRM1-mediated Rev nuclear export. By using a quantitative in vitro CRM1-NES cargo-binding assay, we could demonstrate that PKF050-638 disrupts CRM1-NES interaction. This mode of action is confirmed in cell culture because the drug reversibly interferes with the colocalization of CRM1 and Rev in the nucleolus of the cell. In addition, we prove that the inhibition is through direct interaction of the compound with Cys-539 of CRM1. These effects are similar to those of the known CRM1 inhibitor leptomycin B and suggest that the inhibitory effect of the compound is caused by binding to CRM1 at a similar site. The compound displayed strict structural requirements for its activity, as its enantiomer was inactive in all assays tested. These results show that we identified a drug that interferes with the CRM1-mediated nuclear export of Rev through inhibition of the CRM1-NES complex formation. The reversibility of its binding to CRM1 and its availability through chemical synthesis could make it useful for studying CRM1-mediated export pathways.

Nuclear transport kinetics in microarrays of nuclear envelope patches

Optical Single Transporter Recording (OSTR) is a technique for analyzing membrane transport kinetics at high sensitivity, selectivity, and spatial resolution. Cellular membranes are firmly attached to microarrays of small test compartments (TCs) with diameters between approximately 0.1 and 100 microm and depths between approximately 10 and 100 microm. This permits to generate either "small" membrane patches containing few transporters or "large" patches containing many transporters. Transport of substrates across membrane patches is recorded by confocal microscopy. The present article reviews recent applications of OSTR to the nuclear pore complex (NPC). The results show that the transport functions of the NPC, previously studied almost exclusively in intact and permeabilized cells, are conserved in isolated nuclei and can be fully reconstituted in purified nuclear envelopes by addition of recombinant transport factors. This opens new avenues to the analysis of nuclear transport including the export of nucleic-acid-protein and ribosomal particles.

Reconstitution of nuclear protein export in isolated nuclear envelopes

Signal-dependent nuclear protein export was studied in perforated nuclei and isolated nuclear envelopes of Xenopus oocytes by optical single transporter recording. Manually isolated and purified oocyte nuclei were attached to isoporous filters and made permeable for macromolecules by perforation. Export of a recombinant protein (GG-NES) containing the nuclear export signal (NES) of the protein kinase A inhibitor through nuclear envelope patches spanning filter pores could be induced by the addition of GTP alone. Export continued against a concentration gradient, and was NES dependent and inhibited by leptomycin B and GTPgammaS, a nonhydrolyzable GTP analogue. Addition of recombinant RanBP3, a potential cofactor of CRM1-dependent export, did not promote GG-NES export at stoichiometric concentration but gradually inhibited export at higher concentrations. In isolated filter-attached nuclear envelopes, export of GG-NES was virtually abolished in the presence of GTP alone. However, a preformed export complex consisting of GG-NES, recombinant human CRM1, and RanGTP was rapidly exported. Unexpectedly, export was strongly reduced when the export complex contained RanGTPgammaS or RanG19V/Q69L-GTP, a GTPase-deficient Ran mutant. This paper shows that nuclear transport, previously studied in intact and permeabilized cells only, can be quantitatively analyzed in perforated nuclei and isolated nuclear envelopes.

Importin alpha binds to an unusual bipartite nuclear localization signal in the heterogeneous ribonucleoprotein type I

The heterogeneous nuclear ribonucleoprotein (hnRNP) type I, a modulator of alternative splicing, localizes in the nucleoplasm of mammalian cells and in a discrete perinucleolar structure. HnRNP I contains a novel type of bipartite nuclear localization signal (NLS) at the N-terminus of the protein that we have previously named nuclear determinant localization type I (NLD-I). Recently, a neural counterpart of hnRNP I has been identified that contains a putative NLS with two strings of basic amino acids separated by a spacer of 30 residues. In the present study we show that the neural hnRNP I NLS is necessary and sufficient for nuclear localization and represents a variant of the novel bipartite NLS present in the NLD-I domain. Furthermore, we demonstrate that the NLD-I is transported into the nucleus by cytoplasmic factor(s) with active transport modality. Binding assays using recombinant importin alpha show an interaction with NLD-I similar to that of SV40 large T antigen NLS. Deletion analysis indicates that both stretches of basic residues are necessary for binding to importin alpha. The above experimental results lead to the conclusion that importin alpha acts as cytoplasmic receptor for proteins characterized by a bipartite NLS signal that extends up to 37 residues.

