Expression of chicken lamin B2 in Escherichia coli: characterization of its structure, assembly, and molecular interactions

Peripheral framework of carrot cell nucleus contains a novel protein predicted to exhibit a long alpha-helical domain

A monoclonal antibody, CML-1, raised against carrot (Daucus carota L.) nuclear-matrix proteins selectively labeled the nuclear periphery of carrot protoplasts when visualized by confocal and electron microscopy. To identify the constituent proteins of higher plant cells structurally homologous to the vertebrate nuclear lamina, we cloned overlapping cDNAs partially encoding a CML-1-recognized protein and determined the entire sequence including the open reading frame. When the deduced amino acid sequence was compared with other known protein sequences contained in major databases, no protein was found to show high sequence identity across the whole region of the protein, while the partial sequence showed strong similarities with myosin, tropomyosin, and some intermediate filament proteins. The protein, designated NMCP1, had an estimated molecular mass of 133.6 kDa and showed three characteristic domains. The central domain contains long alpha-helices exhibiting heptad repeats of apolar residues, demonstrating structural similarity to that of filament-forming proteins. The terminal domains are predominantly nonhelical and contain potential sequence motifs for nuclear localization signals. NMCP1 has many recognition motifs for different types of protein kinases, including cdc2 kinase and PKC. These results suggest that NMCP1 protein forms coiled-coil filaments and is a constituent of the peripheral architecture of the higher plant cell nucleus.

Degradation of nuclear matrix and DNA cleavage in apoptotic thymocytes

In dexamethasone-treated thymocyte cultures an increase in nuclear proteolytic activity paralleled chromatin fragmentation and the appearance of small apoptotic cells. The elevation of nuclear proteolytic activity was accompanied by site-specific degradation of nuclear mitotic apparatus protein and lamin B, two essential components of the nuclear matrix. Nuclear mitotic apparatus protein phosphorylation and cleavage into 200 and 48 kDa fragments occurred within 30 minutes of dexamethasone treatment. Cleavage of lamin B, which generated a fragment of 46 kDa consistent with the central rod domain of the protein, was also detected after 30 minutes of exposure to the steroid hormone. The level of lamin B phosphorylation did not change as a result of the dexamethasone treatment and the lamina did not solubilize until the later stages of apoptosis. Initial DNA breaks, detected by the terminal transferase-mediated dUTP-biotin nick end labeling assay, occurred throughout the nuclei and solubilization of lamina was not required for this process to commence. The data presented in this paper support a model of apoptotic nuclear destruction brought about by the site-specific proteolysis of key structural proteins. Both the nuclear mitotic apparatus protein and lamin B were specifically targeted by protease(s) at early stages of the cell death pathway, which possibly initiate the cascade of degradative events in apoptosis.

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.

A chromatin binding site in the tail domain of nuclear lamins that interacts with core histones

Interaction of chromatin with the nuclear envelope and lamina is thought to help determine higher order chromosome organization in the interphase nucleus. Previous studies have shown that nuclear lamins bind chromatin directly. Here we have localized a chromatin binding site to the carboxyl-terminal tail domains of both A- and B-type mammalian lamins, and have characterized the biochemical properties of this binding in detail. Recombinant glutathione-S-transferase fusion proteins containing the tail domains of mammalian lamins C, B1, and B2 were analyzed for their ability to associate with rat liver chromatin fragments immobilized on microtiter plate wells. We found that all three lamin tails specifically bind to chromatin with apparent KdS of 120-300 nM. By examining a series of deletion mutants, we have mapped the chromatin binding region of the lamin C tail to amino acids 396-430, a segment immediately adjacent to the rod domain. Furthermore, by analysis of chromatin subfractions, we found that core histones constitute the principal chromatin binding component for the lamin C tail. Through cooperativity, this lamin-histone interaction could be involved in specifying the high avidity attachment of chromatin to the nuclear envelope in vivo.

