The inner nuclear membrane protein p58 associates in vivo with a p58 kinase and the nuclear lamins

Interaction between an integral protein of the nuclear envelope inner membrane and human chromodomain proteins homologous to Drosophila HP1

At the nuclear envelope in higher eukaryotic cells, the nuclear lamina and the heterochromatin are adjacent to the inner nuclear membrane, and their attachment is presumably mediated by integral membrane proteins. In a yeast two-hybrid screen, the nucleoplasmic domain of lamin B receptor (LBR), an integral protein of the inner nuclear membrane, associated with two human polypeptides homologous to Drosophila HP1, a heterochromatin protein involved in position-effect variegation. LBR fusion proteins bound to HP1 proteins synthesized by in vitro translation and present in cell lysates. Antibodies against LBR also co-immunoprecipitated HP1 proteins from cell extracts. LBR can interact with chromodomain proteins that are highly conserved in eukaryotic species and may function in the attachment of heterochromatin to the inner nuclear membrane in cells.

Characterization of p18, a component of the lamin B receptor complex and a new integral membrane protein of the avian erythrocyte nuclear envelope

Employing avian erythrocytes, we have previously isolated a multimeric complex consisting of the lamin B receptor (LBR, or p58), the nuclear lamins, an LBR-specific kinase, a 34-kDa protein, and an 18-kDa polypeptide termed p18. As the LBR kinase and the 34-kDa component have been recently characterized, we now proceed in the characterization of p18. We show here that p18 is an integral membrane protein specific to the erythrocyte nuclear envelope which binds to LBR and B-type lamins. NH2-terminal sequencing indicates that p18 is distinct from other nuclear envelope components, but has similarity to the mitochondrial isoquinoline-binding protein. In situ analysis by immunoelectron microscopy and examination of digitonin-permeabilized cells by indirect immunofluorescence show that p18, unlike LBR and other lamin-binding proteins, is equally distributed between the inner and outer nuclear membrane. Furthermore, cycloheximide inhibition experiments reveal that the fraction of p18 that resides in the outer nuclear membrane does not represent nascent chains en route to the inner nuclear membrane, but rather material in equilibrium with the p18 that partitions with the inner nuclear membrane. The paradigm of p18 suggests that transmembrane complexes formed by the nuclear lamins and LBR provide potential docking sites for integral membrane proteins of the nuclear envelope that equilibrate between the rough endoplasmic reticulum and the inner nuclear membrane.

A nuclear envelope-associated kinase phosphorylates arginine-serine motifs and modulates interactions between the lamin B receptor and other nuclear proteins

Previous studies have identified a subassembly of nuclear envelope proteins, termed "the LBR complex." This complex includes the lamin B receptor protein (LBR or p58), a kinase which phosphorylates LBR in a constitutive fashion (LBR kinase), the nuclear lamins A and B, an 18-kDa polypeptide (p18), and a 34-kDa protein (p34/p32). The latter polypeptide has been shown to interact with the HIV-1 proteins Rev and Tat and with the splicing factor 2 (SF2). Using recombinant proteins produced in bacteria and synthetic peptides representing different regions of LBR, we now show that the LBR kinase modifies specifically arginine-serine (RS) dipeptide motifs located at the nucleoplasmic, NH2-terminal domain of LBR and in members of the SR family of splicing factors. Furthermore, we show that the NH2-terminal domain of LBR binds to p34/p32, whereas a mutated domain lacking the RS region does not. Phosphorylation of LBR by the RS kinase completely abolishes binding of p34/p32, suggesting that this enzyme regulates interactions among the components of the LBR complex.

Nuclear matrix and the cell cycle

The facts that the nuclear matrix represents a structural framework of the cell nucleus and that nuclear events, such as DNA replication, transcription, and DNA repair, are associated with this skeletal structure suggest that its components are subject to cell cycle-regulatory mechanisms. Cell cycle regulation has been shown for nuclear lamina assembly and disassembly during mitosis and chromatin reorganization. Little attention has so far been paid to internal nuclear matrix proteins and matrix-associated proteins with respect to the cell cycle. This survey attempts to summarize available data and presents experimental evidence that important metabolic functions of the nucleus are regulated by the transient, cell cycle-dependent attachment of enzymes and regulatory proteins to the nuclear matrix. Results on thymidine kinase and RNA polymerase during the synchronous cell cycle of Physarum polycephalum demonstrate that reversible binding to the nuclear matrix represents an additional level of regulation for nuclear processes.