Molecular cloning of a novel importin alpha homologue from rice, by which constitutive photomorphogenic 1 (COP1) nuclear localization signal (NLS)-protein is preferentially nuclear imported

Nuclear import of proteins that contain classical nuclear localization signals (NLS) is initiated by importin alpha, a protein that recognizes and binds to the NLS in the cytoplasm. In this paper, we have cloned a cDNA for a novel importin alpha homologue from rice which is in addition to our previously isolated rice importin alpha1a and alpha2, and we have named it rice importin alpha1b. In vitro binding and nuclear import assays using recombinant importin alpha1b protein demonstrate that rice importin alpha1b functions as a component of the NLS-receptor in plant cells. Analysis of the transcript levels for all three rice importin alpha genes revealed that the genes were not only differentially expressed but that they also responded to dark-adaptation in green leaves. Furthermore, we also show that the COP1 protein bears a bipartite-type NLS and its nuclear import is mediated preferentially by the rice importin alpha1b. These data suggest that each of the different rice importin alpha proteins carry distinct groups of nuclear proteins, such that multiple isoforms of importin alpha contribute to the regulation of plant nuclear protein transport.

Dissection of a nuclear localization signal

The regulated process of protein import into the nucleus of a eukaryotic cell is mediated by specific nuclear localization signals (NLSs) that are recognized by protein import receptors. This study seeks to decipher the energetic details of NLS recognition by the receptor importin alpha through quantitative analysis of variant NLSs. The relative importance of each residue in two monopartite NLS sequences was determined using an alanine scanning approach. These measurements yield an energetic definition of a monopartite NLS sequence where a required lysine residue is followed by two other basic residues in the sequence K(K/R)X(K/R). In addition, the energetic contributions of the second basic cluster in a bipartite NLS ( approximately 3 kcal/mol) as well as the energy of inhibition of the importin alpha importin beta-binding domain ( approximately 3 kcal/mol) were also measured. These data allow the generation of an energetic scale of nuclear localization sequences based on a peptide's affinity for the importin alpha-importin beta complex. On this scale, a functional NLS has a binding constant of approximately 10 nm, whereas a nonfunctional NLS has a 100-fold weaker affinity of 1 microm. Further correlation between the current in vitro data and in vivo function will provide the foundation for a comprehensive quantitative model of protein import.

Ran alters nuclear pore complex conformation

Transport across the nuclear membranes occurs through the nuclear pore complex (NPC), and is mediated by soluble transport factors including Ran, a small GTPase that is generally GDP-bound during import and GTP-bound for export. The dynamic nature of the NPC structure suggests a possible active role for it in driving translocation. Here we show that RanGTP but not RanGDP causes alterations of NPC structure when injected into the cytoplasm of Xenopus oocytes, including compaction of the NPC and extension of the cytoplasmic filaments. RanGTP caused accumulation of nucleoplasmin-gold along the length of extended cytoplasmic filaments, whereas RanGDP caused accumulation around the cytoplasmic rim of the NPC. This suggests a possible role for Ran in altering the conformation of the cytoplasmic filaments during transport.

Yeast Ran-binding protein 1 (Yrb1) shuttles between the nucleus and cytoplasm and is exported from the nucleus via a CRM1 (XPO1)-dependent pathway

The RanGTP-binding protein RanBP1, which is located in the cytoplasm, has been implicated in release of nuclear export complexes from the cytoplasmic side of the nuclear pore complex. Here we show that Yrb1 (the yeast homolog of RanBP1) shuttles between the nucleus and the cytoplasm. Nuclear import of Yrb1 is a facilitated process that requires a short basic sequence within the Ran-binding domain (RBD). By contrast, nuclear export of Yrb1 requires an intact RBD, which forms a ternary complex with the Xpo1 (Crm1) NES receptor in the presence of RanGTP. Nuclear export of Yrb1, however, is insensitive towards leptomycin B, suggesting a novel type of substrate recognition between Yrb1 and Xpo1. Taken together, these data suggest that ongoing nuclear import and export is an important feature of Yrb1 function in vivo.