Disassembly of the Drosophila nuclear lamina in a homologous cell-free system

Stage 14 Drosophila oocytes are arrested in first meiotic metaphase. A cell-free extract of these oocytes catalyzes apparent disassembly of purified Drosophila nuclei as well as of nuclear lamin polymers formed in vitro from isolated interphase lamins. Biochemically, the oocyte extract catalyzes lamin solubilization and phosphorylation as well as characteristic changes in one- and two-dimensional gel mobility. A previously unidentified soluble lamin isoform is easily seen after in vitro disassembly. This isoform is detectable but present only in very small quantities in vivo and is apparently derived specifically from one of the two interphase lamin isoforms. Cell-free nuclear lamina disassembly is ATP-dependent and addition of calcium to extracts blocks disassembly as judged both morphologically and biochemically. This system will allow enzymological characterization of cell-free lamina disassembly as well as molecular analysis of specific Drosophila mutants.

Determinants for intracellular sorting of cytoplasmic and nuclear intermediate filaments

The mechanism by which nuclear and cytoplasmic filaments are sorted in vivo was studied by examining which lamin sequences are required to target an otherwise cytoplasmic IF protein, the small neurofilament subunit (NF-L), to the nuclear lamina. By swapping corresponding domains between NF-L and lamin A, nuclear envelope targeting of NF-L was shown to require the presence of the "head" domain, a 42-amino acid sequence unique to lamin rod domains, a nuclear localization signal and the CAAX motif. Replacement of the entire COOH-terminal tail of lamin A with that of NF-L had no discernible effect on nuclear localization of lamin A, provided the substituted NF-L tail contained a NLS and a CAAX motif. This chimeric protein exhibited characteristics more typical of lamin B than that of the parental lamin A. With regard to cytoplasmic assembly properties, substitution of the head domain of lamin A for that of NF-L did not substantially affect the ability of NF-L to coassemble with vimentin in the cytoplasm. In contrast, insertion of a 42-amino acid sequence unique to lamin rod domains into NF-L profoundly affected NF-L coassembly with vimentin indicating that the 42-amino acid insertion in lamins may be important for sorting lamins from cytoplasmic IF proteins.

Troponin T is capable of binding dystrophin via a leucine zipper

Using genetic and physical assays for protein-protein interactions, we identified a fast isoform of troponin T that binds to dystrophin. Troponin T specifically bound to the first of two highly conserved leucine zipper motifs in the carboxy terminus of dystrophin [1,2]. Single amino acid changes in the zipper predicted to disrupt alpha-helix formation or cause steric hindrance abolished this binding. These data support the hypothesis that dystrophin couples the contractile apparatus to the sarcolemma and indicate that leucine zipper mediated protein-protein interactions are functionally important in the cytoskeleton as well as the nucleus.

Mechanism of S100 protein-dependent inhibition of glial fibrillary acidic protein (GFAP) polymerization

S100 protein, a subfamily of Ca(2+)-binding proteins of the EF-hand type, was recently shown to bind to and to inhibit the polymerization of the glial fibrillary acidic protein (GFAP), the intermediate filament component of astroglial cells, in the presence of micromolar levels of Ca2+ (J. Biol. Chem. 268, 12669-12674). By a sedimentation assay and viscometry we show here that S100 protein interferes with the very early steps of GFAP polymerization (nucleation) and with the GFAP polymer growth, thereby retarding the onset of GFAP assembly, reducing the rate and the extent of GFAP assembly, and increasing the critical concentration of GFAP assembly. Moreover, S100 protein disassembles preformed glial filaments. All the above effects can be explained by sequestration of soluble GFAP by S100 protein, as also indicated by the stoichiometry of S100 protein binding to GFAP and of S100 protein effects on GFAP assembly. Our data suggest that S100 protein might serve the function of avoiding excess GFAP polymerization and might participate in remodeling of glial filaments following elevation of the intracellular free Ca2+ concentration. Also, our data lend support to the notion that intermediate filaments are dynamic cytoskeleton structures that assemble and disassemble, and to the existence of cytoplasmic factors implicated in the regulation of the state of assembly of intermediate filaments.

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.

Role of different domains in the self-association of rat nucleoporin p62

We have expressed rat nucleoporin p62 cDNA in Escherichia coli to obtain material for structural and self-association studies. Electron microscopy and circular dichroism spectroscopy are consistent with a rod-shaped molecule with an alpha-helical coiled-coil domain at its C terminus and a cross-beta structure at its N terminus, separated by a threonine-rich linker, which has a less-defined secondary structure. Electron microscopy and the solubility properties of fragments produced using thrombin and CNBr digestion indicate that p62 molecules associate to form linear chains and that a small region near the C terminus is an important determinant of assembly. This association may have important consequences for pore structure and function; for example, one way p62 could associate would be to form rings in nuclear pores that could function like barrel hoops.

