Electrical dimension of the nuclear envelope

Eukaryotic chromosomes are confined to the nucleus, which is separated from the rest of the cell by two concentric membranes known as the nuclear envelope (NE). The NE is punctuated by holes known as nuclear pore complexes (NPCs), which provide the main pathway for transport of cellular material across the nuclear-cytoplasmic boundary. The single NPC is a complicated octameric structure containing more than 100 proteins called nucleoporins. NPCs function as transport machineries for inorganic ions and macromolecules. The most prominent feature of an individual NPC is a large central channel, ~7 nm in width and 50 nm in length. NPCs exhibit high morphological and functional plasticity, adjusting shape to function. Macromolecules ranging from 1 to >100 kDa travel through the central channel into (and out of) the nucleoplasm. Inorganic ions have additional pathways for communication between cytosol and nucleus. NE can turn from a simple sieve that separates two compartments by a given pore size to a smart barrier that adjusts its permeabiltiy to the metabolic demands of the cell. Early microelectrode work characterizes the NE as a membrane barrier of highly variable permeability, indicating that NPCs are under regulatory control. Electrical voltage across the NE is explained as the result of electrical charge separation due to selective barrier permeability and unequal distribution of charged macromolecules across the NE. Patch-clamp work discovers NE ion channel activity associated with NPC function. From comparison of early microelectrode work with patch-clamp data and late results obtained by the nuclear hourglass technique, it is concluded that NPCs are well-controlled supramolecular structures that mediate transport of macromolecules and small ions by separate physical pathways, the large central channel and the small peripheral channels, respectively. Electrical properties of the two pathways are still unclear but could have great impact on the understanding of signal transfer across NE and gene expression.     

Redistribution of nuclear pores during formation of the redundant nuclear envelope in mouse spermatids

Extensive morphological modification occurs during mammalian spermiogenesis when spermatids change their spherical shape into cells with a compact head and a long tail. In this study, freeze-fracture was used to elucidate the alteration of the nuclear envelope during this process. Nuclear condensation resulted in a great reduction of spermatid nuclear volume and the formation of the redundant nuclear envelope. During nuclear condensation, distribution patterns of nuclear pores were greatly affected by the developing acrosome and manchette. As the acrosome enlarged to cap the nucleus, the pores redistributed caudally in the nuclear membranes and became exclusively localized to the redundant nuclear envelope. Manchette microtubules play an important role in shaping the nucleus, and formation of the manchette was associated with exclusion of nuclear pores from the underlying nuclear envelope; therefore, it is likely that the redistribution of nuclear pores was aided by manchette development. The appearance of an electron-lucent nuclear region surrounded by the nascent redundant nuclear envelope indicated a pathway for transporting degradation products through the nuclear pores to the residual cytoplasm. The packaging of the nuclear pores into the redundant nuclear envelope suggests that they play a role in late stages of sperm maturation or in fertilization, as most other unnecessary organelles of sperm are discarded during spermiogenesis or during shedding of the cytoplasmic droplet.     

Properties of the nuclear polyhedrosis virus

Four passages of Barathra brassicae L nuclear polyhedrosis virus (NPV) in Heliothis armigera Hbn caterpillars were carried out. B. brassicae NPV was found to be highly infective for caterpillars of both species. When H. armigera caterpillars were infected with B. brassicae NPV, reproduction of the latter occurred alongside with the induction of H. armigera latent virus. In subsequent passages the latent virus replaced the foreign NPV. It was noted in mixed infection that in the 2nd passage the activity of NPV decreased apparently due to interference of the viruses.     

Control of the nuclear localization of Extradenticle by competing nuclear import and export signals