GTP hydrolysis by Ran is required for nuclear envelope assembly

Nuclear formation in Xenopus egg extracts requires cytosol and is inhibited by GTP gamma S, indicating a requirement for GTPase activity. Nuclear envelope (NE) vesicle fusion is extensively inhibited by GTP gamma S and two mutant forms of the Ran GTPase, Q69L and T24N. Depletion of either Ran or RCC1, the exchange factor for Ran, from the assembly reaction also inhibits this step of NE formation. Ran depletion can be complemented by the addition of Ran loaded with either GTP or GDP but not with GTP gamma S. RCC1 depletion is only complemented by RCC1 itself or by RanGTP. Thus, generation of RanGTP by RCC1 and GTP hydrolysis by Ran are both required for the extensive membrane fusion events that lead to NE formation.

RanGTP-regulated interactions of CRM1 with nucleoporins and a shuttling DEAD-box helicase

CRM1 is an export receptor mediating rapid nuclear exit of proteins and RNAs to the cytoplasm. CRM1 export cargoes include proteins with a leucine-rich nuclear export signal (NES) that bind directly to CRM1 in a trimeric complex with RanGTP. Using a quantitative CRM1-NES cargo binding assay, significant differences in affinity for CRM1 among natural NESs are demonstrated, suggesting that the steady-state nucleocytoplasmic distribution of shuttling proteins could be determined by the relative strengths of their NESs. We also show that a trimeric CRM1-NES-RanGTP complex is disassembled by RanBP1 in the presence of RanGAP, even though RanBP1 itself contains a leucine-rich NES. Selection of CRM1-binding proteins from Xenopus egg extract leads to the identification of an NES-containing DEAD-box helicase, An3, that continuously shuttles between the nucleus and the cytoplasm. In addition, we identify the Xenopus homologue of the nucleoporin CAN/Nup214 as a RanGTP- and NES cargo-specific binding site for CRM1, suggesting that this nucleoporin plays a role in export complex disassembly and/or CRM1 recycling.

Structure of the nuclear transport complex karyopherin-beta2-Ran x GppNHp

Transport factors in the karyopherin-beta (also called importin-beta) family mediate the movement of macromolecules in nuclear-cytoplasmic transport pathways. Karyopherin-beta2 (transportin) binds a cognate import substrate and targets it to the nuclear pore complex. In the nucleus, Ran x GTP binds karyopherin-beta2 and dissociates the substrate. Here we present the 3.0 A structure of the karyopherin-beta2-Ran x GppNHp complex where GppNHp is a non-hydrolysable GTP analogue. Karyopherin-beta2 contains eighteen HEAT repeats arranged into two continuous orthogonal arches. Ran is clamped in the amino-terminal arch and substrate-binding activity is mapped to the carboxy-terminal arch. A large loop in HEAT repeat 7 spans both arches. Interactions of the loop with Ran and the C-terminal arch implicate it in GTPase-mediated dissociation of the import-substrate. Ran x GppNHp in the complex shows extensive structural rearrangement, compared to Ran GDP, in regions contacting karyopherin-beta2. This provides a structural basis for the specificity of the karyopherin-beta family for the GTP-bound state of Ran, as well as a rationale for interactions of the karyopherin-Ran complex with the regulatory proteins ranGAP, ranGEF and ranBP1.

beta-catenin can be transported into the nucleus in a Ran-unassisted manner

The nuclear accumulation of beta-catenin plays an important role in the Wingless/Wnt signaling pathway. This study describes an examination of the nuclear import of beta-catenin in living mammalian cells and in vitro semi-intact cells. When injected into the cell cytoplasm, beta-catenin rapidly migrated into the nucleus in a temperature-dependent and wheat germ agglutinin-sensitive manner. In the cell-free import assay, beta-catenin rapidly migrates into the nucleus without the exogenous addition of cytosol, Ran, or ATP/GTP. Cytoplasmic injection of mutant Ran defective in its GTP hydrolysis did not prevent beta-catenin import. Studies using tsBN2, a temperature-sensitive mutant cell line that possesses a point mutation in the RCC1 gene, showed that the import of beta-catenin is insensitive to nuclear Ran-GTP depletion. These results show that beta-catenin possesses the ability to constitutively translocate through the nuclear pores in a manner similar to importin beta in a Ran-unassisted manner. We further showed that beta-catenin also rapidly exits the nucleus in homokaryons, suggesting that the regulation of nuclear levels of beta-catenin involves both nuclear import and export of this molecule.