Making heads and tails of intermediate filament assembly, dynamics and networks

Thus far, intermediate filaments (IFs) have been the least understood of the three cytoskeletal filament systems with regard to their structure, assembly, network formation, and dynamics. This picture is now slowly but definitely changing, as recent in vivo and in vitro experiments, including generation of transgenic animals, have yielded important new data shedding light on the following areas: the molecular architecture of IFs; the role of the highly variable end domains during IF assembly and network formation; the factors that govern whether IF proteins are involved in de novo filament formation or are incorporated into a pre-existing IF network; and the effects of post-translational modifications, such as phosphorylation and glycosylation of IF polypeptides, on filament assembly, dynamics and turnover.

The rod domain of NF-L determines neurofilament architecture, whereas the end domains specify filament assembly and network formation

Neurofilaments, assembled from NF-L, NF-M, and NF-H subunits, are the most abundant structural elements in myelinated axons. Although all three subunits contain a central, alpha-helical rod domain thought to mediate filament assembly, only NF-L self-assembles into 10-nm filaments in vitro. To explore the roles of the central rod, the NH2-terminal head and the COOH-terminal tail domain in filament assembly, full-length, headless, tailless, and rod only fragments of mouse NF-L were expressed in bacteria, purified, and their structure and assembly properties examined by conventional and scanning transmission electron microscopy (TEM and STEM). These experiments revealed that in vitro assembly of NF-L into bona fide 10-nm filaments requires both end domains: whereas the NH2-terminal head domain promotes lateral association of protofilaments into protofibrils and ultimately 10-nm filaments, the COOH-terminal tail domain controls lateral assembly of protofilaments so that it terminates at the 10-nm filament level. Hence, the two end domains of NF-L have antagonistic effects on the lateral association of protofilaments into higher-order structures, with the effect of the COOH-terminal tail domain being dominant over that of the NH2-terminal head domain. Consideration of the 21-nm axial beading commonly observed with 10-nm filaments, the approximate 21-nm axial periodicity measured on paracrystals, and recent cross-linking data combine to support a molecular model for intermediate filament architecture in which the 44-46-nm long dimer rods overlap by 1-3-nm head-to-tail, whereas laterally they align antiparallel both unstaggered and approximately half-staggered.

The central region of the synaptonemal complex in Blaps cribrosa studied by electron microscope tomography

The synaptonemal complex (SC) in the beetle Blaps cribrosa contains a highly organized central element (CE), two flanking lateral elements (LEs), and a number of regularly spaced transverse filaments (TFs) crossing the central region. The CE is built like a ladder with two longitudinal components running in parallel and a number of regularly spaced transverse CE components, bridging the two longitudinal components. The CE is multi-layered with the ladders of the individual layers more or less in register. Essentially every TF originates in one of the LEs, crosses the CE through a transverse CE component and reaches the opposite LE; every transverse CE component in a given layer corresponds to one, and only one, TF. In a CE layer, short irregular pillars form the junctions between the transverse and longitudinal CE components. Adjacent pillars are connected to each other by fine fibrous bridges: the two pillars in the same transverse CE component are linked, and so are the pillars along each longitudinal component, and also more occasionally adjacent pillars in separate CE layers. It is proposed that a TF with the two associated short pillars represents the structural unit in the central region. The ordered structure of the CE is accomplished by linking adjacent pillars to each other into the well-defined three-dimensional organization of the CE.

Covalent binding of biological samples to solid supports for scanning probe microscopy in buffer solution

Scanning force microscopy allows imaging of biological molecules in their native state in buffer solution. To this end samples have to be fixed to a flat solid support so that they cannot be displaced by the scanning tip. Here we describe a method to achieve the covalent binding of biological samples to glass surfaces. Coverslips were chemically modified with the photoactivatable cross-linker N-5-azido-2-nitrobenzoyloxysuccinimide. Samples are squeezed between derivatized coverslips and then cross-linked to the glass surface by irradiation with ultraviolet light. Such samples can be imaged repeatedly by the scanning force microscope without loss of image quality, whereas identical but not immobilized samples are pushed away by the stylus.

