An ATP-dependent activity that releases RanGDP from NTF2

Nuclear transport proteins: structure, function, and disease relevance

Proper subcellular localization is crucial for the functioning of biomacromolecules, including proteins and RNAs. Nuclear transport is a fundamental cellular process that regulates the localization of many macromolecules within the nuclear or cytoplasmic compartments. In humans, approximately 60 proteins are involved in nuclear transport, including nucleoporins that form membrane-embedded nuclear pore complexes, karyopherins that transport cargoes through these complexes, and Ran system proteins that ensure directed and rapid transport. Many of these nuclear transport proteins play additional and essential roles in mitosis, biomolecular condensation, and gene transcription. Dysregulation of nuclear transport is linked to major human diseases such as cancer, neurodegenerative diseases, and viral infections. Selinexor (KPT-330), an inhibitor targeting the nuclear export factor XPO1 (also known as CRM1), was approved in 2019 to treat two types of blood cancers, and dozens of clinical trials of are ongoing. This review summarizes approximately three decades of research data in this field but focuses on the structure and function of individual nuclear transport proteins from recent studies, providing a cutting-edge and holistic view on the role of nuclear transport proteins in health and disease. In-depth knowledge of this rapidly evolving field has the potential to bring new insights into fundamental biology, pathogenic mechanisms, and therapeutic approaches.

ARL3, a small GTPase with a functionally conserved role in primary cilia and immune synapses

The primary cilium and the immunological synapse are both specialized functional plasma membrane domains that share several similarities. Signalling output of membrane domains is regulated, spatially and temporally, by segregating and focusing lipids and proteins. ARL3, a small GTPase, plays a major role in concentrating lipid-modified proteins in both the immunological synapse and the primary cilia. Here in this review we will introduce the role of ARL3 in health and disease and its role in polarizing signalling at the primary cilia and immunological synapses.

Reduction of exportin 6 activity leads to actin accumulation via failure of RanGTP restoration and NTF2 sequestration in the nuclei of senescent cells

We have previously reported that G-actin accumulation in nuclei is a universal phenomenon of cellular senescence. By employing primary culture of human diploid fibroblast (HDF) and stress-induced premature senescence (SIPS), we explored whether the failure of actin export to cytoplasm is responsible for actin accumulation in nuclei of senescent cells. Expression of exportin 6 (Exp6) and small G-protein, Ran, was significantly reduced in the replicative senescence, but not yet in SIPS, whereas nuclear import of actin by cofilin was already increased in SIPS. After treatment of young HDF cells with H(2)O(2), rapid reduction of nuclear RanGTP was observed along with cytoplasmic increase of RanGDP. Furthermore, significantly reduced interaction of Exp6 with RanGTP was found by GST-Exp6 pull-down analysis. Failure of RanGTP restoration was accompanied with inhibition of ATP synthesis and NTF2 sequestration in the nuclei along with accordant change of senescence morphology. Indeed, knockdown of Exp6 expression significantly increased actin molecule in the nuclei of young HDF cells. Therefore, actin accumulation in nuclei of senescent cells is most likely due to the failure of RanGTP restoration with ATP deficiency and NTF2 accumulation in nuclei, which result in the decrease of actin export via Exp6 inactivation, in addition to actin import by cofilin activation.

Shiga toxin is transported into the nucleoli of intestinal epithelial cells via a carrier-dependent process

Shiga toxin (Stx) produced by the invasive Shigella dysenteriae serotype 1 (S. dysenteriae1) causes gastrointestinal and kidney complications. It has been assumed that Stx is released intracellularly after enterocyte invasion by S. dysenteriae1. However, there is little information about Stx distribution inside S. dysenteriae1-infected enterocytes. Here, we use intestinal epithelial T84 cells to characterize the trafficking of Stx delivered into the cytosol, in ways that mimic aspects of S. dysenteriae1 infection. We find that cytoplasmic Stx is transported into nucleoli. Stx nucleolar movement is carrier- and energy-dependent. Stx binding to the nucleoli of normal human enterocytes in vitro supports possible roles for nucleolar trafficking in toxin-induced intestinal pathology.

ATP modulates PTEN subcellular localization in multiple cancer cell lines

The tumour suppressor gene PTEN plays an important somatic role in both hereditary and sporadic breast carcinogenesis. While the role of PTEN's lipid phosphatase activity, as a negative regulator of the cytoplasmic phosphatidylinositol-3-kinase/Akt pathway is well known, it is now well established that PTEN exists and functions in the nucleus. Multiple mechanisms of regulating PTEN's subcellular localization have been reported. However none are ubiquitous across multiple cancer cell lines and tissue types. We show here that adenosine triphosphate (ATP) regulates PTEN subcellular localization in a variety of different cancer cell lines, including those derived from breast, colon and thyroid carcinomas. Cells deficient in ATP show an increased level of nuclear PTEN protein. This increase in PTEN is reversed when cells are supplemented with ATP, ADP or AMP. In contrast, the addition of the non-hydrolyzable analogue ATPgammaS, did not reverse nuclear PTEN protein levels in all the cell types tested. To our knowledge, this is the first report that describes a regulation of PTEN subcellular localization that is not specific to one cell line or tissue type, but appears to be common across a variety of cell lineages.

