P446L-importin-beta inhibits nuclear envelope assembly by sequestering nuclear envelope assembly factors to the microtubules

More than a zip code: global modulation of cellular function by nuclear localization signals

Extensive structural and functional studies have been carried out in the field of nucleocytoplasmic transport. Nuclear transport factors, such as Importin-α/-β, recognize nuclear localization signals (NLSs) on cargo, and together with the small GTPase Ran, facilitate their nuclear localization. However, it is now emerging that binding of nuclear transport factors to NLSs not only mediates nuclear transport but also contributes to a variety of cellular functions in eukaryotes. Here, we describe recent advances that reveal how NLSs facilitate diverse cellular functions beyond nuclear transport activity. We review separately NLS-mediated regulatory mechanisms at different levels of biological organization, including (a) assembly of higher-order structures; (b) cellular organelle dynamics; and (c) modulation of cellular stress responses and viral infections. Finally, we provide mechanistic insights into how NLSs can regulate such a broad range of functions via their structural and biochemical properties.

Changing expression and subcellular distribution of karyopherins during murine oogenesis

Mammalian oocyte growth and development is driven by a strict program of gene expression that relies on the timely presence of transcriptional regulators via nuclear pores. By targeting specific cargos for nucleo-cytoplasmic transport, karyopherin (KPN) proteins are key to the relocation of essential transcription factors and chromatin-remodelling factors into and out of the nucleus. Using multiple complementary techniques, here we establish that KPNA genes and proteins are dynamically expressed and relocalised throughout mouse oogenesis and folliculogenesis. Of the KPNAs examined (Kpna1, Kpna2, Kpna3, Kpna4, Kpna6, Kpna7, Kpnb1, Ipo5 and Xpo1), all were expressed in the embryonic ovary with up-regulation of protein levels concomitant with meiotic entry for KPNA2, accompanied by the redistribution of the cellular localisation of KPNA2 and XPO1. In contrast, postnatal folliculogenesis revealed significant up-regulation of Kpna1, Kpna2, Kpna4, Kpna6 and Ipo5 and down-regulation of Kpnb1, Kpna7 and Xpo1 at the primordial to primary follicle transition. KPNAs exhibited different localisation patterns in both oocytes and granulosa cells during folliculogenesis, with three KPNAs – KPNA1, KPNA2 and IPO5 – displaying marked enrichment in the nucleus by antral follicle stage. Remarkably, varied subcellular expression profiles were also identified in isolated pre-ovulatory oocytes with KPNAs KPNA2, KPNB1 and IPO5 detected in the cytoplasm and at the nuclear rim and XPO1 in cytoplasmic aggregates. Intriguingly, meiotic spindle staining was also observed for KPNB1 and XPO1 in meiosis II eggs, implying roles for KPNAs outside of nucleo-cytoplasmic transport. Thus, we propose that KPNAs, by targeting specific cargoes, are likely to be key regulators of oocyte development.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

'Poking' microtubules bring about nuclear wriggling to position nuclei

Nuclei wriggle in the cells of the follicle epithelium of the Drosophila pre-vitellogenic egg primordia. Although similar phenomena have been reported for a number of cultured cell types and some neurons in the zebrafish embryo, the mechanism and importance of the process have remained unexplained. Wriggling involves successive sudden and random minor turns of the nuclei, approximately three twists per minute with roughly 12° per twist, one of which lasts typically for 14 seconds. Wriggling is generated by the growing microtubules seeded throughout the cell cortex, which, while poking the nuclei, buckle and exert 5-40 piconewtons over ∼16 seconds. While wriggling, the nuclei drift ∼5 µm in a day in the immensely growing follicle cells along the apical-basal axis from the apical to the basal cell region. A >2-fold excess of the microtubules nucleated in the apical cell region, as compared with those seeded in the basal cell cortex, makes the nuclei drift along the apical-basal axis. Nuclear wriggling and positioning appear to be tightly related processes: they cease simultaneously when the nuclei become anchored by the actin cytoskeleton; moreover, colchicine or taxol treatment eliminates both nuclear wriggling and positioning. We propose that the wriggling nuclei reveal a thus far undescribed nuclear positioning mechanism.