Missing links: Weber-Cockayne keratin mutations implicate the L12 linker domain in effective cytoskeleton function

We have identified mutations in keratins K5 (Arg331Cys) and K14 (Val270Met) in two kinships affected by the dominantly-inherited skin blistering disease, Weber-Cockayne epidermolysis bullosa simplex (EBS-WC). Linkage analysis, DNA sequencing and clinical and ultrastructural analysis are combined to provide the first detailed description of classical EBS-WC. Both phenotypes show similar blistering on trauma, indicating that both mutations compromise the structural resilience of the basal keratinocytes by affecting the keratin cytoskeleton. The location of these mutations in the L12 linker, which bisects the alpha-helical rod region of intermediate filament proteins, identifies another keratin mutation cluster leading to hereditary skin fragility syndromes.

Assembly of intermediate filaments

The assembly of intermediate filaments is a fundamental property of the central rod domain of the individual subunit proteins. This rod domain, with its high propensity for alpha-helix formation, is the common and identifying feature of this family of proteins. Assembly occurs in vitro in the absence of other proteins or exogenous sources of energy; in vivo, it appears as if other factors, as yet poorly understood, modulate the assembly of intermediate filaments. Parallel, in-register dimers form via coiled-coil interactions of the rod domain. Tetramers may form from staggered arrays of parallel or antiparallel arrangements of dimers. Higher-order polymerization, which occurs spontaneously if the ionic strength of a mixture of dimers and tetramers is raised, proceeds rapidly through poorly described intermediates to the final 10 nm filament. This process is dependent on and modulated by the non-alpha-helical end domains, as well as those amino acids present at the very beginning and end of the rod domain. The interactions governing tetramer formation are most probably the same ones that are responsible for the lateral and longitudinal associations within intermediate filaments.

Phosphorylation on protein kinase C sites inhibits nuclear import of lamin B2

The nuclear lamina is a karyoskeletal structure at the nucleoplasmic surface of the inner nuclear membrane. Its assembly state is regulated by phosphorylation of the intermediate filament type lamin proteins. Strong evidence has been obtained for a causal link between phosphorylation of lamins by the p34cdc2 protein kinase and disassembly of the nuclear lamina during mitosis. In contrast, no information is currently available on the role of lamin phosphorylation during interphase of the cell cycle. Here, we have identified four protein kinase C phosphorylation sites in purified chicken lamin B2 as serines 400, 404, 410, and 411. In vivo, the tryptic peptide containing serines 400 and 404 is phosphorylated throughout interphase, whereas serines 410 and 411 become phosphorylated specifically in response to activation of protein kinase C by phorbol ester. Prompted by the close proximity of serines 410/411 to the nuclear localization signal of lamin B2, we have studied the influence of phosphorylation of these residues on nuclear transport. Using an in vitro assay, we show that phosphorylation of lamin B2 by protein kinase C strongly inhibits transport to the nucleus. Moreover, phorbol ester treatment of intact cells leads to a substantial reduction of the rate of nuclear import of newly synthesized lamin B2 in vivo. These findings have implications for the dynamic structure of the nuclear lamina, and they suggest that the modulation of nuclear transport rates by cytoplasmic phosphorylation may represent a general mechanism for regulating nuclear activities.

Dynamics of intermediate filaments. Recent progress and unanswered questions

Intermediate filaments (IFs) have always been considered as the most static and 'skeletal' cellular elements. This view is now changing: new information reveals that IFs exchange subunits at steady-state, that IF networks can be assembled de novo, and that IF proteins are subject to elaborate chemical modification and de-modification during mitosis. I describe below some of the key observations which have made us realize that IFs are dynamic structures. I also discuss some of the remaining questions pertinent to the pathways of IF assembly under in vivo conditions.

Intermediate filament structure and assembly

Intermediate filaments are constructed from two-chain alpha-helical coiled-coil molecules arranged on an imperfect helical lattice. Filament structure and assembly can be influenced at several different structural levels, including molecular structure, oligomer formation and filament nucleation and elongation. Consequently, it can sometimes be difficult to interpret mutagenesis data unequivocally, although regions near the amino and carboxyl termini of the rod domain of the molecule are known to be important for the production of native filaments. Imperfections in molecular packing may be important in filament assembly and dynamics.