The dynamic properties and possible functions of nuclear lamins

The nuclear lamins are thought to form a thin fibrous layer called the nuclear lamina, underlying the inner nuclear envelope membrane. In this review, we summarize data on the dynamic properties of nuclear lamins during the cell cycle and during development. We discuss the implications of dynamics for lamin functions. The lamins may be involved in DNA replication, chromatin organization, differentiation, nuclear structural support, and nuclear envelope reassembly. Emphasis is placed on recent data that indicate that the lamina, contrary to previous views, is not a static structure. For example, the lamins form nucleoplasmic foci, distinct from the peripheral lamina, which vary in their patterns of distribution as well as their composition in a cell cycle-dependent manner. During the S phase, these foci colocalize with chromatin and sites of DNA replication. At other points during the cell cycle, they may represent sites of lamin post-translation processing that take place prior to incorporation into the lamina. Secondary modifications of the lamins such as isoprenylation and phosphorylation are involved in the regulation of the dynamic properties and the assembly of lamins. In addition, a number of lamin-associated proteins have been recently identified and these are described along with their potential functions.

Integration of intermediate filaments into cellular organelles

The intermediate filaments represent core components of the cytoskeleton and are known to interact with several membranous organelles. Classic examples of this are the attachment of keratin filaments to the desmosomes and the association of the lamin filament meshwork with the inner nuclear membrane. At this point, the molecular mechanisms by which the filaments link to membranes are not clearly understood. However, since a substantial body of information has been amassed, the time is now ripe for comparing notes and formulating working hypotheses. With this objective in mind, we review here pioneering studies on this subject, together with work that has appeared more recently in the literature.

Immunolocalization of lamins and nuclear pore complex proteins by atomic force microscopy

The nuclear envelope functions as a selective barrier separating the nuclear from the cytosolic compartment. Nuclear pore complexes (NPCs) mediate nuclear import and export of macromolecules and, therefore, are potential regulators of gene expression. In this study we applied atomic force microscopy (AFM) to visualize the three dimensional (3D) structure of individual NPCs in the absence and presence of two different antibodies, one directed against a pore protein (gp62) and another directed against Xenopus lamin LIII, a component of the nuclear lamina, a filament meshwork localized on the nucleoplasmic side of the nuclear envelope (NE) adjacent to and interacting with NPCs. Using 12-nm gold-labelled secondary antibodies and transmission electron microscopy we could clearly localize the primary single anti-gp62 antibody on NPCs and the primary single anti-LIII antibody between NPCs. Using AFM, the secondary antibodies against anti-gp62 could be detected as particles 7 nm in height on the nucleoplasmic face of NPCs. The secondary antibodies against anti-LIII could be clearly identified between NPCs. The secondary antibodies, attached to a 12-nm colloidal gold particle and visualized on glass, revealed similar shapes and heights as found on NEs. According to the 3D images, the volume of a single gold particle conjugated with secondary antibodies was 10203 nm3. This volume is equivalent to the volume of 38 IgG molecules associated with one individual gold particle. A similar volume of 11987 nm3 was calculated from a model assuming that the 150-kDa IgG molecules perfectly cover the spherical gold particle. We conclude that AFM can be used for identifying antibodies or other macromolecules associated with biomembranes.

Differential mitotic phosphorylation of proteins of the nuclear pore complex

During each cell cycle, the nucleus of higher eukaryotes undergoes a dramatic assembly and disassembly. These events can be faithfully reproduced in vitro using cell-free extracts derived from Xenopus eggs. Such extracts contain three major N-acetylglucosaminylated proteins, p200, p97, and p60. All three become assembled into reconstituted nuclear pores. Here we show that p200, p97, and p60 exist in eggs in soluble high molecular mass complexes of 1000, 450, and 600 kDA, respectively. The bulk of p60 is stably associated with proteins of 58 and 54 kDa, while p200 is associated with a fraction of p60 in a separate complex lacking p58 and p54. Upon examining the behavior of these proteins in the cell cycle, we find that p200 and p97 are highly phosphorylated at mitosis, both in vivo and in vitro. Moreover, in extracts that cycle between interphase and mitosis, p200 and p97 are specifically phosphorylated at mitosis. Corresponding with their mitotic phosphorylation, both p200 and p97 are specific substrates for purified mitotic Cdc2 kinase, whereas nucleoporin p60 is not. Analysis indicates that the size of the complexes containing the pore N-acetylglucosamine glycoproteins does not change during mitosis, suggesting that such complexes represent stable multicomponent modules into which the nucleus disassembles at mitosis.

The lamin B receptor-associated protein p34 shares sequence homology and antigenic determinants with the splicing factor 2-associated protein p32

The lamin B receptor (p58) is an inner nuclear membrane protein that forms an in vivo complex with the nuclear lamins, a nuclear envelope kinase, and two other nuclear proteins with apparent M(r) of 18,000 (p18) and 34,000 (p34). We now report the isolation of p34 by partial dissociation of the immunoaffinity-purified p58 protein complex. Determination of the N-terminal amino acid sequence of purified p34 shows that this polypeptide is homologous to p32, a splicing factor 2 (SF2)-associated protein. The relatedness between p34 and p32 can be further established by showing that antibodies raised against N- and C-terminal peptides of p32 cross-react with purified p34. As the amino acid sequence of p58 contains an arginine/serine (RS)-rich region similar to the RS-rich region found in SF 2, we speculate that these domains provide binding sites for p34 and that this protein may be a linker between the nuclear membrane and intranuclear spliceosomal substructures.