A monoclonal antibody to the COOH-terminal acidic portion of Ran inhibits both the recycling of Ran and nuclear protein import in living cells

A small GTPase Ran is a key regulator for active nuclear transport. In immunoblotting analysis, a monoclonal antibody against recombinant human Ran, designated ARAN1, was found to recognize an epitope in the COOH-terminal domain of Ran. In a solution binding assay, ARAN1 recognized Ran when complexed with importin beta, transportin, and CAS, but not the Ran-GTP or the Ran-GDP alone, indicating that the COOH-terminal domain of Ran is exposed via its interaction with importin beta-related proteins. In addition, ARAN1 suppressed the binding of RanBP1 to the Ran-importin beta complex. When injected into the nucleus of BHK cells, ARAN1 was rapidly exported to the cytoplasm, indicating that the Ran-importin beta-related protein complex is exported as a complex from the nucleus to the cytoplasm in living cells. Moreover, ARAN1, when injected into the cultured cells induces the accumulation of endogenous Ran in the cytoplasm and prevents the nuclear import of SV-40 T-antigen nuclear localization signal substrates. From these findings, we propose that the binding of RanBP1 to the Ran-importin beta complex is required for the dissociation of the complex in the cytoplasm and that the released Ran is recycled to the nucleus, which is essential for the nuclear protein transport.

In vitro characterization of rice importin beta1: molecular interaction with nuclear transport factors and mediation of nuclear protein import

We recently isolated two cDNAs encoding importin 3 homologues (rice importin beta1 and beta2), the first such homologues identified in plants. To address the function of rice importin beta1 in the process of nuclear import of proteins, we carried out in vitro binding and nuclear import assays. Recombinant protein of rice importin beta1 assembled a complex (PTAC) with rice importin alpha1 and NLS protein, and also bound to the nuclear envelope of tobacco BY-2 cells. Ran-GTP, but not Ran-GDP, interacted with rice importin beta1 and dissociated the heterodimer formed between rice importin alpha1 and rice importin beta1. An in vitro nuclear import assay using digitonin-permeabilized HeLa cells revealed that rice importin beta1 can mediate nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. These data strongly suggest that rice importin beta1 functions as a component of the NLS receptor in plant cells.

The role of the ran GTPase in nuclear assembly and DNA replication: characterisation of the effects of Ran mutants

The Ran GTPase plays a critical role in nucleocytoplasmic transport and has been implicated in the maintenance of nuclear structure and cell cycle control. Here, we have investigated its role in nuclear assembly and DNA replication using recombinant wild-type and mutant Ran proteins added to a cell-free system of Xenopus egg extracts. RanQ69L and RanT24N prevent lamina assembly, PCNA accumulation and DNA replication. These effects may be due to the disruption of nucleocytoplasmic transport, since both mutants inhibit nuclear import of a protein carrying a nuclear localisation signal (NLS). RanQ69L, which is deficient in GTPase activity, sequesters importins in stable complexes that are unable to support the docking of NLS-proteins at the nuclear pore complex (NPC). RanT24N, in contrast to wild-type Ran-GDP, interacts only weakly with importin alpha and nucleoporins, and not at all with the import factor p10, consistent with its poor activity in nuclear import. However, RanT24N does interact stably with importin beta, Ran binding protein 1 and RCC1, an exchange factor for Ran. We show that Ran-GDP is essential for proper nuclear assembly and DNA replication, the requirement being primarily before the initiation of DNA replication. Ran-GDP therefore mediates the active transport of necessary factors or otherwise controls the onset of S-phase in this system.

Functional characterization of a plant importin alpha homologue. Nuclear localization signal (NLS)-selective binding and mediation of nuclear import of nls proteins in vitro

Nuclear import of most nuclear proteins is initiated by recognition of the nuclear localization signal (NLS) by importin alpha. We recently isolated an importin alpha homologue from rice (rice importin alpha1) and demonstrated that transcription of the gene is down-regulated by light in rice leaves. To address the function of rice importin alpha1 in the process of nuclear import of proteins, we performed in vitro binding and nuclear import assays. The rice importin alpha1 showed specific binding to fusion proteins containing either monopartite or bipartite NLSs, but not to a fusion protein containing a Matalpha-2-type NLS, suggesting that there exists selective binding of rice importin alpha1 to different plant NLSs. The rice importin alpha1 is also capable of forming a complex with mouse importin beta and NLS protein in vitro. An in vitro nuclear import assay using permeabilized HeLa cells revealed that rice importin alpha1, in conjunction with other vertebrate transport factors, mediates the nuclear envelope docking of NLS proteins and their subsequent translocation into the nucleus. These data provide strong, direct evidence suggesting that rice importin alpha1 functions as a component of the NLS receptor in plant cells.