High resolution scanning electron microscopy of the nuclear envelope: demonstration of a new, regular, fibrous lattice attached to the baskets of the nucleoplasmic face of the nuclear pores

The nuclear envelope (NE) of amphibian oocytes can be readily isolated in relatively structurally intact and pure form and has been used extensively for structural studies. Using high resolution scanning electron microscopy (HRSEM), both surfaces of the NE can be visualized in detail. Here, we demonstrate the use of HRSEM to obtain high resolution information of NE structure, confirming previous data and providing some new information. NEs, manually isolated from Triturus cristatus oocytes, have been mounted on conductive silicon chips, fixed, critical point dried and coated with a thin, continuous film of chromium or tantalum and viewed at relatively high accelerating voltage in a field emission scanning electron microscope with the sample within the objective lens. Both nucleoplasmic and cytoplasmic surfaces of the nuclear pore complexes (NPC) have been visualized, revealing the cytoplasmic coaxial ring, associated particles, central plug/transporter and spokes. The nucleoplasmic face is dominated by the previously described basketlike structure attached to the nucleoplasmic coaxial ring. In Triturus, a novel, highly regular flat sheet of fibers, termed the NE lattice (NEL) has been observed attached to the distal ring of the NPC basket. The NEL appears to be distinct from the nuclear lamina. Evidence for the NEL is also presented in thin TEM sections from Triturus oocytes and GVs and in spread NEs from Xenopus. A model is presented for NEL structure and its interaction with the NPCs is discussed.

Pathway of incorporation of microinjected lamin A into the nuclear envelope

When microinjected into the cytoplasm of 3T3 cells, biotinylated human lamin A rapidly enters the nucleus and gradually becomes incorporated into the nuclear lamina region as determined by immunofluorescence. The incorporation of the microinjected material takes several hours and progresses through a series of morphologically identifiable stages. Within minutes after microinjection, lamin A is found in spots distributed throughout the nucleus, except in nucleolar regions. Over a time course of up to 6 h, these spots appear to decrease in size and number as the biotinylated lamin A becomes associated with the endogenous nuclear lamina. Eventually, the typical nuclear rim staining pattern normally revealed by immunofluorescence with nuclear lamin antibodies is seen with antibiotin. This latter rim staining property is passed on to daughter cells following mitosis. These results indicate that the microinjected biotinylated nuclear lamin A retains those properties required for its integration into the lamina, as well as those necessary for the disassembly and subsequent reassembly of the nuclear lamina during cell division. The initial rapid accumulation into foci and the subsequent slower incorporation into the nuclear lamina appear to be analogous to the stages of incorporation following the microinjection of cytoskeletal intermediate filament proteins such as vimentin and keratin (Vikstrom, K., G. G. Borisy, and R. D. Goldman. 1989. Proc. Natl. Acad. Sci. USA. 86:549-553; Miller, R. K., K. Vikstrom, and R. D. Goldman. 1991. J. Cell Biol. 113:843-855). Foci are also observed in some uninjected cells using nuclear lamin antibodies, indicating that these features are a genuine component of nuclear substructure. Evidence is presented that shows the appearance of these nuclear structures is cell cycle dependent.

The roles of K5 and K14 head, tail, and R/K L L E G E domains in keratin filament assembly in vitro

Type I and type II keratins form obligatory heterodimers, which self-assemble into 10-nm intermediate filaments (IFs). Like all IF proteins, they have a central alpha-helical rod domain, flanked by nonhelical head and tail domains. The IF rod is more highly conserved than head and tail, and within the rod, the carboxy R/K L L E G E sequence is more highly conserved than most other regions. Mutagenesis studies have shed some light on the roles of the head, tail, and R/K L L E G E sequence in 10-nm filament structure. However, interpretations have often been complicated in part because many of these studies have focused on transfected cells, where filament structure cannot be evaluated. Of the few in vitro assembly studies thus far conducted, comparison of keratin mutants with other IF mutants have often been difficult, due to the obligatory heteropolymeric nature of keratin IFs. In this report, we describe in vitro filament assembly studies on headless, tailless, headless/tailless, and R/K L L E G E truncated mutants of keratin 5 and its partner keratin 14. Using varying conditions of ionic strength and pH, we examine effects of analogous K5 and K14 mutations on the stability of 10-nm filament structure. Using EM, we examine effects of mutations on the ability of subunits/protofibrils to (a) elongate and (b) laterally associate. Our results demonstrate that (a) tails of K5 and K14 are required for filament stabilization; (b) the head of K5, but not of K14, is required for filament elongation and lateral alignments; and (c) the R/K L L E G E domains are required for lateral alignments, but not for filament elongation.