Ran-binding protein 3 phosphorylation links the Ras and PI3-kinase pathways to nucleocytoplasmic transport

The major participants of the Ras/ERK and PI3-kinase (PI3K) pathways are well characterized. The cellular response to activation of these pathways, however, can vary dramatically. How differences in signal strength, timing, spatial location, and cellular context promote specific cell-fate decisions remains unclear. Nuclear transport processes can have a major impact on the determination of cell fate; however, little is known regarding how nuclear transport is regulated by or regulates these pathways. Here we show that RSK and Akt, which are activated downstream of Ras/ERK and PI3K, respectively, modulate the Ran gradient and nuclear transport by interacting with, phosphorylating, and regulating Ran-binding protein 3 (RanBP3) function. Our findings highlight an important link between two major cell-fate determinants: nuclear transport and the Ras/ERK/RSK and PI3K/Akt signaling pathways.

A new role for nuclear transport factor 2 and Ran: nuclear import of CapG

The small GTPase Ran plays a central role in nucleocytoplasmic transport. Nuclear transport of Ran itself depends on nuclear transport factor 2 (NTF2). Here, we report that NTF2 and Ran control nuclear import of the filamentous actin capping protein CapG. In digitonin-permeabilized cells, neither GTPgammaS nor the GTP hydrolysis-deficient Ran mutant RanQ69L affect transit of CapG to the nucleus in the presence of cytosol. Obstruction of nucleoporins prevents nuclear transport of CapG, and we show that CapG binds to nucleoporin62. In addition, CapG interacts with NTF2, associates with Ran and is furthermore able to bind the NTF2-Ran complex. NTF2-Ran interaction is required for CapG nuclear import. This is corroborated by a NTF2 mutant with reduced affinity for Ran and a Ran mutant that does not bind NTF2, both of which prevent CapG import. Thus, a ubiquitously expressed protein shuttles to the nucleus through direct association with NTF2 and Ran. The role of NTF2 may therefore not be solely confined to sustaining the Ran gradient in cells.

Identification of novel suppressors for Mog1 implies its involvement in RNA metabolism, lipid metabolism and signal transduction

Mog1 is conserved from yeast to mammal, but its function is obscure. We isolated yeast genes that rescued a temperature-sensitive death of S. cerevisiae Scmog1Delta, and of S. pombe Spmog1(ts). Scmog1Delta was rescued by Opi3p, a phospholipid N-methyltransferase, in addition to S. cerevisiae Ran-homologue Gsp1p, and a RanGDP binding protein Ntf2p. On the other hand, Spmog1(ts) was rescued by Cid13 that is a poly (A) polymerase specific for suc22(+) mRNA encoding a subunit of ribonucleotide reductase, Ssp1 that is a protein kinase involved in stress response pathway, and Crp79 that is required for mRNA export, in addition to Spi1, S. pombe Ran-homologue, and Nxt2, S. pombe homologue of Ntf2p. Consistent with the identification of those suppressors, lack of ScMog1p dislocates Opi3p from the nuclear membrane and all of Spmog1(ts) showed the nuclear accumulation of mRNA. Furthermore, SpMog1 was co-precipitated with Nxt2 and Cid13.

Identification and characterization of the Arabidopsis orthologs of nuclear transport factor 2, the nuclear import factor of ran

Ran is a multifunctional small GTPase that is involved in nucleocytoplasmic transport, mitotic spindle assembly, and nuclear envelope formation. Nuclear import of Ran relies on a small RanGDP-binding protein, Nuclear Transport Factor 2 (NTF2). Three proteins are expressed in Arabidopsis (Arabidopsis thaliana) that show significant sequence similarity to human and yeast (Saccharomyces cerevisiae) NTF2. Here, we demonstrate that two of them, AtNTF2a and AtNTF2b, can functionally replace the essential NTF2 gene in yeast. Consistent with this finding, both AtNTF2a and AtNTF2b interact with yeast and Arabidopsis Ran. The third NTF2-related protein, AtNTL, does not functionally replace NTF2 in yeast. Similar to yeast NTF2-green fluorescent protein (GFP), AtNTF2a-GFP and AtNTF2b-GFP accumulate at the nuclear rim. The AtNTF2a E38K and E91K mutants, which fail to bind Ran, are not functional in yeast, indicating conservation of the requirement for these key amino acids in plants and yeast. AtNTF2a overexpression, but not AtNTF2aE38K overexpression, blocks nuclear import of a plant transcription factor in Nicotiana benthamiana leaves, indicating that excess AtNTF2a disrupts nuclear import in a Ran-binding-dependent manner. On the basis of these results, we propose that AtNTF2a and AtNTF2b function in Ran import in Arabidopsis and that nuclear import of Ran is functionally conserved in plants.

Dissociation of recruitment and activation of the small G-protein Rac during Fcgamma receptor-mediated phagocytosis

Rho-family proteins play a central role in most actin-dependent processes, including the control and maintenance of cell shape, adhesion, motility, and phagocytosis. Activation of these GTP-binding proteins is tightly regulated spatially and temporally; however, very little is known of the mechanisms involved in their recruitment and activation in vivo. Because of its inducible, restricted signaling, phagocytosis offers an ideal physiological system to delineate the pathways linking surface receptors to actin remodeling via Rho GTPases. In this study, we investigated the involvement of early regulators of Fcgamma receptor signaling in Rac recruitment and activation. Using a combination of receptor mutagenesis, cellular, molecular, and pharmacological approaches, we show that Src family and Syk kinases control Rac and Vav function during phagocytosis. Importantly, both the immunoreceptor tyrosine-based activation motif within Fcgamma receptor cytoplasmic domain and Src kinase control the recruitment of Vav and Rac. However, Syk activity is dispensable for Vav and Rac recruitment. Moreover, we show that Rac and Cdc42 activities coordinate F-actin accumulation at nascent phagosomes. Our results provide new insights in the understanding of the spatiotemporal regulation of Rho-family GTPase function, and of Rac in particular, during phagocytosis. We believe they will contribute to a better understanding of more complex cellular processes, such as cell adhesion and migration.