The Drosophila PBC protein Extradenticle (Exd) is regulated at the level of its subcellular distribution: It is cytoplasmic in the absence of Homothorax (Hth), a Meis family member, and nuclear in the presence of Hth. Here we present evidence that, in the absence of Hth, Exd is exported from nuclei due to the activity of a nuclear export signal (NES). The activity of this NES is inhibited by the antibiotic Leptomycin B, suggesting that Exd is exported by a CRM1/exportin1-related export pathway. By analyzing the subcellular localization of Exd deletion mutants in imaginal discs and cultured cells, we identified three elements in Exd, a putative NES, a nuclear localization sequence (NLS), and a region required for Hth-mediated nuclear localization. This latter region coincides with a domain in Exd that binds Hth protein in vitro. When Exd is uncomplexed with Hth, the NES dominates over the NLS. When Exd is expressed together with Hth, or when the NES is deleted, Exd is nuclear. Thus, Hth is required to overcome the influence of the NES, possibly by inducing a conformational change in Exd. Finally, we provide evidence that Hth and Exd normally interact in the cytoplasm, and that Hth also has an NLS. We propose that in Exd there exists a balance between the activities of an NES and an NLS, and that Hth alters this balance in favor of the NLS.     

Autoantigens of the nuclear pore complex

The nuclear envelope (NE) is one of many intracellular targets of the autoimmune response in patients with autoimmune liver disease, systemic lupus erythematosus, and related conditions. In eukaryotic organisms the NE consists of five interconnected regions: an outer nuclear membrane (ONM) that is continuous with the endoplasmic reticulum, an intermembrane or perinuclear space, an inner nuclear membrane (INM) with a unique set of integral membrane proteins, the underlying nuclear lamina, and the pore domains that are regions where the ONM and INM come together. The pore domains are sites of regulated continuity between the cytoplasm and nucleus that are occupied by supramolecular structures, termed nuclear pore complexes (NPCs). Human autoantibodies identified to date bind to specific components in three of the five NE compartments. Autoantigen targets include the lamins A, B, and C of the nuclear lamina, gp210, p62 complex proteins, Nup153, and Tpr within the NPC, and LBR, MAN1, LAP1, and LAP2 that are integral proteins of the INM. Autoantibodies to these NE targets have been shown to be correlated with various autoimmune diseases such as primary biliary cirrhosis, other autoimmune liver diseases and systemic rheumatic diseases. Now that the proteome of the NE is more clearly defined, other autoantibodies to components in this cell compartment are likely to be defined.Abbreviations NE Nuclear envelope - ONM Outer nuclear membrane - INM Inner nuclear membrane - NPC Nuclear pore complex - IIF Indirect immunofluorescence - SLE Systemic lupus erythematosus - PBC Primary biliary cirrhosis - CFS Chronic fatigue syndrome     

Association of Autographa californica nuclear polyhedrosis virus (AcMNPV) with the nuclear matrix

Nuclear matrices from uninfected Spodoptera frugiperda cells and those infected with Autographa californica nuclear polyhedrosis virus (AcMNPV) were isolated and their protein constituents were compared. Proteins were characterized according to size and several different antibodies to Drosophila nuclear proteins were employed in an attempt to identify the proteins comprising this nuclear substructure. Three species of lamins were identified as major constituents of the nuclear matrix of Spodoptera cells. Two DNA-binding proteins having molecular weights of 54 and 36 kDa were also identified as components of the nuclear matrix of uninfected cells. Infection resulted in a superimposition of viral proteins upon the nuclear matrix of the host cell. Polyhedrin, the basic viral DNA-binding protein (p6.9), and the major capsid protein of AcMNPV were identified immunologically as components of the nuclear matrix fraction of infected cells. Infection also resulted in the increased association of cellular histones with the nuclear matrix. DNA-binding assays demonstrated histones and p6.9 were the predominant DNA-binding proteins associated with the nuclear matrix of infected cells. Nuclear matrices from uninfected cells and cells infected with AcMNPV for 10 and 24 hr were examined using transmission electron microscopy. Morphologically, the nuclear matrix of the uninfected cell consists of the outer nuclear lamina (including nuclear pore complexes), an internal fibrogranular protein constituent, and a residual nucleolar structure. Numerous viral capsids were observed associated with the nuclear matrix in cells infected with either wild-type AcMNPV or a polyhedrin-deletion mutant by 10 hr p.i. The capsids appeared to be attached in an end-on association with the internal fibrogranular protein network of the nuclear matrix. The matrix-associated capsids were similar in width and length to those packaged within the polyhedra. In addition to the capsids, polyhedra in various stages of maturation were seen at 24 hr following infection of the cells with the wild-type virus. The nuclear matrix of the infected cell appears to play an important role in baculovirus assembly.