The gene structure of Xenopus nuclear lamin A: a model for the evolution of A-type from B-type lamins by exon shuffling

Nuclear lamins are intermediate filament (IF) type proteins that form a fibrillar network underlying the inner nuclear membrane. The existence of multiple subtypes of lamins in vertebrates has been interpreted in terms of functional specialization during cell division and differentiation. The structure of a gene encoding an A-type lamin of Xenopus laevis was analysed. Comparison with that of a B-type lamin of the same species shows remarkable conservation of the exon/intron pattern. In both genes the last exon, only 9-12 amino acids in length, encodes the complete information necessary for membrane targeting of lamins, i.e. a ras-related CaaX motif. The lamin A specific extension of the tail domain is encoded by a single additional exon. The 5' boundary of this exon coincides with the sequence divergence between human lamins A and C, for which an alternative splice mechanism had previously been suggested. Arguments are presented suggesting that B-type lamins represent the ancestral type of lamins and that A-type lamins derived there from by exon shuffling. The acquisition of the new exon might explain the different fates of A- and B-types lamins during cell division.

Assembly and cell cycle dynamics of the nuclear lamina

The nuclear lamina is a karyoskeletal structure composed of intermediate filament type proteins. It underlies the inner nuclear membrane and confers mechanical stability to the nuclear envelope. In addition, it interacts with chromatin and may thereby participate in determining the three-dimensional organization of the interphase nucleus. During mitosis, the nuclear lamina is transiently disassembled, most probably through hyperphosphorylation of lamin proteins by the protein kinase p34cdc2, a key regulator of the eukaryotic cell cycle. Mitotic disassembly of the lamina is necessary but not sufficient for nuclear envelope breakdown. Electron microscopic analyses have begun to provide insights into the principles that govern lamina assembly in vitro, and sequence motifs required for targeting newly synthesized lamins to the nuclear envelope have been identified. Of particular interest, lamins were shown to undergo a type of hydrophobic modification known as isoprenylation. Finally, recent studies addressing the nature of lamin-chromatin interactions may provide the basis for elucidating the role of lamins in organizing the distribution of interphase chromatin.

An extended view of nuclear lamin structure, function, and dynamics

Molecularly-based studies of nuclear lamins have progressed at a rapid rate in the last decade. However, we still have no answer to the most important question: what are the functions of lamins? In this review we describe recent experiments which challenge traditional views of lamin function and structure. These surprising results indicate that much lamin functionality remains to be discovered, and that more global approaches to lamin structure and function are especially appropriate at this time.

Phorbol esters induce transient changes in the accessibility of the carboxy-terminal domain of nuclear lamin A

Treatment of human epithelial cells in culture with phorbol esters (TPA) gives rise to a transient and reversible loss of accessibility to antibodies of the nonhelical carboxy-terminal domain of nuclear lamin A that distinguishes it from lamin C. No change in the accessibility of epitopes present in the common domain of lamins A and C was observed. Loss of accessibility of lamin A was not due to proteolytic degradation nor to modification of the isoelectric point of lamin A and did not depend upon protein kinase C activation nor protein synthesis. Perturbation of desmosome organization by growth in low calcium blocked the effect of TPA on lamin A. Prolonged exposure to nocodazole, one of the effects of which is a perinuclear collapse of intermediate filaments, also blocked the effect of TPA on lamin A. These results suggest that the initial target of TPA may be at the level of cell-cell contacts and that the perturbation induced by TPA may be propagated via the structural link formed by intermediate filaments between the cell surface and the nucleus, giving rise to a change in conformation of the carboxy-terminal domain of lamin A or to an interaction of this domain with another nuclear component. These results form the basis for the hypothesis that the interphase nuclear lamina may play an active role in the process of mechanochemical signal transduction.