Nuclear reformation after mitosis requires downregulation of the Ran GTPase effector RanBP1 in mammalian cells

The GTPase Ran regulates nucleocytoplasmic transport in interphase and spindle organisation in mitosis via effectors of the importin beta superfamily. Ran-binding protein 1 (RanBP1) regulates guanine nucleotide turnover on Ran, as well as its interactions with effectors. Unlike other Ran network members that are steadily expressed, RanBP1 abundance is modulated during the mammalian cell cycle, peaking in mitosis and declining at mitotic exit. Here, we show that RanBP1 downregulation takes place in mid to late telophase, concomitant with the reformation of nuclei. Mild RanBP1 overexpression in murine cells causes RanBP1 to persist in late mitosis and hinders a set of events underlying the telophase to interphase transition, including chromatin decondensation, nuclear expansion and nuclear lamina reorganisation. Moreover, the reorganisation of nuclear pores fails associated with defective nuclear relocalisation of NLS cargoes. Co-expression of importin beta, together with RanBP1, however mitigates these defects. Thus, RanBP1 downregulation is required for nuclear reorganisation pathways operated by importin beta after mitosis.

In vivo dynamics of Drosophila nuclear envelope components

Nuclear pore complexes (NPCs) are multisubunit protein entities embedded into the nuclear envelope (NE). Here, we examine the in vivo dynamics of the essential Drosophila nucleoporin Nup107 and several other NE-associated proteins during NE and NPCs disassembly and reassembly that take place within each mitosis. During both the rapid mitosis of syncytial embryos and the more conventional mitosis of larval neuroblasts, Nup107 is gradually released from the NE, but it remains partially confined to the nuclear (spindle) region up to late prometaphase, in contrast to nucleoporins detected by wheat germ agglutinin and lamins. We provide evidence that in all Drosophila cells, a structure derived from the NE persists throughout metaphase and early anaphase. Finally, we examined the dynamics of the spindle checkpoint proteins Mad2 and Mad1. During mitotic exit, Mad2 and Mad1 are actively imported back from the cytoplasm into the nucleus after the NE and NPCs have reformed, but they reassociate with the NE only later in G1, concomitantly with the recruitment of the basket nucleoporin Mtor (the Drosophila orthologue of vertebrate Tpr). Surprisingly, Drosophila Nup107 shows no evidence of localization to kinetochores, despite the demonstrated importance of this association in mammalian cells.

Importin beta is transported to spindle poles during mitosis and regulates Ran-dependent spindle assembly factors in mammalian cells

Spatial control is a key issue in cell division. The Ran GTPase regulates several fundamental processes for cell life, largely acting through importin molecules. The best understood of these is protein import through the nuclear envelope in interphase, but roles in mitotic spindle assembly are also established. In mammalian cells, in which centrosomes are major spindle organizers, a link is emerging between the Ran network, centrosomes and spindle poles. Here, we show that, after nuclear envelope breakdown, importin beta is transported to the spindle poles in mammalian cells. This localization is temporally regulated from prometaphase until anaphase, when importin beta dissociates from poles and is recruited back around reforming nuclei. Importin beta sediments with mitotic microtubules in vitro and its accumulation at poles requires microtubule integrity and dynamics in vivo. Furthermore, RNA interference-dependent inactivation of TPX2, the major Ran-dependent spindle organizer, abolishes importin beta accumulation at poles. Importin beta has a functional role in spindle pole organization, because overexpression yields mitotic spindles with abnormal, fragmented poles. Coexpression of TPX2 with importin beta mitigates these abnormalities. Together, these results indicate that the balance between importins and spindle regulators of the TPX2 type is crucial for spindle formation. Targeting of TPX2/importin-beta complexes to poles is a key aspect in Ran-dependent control of the mitotic apparatus in mammalian cells.

Importin alpha associates with membranes and participates in nuclear envelope assembly in vitro

Importin alpha is well known as an adaptor that functions with Importin beta in the nuclear import of proteins containing specific nuclear localization signals (NLSs). We show here that either an excess or a lack of Importin alpha blocks nuclear envelope (NE) assembly in vitro, and our data suggest that soluble Importin alpha functions in NE assembly in conjunction with NLS-containing partner proteins. Surprisingly, a significant proportion of Importin alpha is found to fractionate with Xenopus egg membranes. We demonstrate that membrane association of Importin alpha is regulated by phosphorylation. Using mutant forms of Importin alpha that either do not bind membranes or are not released from them by phosphorylation, we provide evidence that membrane-associated Importin alpha is required for NE formation. Unlike other functions of Importin alpha, this membrane-associated activity does not require interaction with NLS proteins.