Lamins and lamin-associated proteins

A variety of morphological and biochemical studies have established that the nuclear lamins play an important role in nuclear structure and dynamics. Recent work reveals the existence of specialized lamin isotypes and novel pathways of modulation of lamin import into the nucleus via phosphorylation by protein kinase C. Other studies also unveil a wide spectrum of molecular interactions between the lamin proteins and integral membrane components of the nuclear envelope.

Towards an understanding of nuclear morphogenesis

In the age of "virtual reality," the imperfect microscopic silhouettes of cells and organelles are gradually being replaced by calligraphic computer drawings. In this context, textbooks and introductory slides often depict the cell nucleus as a smooth-shaped, featureless object. However, in reality, the nuclei of different cells possess distinct sizes and morphological features which develop in a programmed fashion as each cell differentiates. To dissect this complex morphogenetic process, we need to identify the basic elements that determine nuclear architecture and the regulatory factors involved. Recently, clues about the identity of these components have been obtained both by systematic analysis and by serendipity. This review summarizes a few recent findings and ideas that may serve as a first forum for future discussions and, I hope, for further work on this topic.

Type B lamins remain associated with the integral nuclear envelope protein p58 during mitosis: implications for nuclear reassembly

p58 (also referred to as the lamin B receptor) is an integral membrane protein of the nuclear envelope known to form a multimeric complex with the lamins and other nuclear proteins during interphase. To examine the fate of this complex during mitosis, we have investigated the partitioning and the molecular interactions of p58 in dividing chicken hepatoma (DU249) cells. Using confocal microscopy and double immunolabelling, we show here that lamins B1 and B2 co-localize with p58 during all phases of mitosis and co-assemble around reforming nuclei. A close juxtaposition of p58/lamin B-containing vesicles and chromosomes is already detectable in metaphase; however, p58 and lamin reassembly proceeds slowly and is completed in late telophase--G1. Flotation of mitotic membranes in sucrose density gradients and analysis of mitotic vesicles by immunoelectron microscopy confirms that p58 and most of the type B lamins reside in the same compartment. Co-immunoprecipitation of both proteins by affinity-purified anti-p58 antibodies shows that they are physically associated in the context of a mitotic p58 'sub-complex'. This sub-assembly does not include the type A lamins which are fully solubilized during mitosis. Our data provide direct, in vivo and in vitro evidence that the majority of type B lamins remain connected to nuclear membrane 'receptors' during mitosis. The implications of these findings in nuclear envelope reassembly are discussed below.

The role of isoprenylation in membrane attachment of nuclear lamins. A single point mutation prevents proteolytic cleavage of the lamin A precursor and confers membrane binding properties

Mature A- and B-type lamins differ in the extent to which they interact with the nuclear membrane and thus represent an interesting model for studying the role of isoprenylation and carboxyl-methylation in membrane attachment. Both A- and B-type lamins are isoprenylated and carboxyl-methylated shortly after synthesis, but A-type lamins undergo a further proteolytic cleavage which results in the loss of the hydrophobically modified C terminus. Here, we have constructed mutants of chicken lamin A that differ in their abilities to serve as substrates for different post-translational processing events occurring at the C terminus of the wild-type precursor. In addition to studying full-length proteins, we have analyzed C-terminal end domains of lamin A, either alone or after fusion to reporter proteins. Mutant proteins were expressed in mammalian cells, and their membrane association was analyzed by immunofluorescence microscopy and subcellular fractionation. Our results provide information on the substrate specificity and subcellular localization of the lamin-A-specific protease. Moreover, they indicate that hydrophobic modifications of the C-terminal end domains account for the differential membrane-binding properties of A- and B-type lamins. Thus, some of the integral membrane proteins implicated in anchoring B-type lamins to the membrane may function as receptors for the isoprenylated and carboxyl-methylated C terminus.

Nuclear membrane dynamics

Our understanding of nuclear membrane trafficking and protein targeting has increased significantly, due to newly developed assays and tools. We present a conceptual framework for thinking about protein targeting to the inner nuclear membrane, and discuss nuclear envelope assembly in terms of vesicle binding to chromatin, vesicle fusion, structural attachments to the inner membrane, and the mitotic regulation of these attachments.

Lamin dynamics

Nuclear lamins form a highly insoluble structure, the nuclear lamina, which is associated with the nuclear envelope. Recent results suggest, however, that the lamins are more dynamic than originally thought. They accumulate in nucleoplasmic foci in the G1 stage of the cell cycle and later appear mainly in the peripheral lamina. Some of the lamin foci are closely associated with heterochromatin. Furthermore, the various lamin types assemble into the lamina polymer independently of each other. Both the assembly and disassembly of the lamins, as well as the interaction of the lamins with other nuclear structures such as the nuclear membrane, may be mediated by phosphorylation and dephosphorylation.