Nuclear envelope structure

The past 18 months have seen significant advances in our knowledge of the constituents of the nuclear envelope, their interactions during interphase and the mechanisms involved in their mitotic dynamics. Although most of the new data are in general agreement with, and contribute detail to, our traditional image of the nuclear envelope, a few observations appear to mark the beginning of new and important directions in research.

Mitogen-activated protein kinases phosphorylate nuclear lamins and display sequence specificity overlapping that of mitotic protein kinase p34cdc2

Members of the mitogen-activated protein (MAP) kinase family are implicated in mediating entry of cells into the cell cycle, as well as passage through meiotic M phase. These kinases have attracted much interest because their activation involves phosphorylation on both tyrosine and threonine residues, but little is known about their physiological targets. In this study, two distinct members of the MAP kinase family (p44mpk and p42mapk) are shown to phosphorylate chicken lamin B2 at a single site identified as Ser16. Moreover, these MAP kinases cause depolymerization of in-vitro-assembled longitudinal lamin head-to-tail polymers. Ser16 was previously shown to be phosphorylated during mitosis in vivo, and to be a target of the mitotic protein kinase p34cdc2 in vitro. Accordingly, lamins were proposed to be direct in vivo substrates of p34cdc2. This proposal is supported by quantitative analyses indicating that lamin B2, when assayed in vitro, is a substantially better substrate for p34cdc2 than for MAP kinases. Nevertheless, a physiological role of MAP kinases in lamin phosphorylation is not excluded. The observation that members of the MAP kinase family display sequence specificities overlapping that of p34cdc2 raises the possibility that some of the purported substrates of p34cdc2 may actually be physiological substrates of MAP kinases.

The NUF1 gene encodes an essential coiled-coil related protein that is a potential component of the yeast nucleoskeleton

In an attempt to identify structural components of the yeast nucleus, subcellular fractions of yeast nuclei were prepared and used as immunogens to generate complex polyclonal antibodies. One such serum was used to screen a yeast genomic lambda gt11 expression library. A clone encoding a gene called NUF1 (for nuclear filament-related) was identified and extensively characterized. Antibodies to NUF1 fusion proteins were generated, and affinity-purified antibodies were used for immunoblot analysis and indirect immunofluorescence localization. The NUF1 protein is 110 kD in molecular mass and localizes to the yeast nucleus in small granular patches. Intranuclear staining is present in cells at all stages of the cell cycle. The NUF1 protein of yeast is tightly associated with the nucleus; it was not removed by extraction of nuclei with nonionic detergent or salt, or treatment with RNAse and DNAse. Sequence analysis of the NUF1 gene predicts a protein 945 amino acids in length that contains three domains: a large 627 residue central domain predicted to form a coiled-coil structure flanked by nonhelical amino-terminal and carboxy-terminal regions. Disruption of the NUF1 gene indicates that it is necessary for yeast cell growth. These results indicate that NUF1 encodes an essential coiled-coil protein within the yeast nucleus; we speculate that NUF1 is a component of the yeast nucleoskeleton. In addition, immunofluorescence results indicate that mammalian cells contain a NUF1-related nuclear protein. These data in conjunction with those in the accompanying manuscript (Yang et al., 1992) lead to the hypothesis that an internal coiled-coil filamentous system may be a general structural component of the eukaryotic nucleus.