Mediators of nuclear protein import target karyophilic proteins to pore complexes of cytoplasmic annulate lamellae

Nuclear pore complexes are constitutive structures of the nuclear envelope in eukaryotic cells and represent the sites where transport of molecules between nucleus and cytoplasm takes place. However, pore complexes of similar structure, but with largely unknown functional properties, are long known to occur also in certain cytoplasmic cisternae that have been termed annulate lamellae (AL). To analyze the capability of the AL pore complex to interact with the soluble mediators of nuclear protein import and their karyophilic protein substrates, we have performed a microinjection study in stage VI oocytes of Xenopus laevis. In these cells AL are especially abundant and can easily be identified by light and electron microscopy. Following injection into the cytoplasm, fluorochrome-labeled mediators of two different nuclear import pathways, importin beta and transportin, not only associate with the nuclear envelope but also with AL. Likewise, nuclear localization signals (NLS) of the basic and M9 type, but not nuclear export signals, confer targeting and transient binding of fluorochrome-labeled proteins to cytoplasmic AL. Mutation or deletion of the NLS signals prevents these interactions. Furthermore, binding to AL is abolished by dominant negative inhibitors of nuclear protein import. Microinjections of gold-coupled NLS-bearing proteins reveal specific gold decoration at distinct sites within the AL pore complex. These include such at the peripheral pore complex-attached fibrils and at the central "transporter" and closely resemble those of "transport intermediates" found in electron microscopic studies of the nuclear pore complex (NPC). These data demonstrate that AL can represent distinct sites within the cytoplasm of transient accumulation of nuclear proteins and that the AL pore complex shares functional binding properties with the NPC.

RanBP1 is crucial for the release of RanGTP from importin beta-related nuclear transport factors

Nucleocytoplasmic transport appears mediated by shuttling transport receptors that bind RanGTP as a means to regulate interactions with their cargoes. The receptor-RanGTP complexes are kinetically very stable with nucleotide exchange and GTP hydrolysis being blocked, predicting that a specific disassembly mechanism exists. Here we show in three cases receptor RanGTP x RanBP1 complexes to be the key disassembly intermediates, where RanBP1 stimulates the off-rate at the receptor/RanGTP interface by more than two orders of magnitude. The transiently released RanGTP x RanBP1 complex is then induced by RanGAP to hydrolyse GTP, preventing the receptor to rebind RanGTP. The efficient release of importin beta from RanGTP requires importin alpha, in addition to RanBP1.

Transportin-mediated nuclear import of heterogeneous nuclear RNP proteins

Heterogeneous nuclear ribonucleoprotein (hnRNP) A1 is an abundant nuclear protein that plays an important role in pre-mRNA processing and mRNA export from the nucleus. A1 shuttles rapidly between the nucleus and the cytoplasm, and a 38-amino acid domain, M9, serves as the bidirectional transport signal of A1. Recently, a 90-kD protein, transportin, was identified as the mediator of A1 nuclear import. In this study, we show that transportin mediates the nuclear import of additional hnRNP proteins, including hnRNP F. We have also isolated and sequenced a novel transportin homolog, transportin2, which may differ from transportin1 in its substrate specificity. Immunostaining shows that transportin1 is localized both in the cytoplasm and the nucleoplasm, and nuclear rim staining is also observed. The nuclear localization of A1 is dependent on ongoing RNA polymerase II transcription. Interestingly, a pyruvate kinase-M9 fusion, which normally localizes in the nucleus, also accumulates in the cytoplasm when RNA polymerase II is inhibited. Thus, M9 itself is a specific sensor for transcription-dependent nuclear transport. Transportin1-A1 complexes can be isolated from the cytoplasm and the nucleoplasm, but transportin1 is not detectable in hnRNP complexes. RanGTP causes dissociation of A1-transportin1 complexes in vitro. Thus, it is likely that after nuclear import, A1 dissociates from transportin1 by RanGTP and becomes incorporated into hnRNP complexes, where A1 functions in pre-mRNA processing.