NuMA: an unusually long coiled-coil related protein in the mammalian nucleus

A bank of 892 autoimmune sera was screened by indirect immunofluorescence on mammalian cells. Six sera were identified that recognize an antigen(s) with a cell cycle-dependent localization pattern. In interphase cells, the antibodies stained the nucleus and in mitotic cells the spindle apparatus was recognized. Immunological criteria indicate that the antigen recognized by at least one of these sera corresponds to a previously identified protein called the nuclear mitotic apparatus protein (NuMA). A cDNA which partially encodes NuMA was cloned from a lambda gt11 human placental cDNA expression library, and overlapping cDNA clones that encode the entire gene were isolated. DNA sequence analysis of the clones has identified a long open reading frame capable of encoding a protein of 238 kD. Analysis of the predicted protein sequence suggests that NuMA contains an unusually large central alpha-helical domain of 1,485 amino acids flanked by nonhelical terminal domains. The central domain is similar to coiled-coil regions in structural proteins such as myosin heavy chains, cytokeratins, and nuclear lamins which are capable of forming filaments. Double immunofluorescence experiments performed with anti-NuMA and antilamin antibodies indicate that NuMA dissociates from condensing chromosomes during early prophase, before the complete disintegration of the nuclear lamina. As mitosis progresses, NuMA reassociates with telophase chromosomes very early during nuclear reformation, before substantial accumulation of lamins on chromosomal surfaces is evident. These results indicate that the NuMA proteins may be a structural component of the nucleus and may be involved in the early steps of nuclear reformation during telophase.

Assembly-disassembly of the nuclear lamina

Recently, progress in the study of lamins has been made in three areas: signals required for targetting newly synthesized lamins to the correct subnuclear compartment have been identified; information on lamina assembly has been obtained from in vitro studies using bacterially expressed proteins; and a mechanistic explanation for how the nuclear lamina is diassembled at the onset of mitosis is emerging.

The role of the head and tail domain in lamin structure and assembly: analysis of bacterially expressed chicken lamin A and truncated B2 lamins

Nuclear lamins like cytoplasmic intermediate filament proteins exhibit a characteristic tripartite domain structure with a segmented alpha-helical rod domain flanked by an N-terminal head and a C-terminal tail domain. To examine the influence of the head and tail domains on the structure and assembly properties of nuclear lamins, we have engineered "headless," "tailless," and "rod" chicken lamin B2 cDNAs and expressed them in Escherichia coli. A full-length chicken lamin A cDNA was also expressed in E. coli, and the recombinant protein compared with the structure and assembly properties of full-length chicken lamin B2 (E. Heitlinger et al. (1991) J. Cell Biol. 113, 485-495). As with lamin B2, at their first level of structural organization, lamin A and the headless lamin B2 formed myosin-like dimers consisting of a 51- to 52-nm-long tail flanked by two globular heads at one end. Similarly, the tailless and rod lamin B2 fragments formed tropomyosin-like dimers consisting of a 51 to 52-nm-long rod. In contrast to the lateral mode of association of cytoplasmic IF dimers into four-chain tetramers, at their second level of structural organization, lamin A dimers, just as lamin B2 dimers (E. Heitlinger et al. (1991) J. Cell Biol. 113, 485-495), associated longitudinally to form polar head-to-tail polymers. Whereas dimers made of the truncated B2 headless and rod lamins had lost their propensity to associate head-to-tail, tailless lamin B2 dimers revealed an enhanced head-to-tail association. Finally, at their third level of structural organization, rather than assembling into stable 10-nm filaments, both lamin A and the three truncated B2 lamins formed paracrystalline arrays exhibiting distinct transverse banding patterns with axial repeats of either 24 or 48-49 nm depending on the species.

Interaction of Xenopus lamins A and LII with chromatin in vitro mediated by a sequence element in the carboxyterminal domain

Morphological data suggest an interaction of the nuclear lamina with chromatin which markedly changes during the cell cycle. To study the molecular basis of this interaction we developed a novel lamin/chromatin binding assay that quantitated the binding of soluble, radiolabeled lamins to minichromosomes assembled in Xenopus laevis oocyte nuclear extracts. Lamins were derived from couple in vitro transcription and translation of the corresponding cDNAs. Chromatin binding was detected by monitoring the cofractionation with assembled minichromosomes in gel filtration and sucrose gradient centrifugation. Binding of lamins to chromatin increased with chromatin concentration and was accompanied by lamin polymerization. Lamins of the A-(Xenopus LA and human LC) as well as the B-type (Xenopus LI and LII) showed strikingly different chromatin binding capacities. Lamins A and LII bound efficiently of lamins LI and LC was detected. Using site-directed mutagenesis, we were able to define carboxy-terminal sequence elements of LA and LII required for the observed lamin/chromatin interaction that are rich in serine, threonine, and glycine residues. Competition experiments with a synthetic peptide containing the chromatin binding motif of lamin A corroborate the importance of these sequence elements in the lamin/chromatin interaction.