Reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p involves the nuclear envelope

The reassembly of peroxisomes in Hansenula polymorpha pex3 cells on reintroduction of Pex3p was examined. Using a Pex3-green fluorescent protein (Pex3-GFP) fusion protein, expressed under the control of an inducible promoter, it was observed that, initially on induction of Pex3-GFP synthesis, GFP fluorescence was localized to the endoplasmic reticulum and the nuclear envelope. Subsequently, a single organelle developed per cell that increased in size and multiplied by division. At these stages, GFP fluorescence was confined to peroxisomes. Fractionation experiments on homogenates of pex3 cells, in which the endoplasmic reticulum and nuclear envelope were marked with GFP, identified a small amount of GFP in peroxisomes present in the initial stage of peroxisome reassembly. Our data suggest a crucial role for the endoplasmic reticulum/nuclear envelope in peroxisome reintroduction on complementation of pex3 cells by the PEX3 gene.     

OSBP-related protein 8 (ORP8) interacts with Homo sapiens sperm associated antigen 5 (SPAG5) and mediates oxysterol interference of HepG2 cell cycle

We earlier identified OSBP-related protein 8 (ORP8) as an endoplasmic reticulum/nuclear envelope oxysterol-binding protein implicated in cellular lipid homeostasis, migration, and organization of the microtubule cytoskeleton. Here, a yeast two-hybrid screen identified Homo sapiens sperm associated antigen 5 (SPAG5)/Astrin as interaction partner of ORP8. The putative interaction was further confirmed by pull-down and co-immunoprecipitation assays. ORP8 did not colocalize with kinetochore-associated SPAG5 in mitotic HepG2 or HuH7 cells, but overexpressed ORP8 was capable of recruiting SPAG5 onto endoplasmic reticulum membranes in interphase cells. In our experiments, 25-hydroxycholesterol (25OHC) retarded the HepG2 cell cycle, causing accumulation in G2/M phase; ORP8 overexpression resulted in the same phenotype. Importantly, ORP8 knock-down dramatically inhibited the oxysterol effect on HepG2 cell cycle, suggesting a mediating role of ORP8. Furthermore, knock-down of SPAG5 significantly reduced the effects of both ORP8 overexpression and 25OHC on the cell cycle, placing SPAG5 downstream of the two cell-cycle interfering factors. Taken together, the present results suggest that ORP8 may via SPAG5 mediate oxysterol interference of the HepG2 cell cycle.     

Subcellular distribution and regulation of hepatic bilirubin UDP-glucuronyltransferase

We have investigated the subcellular location and regulation of hepatic bilirubin UDP-glucuronyltransferase, which has been presumed to be located largely in the smooth endoplasmic reticulum. Purity of subcellular membrane fractions isolated from rat liver was assessed by electron microscopy and marker enzymes. Bilirubin UDP-glucuronyltransferase activity was measured by radiochemical assay using a physiologic concentration of [14C]bilirubin, and formation rates of bilirubin diglucuronide and monoglucuronides (C-8 and C-12 isomers) were determined. Activity of the enzyme was widely distributed in subcellular membranes, the majority being found in smooth and rough endoplasmic reticulum, with small amounts in nuclear envelope and Golgi membranes. No measurable activity was found in plasma membranes or in cytosol. Synthesis of bilirubin diglucuronide as a percentage of total conjugates and the ratio of C-8/C-12 bilirubin monoglucuronide isomers formed were comparable in all membranes, suggesting that the same enzyme is present in all locations. However, the regulation of bilirubin UDP-glucuronyltransferase activity differed among intracellular membranes; enzyme activity measured in the presence of the allosteric effector uridine 5'-diphospho-N-acetylglucosamine exhibited latency in smooth endoplasmic reticulum and Golgi membranes, but not in rough endoplasmic reticulum and nuclear envelope. Since rough membranes comprise 60% of hepatocyte endoplasmic reticulum and bilirubin UDP-glucuronyltransferase activity in vitro is maximal in this membrane fraction under presumed physiologic conditions, it is likely that the rough endoplasmic reticulum represents the major site of bilirubin glucuronidation in hepatocytes.     

Role of the nuclear envelope in synthesis, processing, and transport of membrane glycoproteins

The outer nuclear membrane is morphologically similar to rough endoplasmic reticulum. The presence of ribosomes bound to its cytoplasmic surface suggests that it could be a site of synthesis of membrane glycoproteins. We have examined the biogenesis of the vesicular stomatitis virus G protein in the nuclear envelope as a model for the biogenesis of membrane glycoproteins. G protein was present in nuclear membranes of infected Friend erythroleukemia cells immediately following synthesis and was transported out of nuclear membranes to cytoplasmic membranes with a time course similar to transport from rough endoplasmic reticulum (t 1/2 = 5-7 min). Temperature-sensitive mutations in viral membrane proteins which block transport of G protein from endoplasmic reticulum also blocked transport of G protein from the nuclear envelope. Friend erythroleukemia cells and NIH 3T3 cells differed in the fraction of newly synthesized G protein found in nuclear membranes, apparently reflecting the relative amount of nuclear membrane compared to endoplasmic reticulum available for glycoprotein synthesis. Nuclear membranes from erythroleukemia cells appeared to have the enzymatic activities necessary for cleavage of the signal sequence and core glycosylation of newly synthesized G protein. Signal peptidase activity was detected by the ability of detergent-solubilized membranes of isolated nuclei to correctly remove the signal sequence of human preplacental lactogen. RNA isolated from the nuclear envelope was highly enriched for G protein mRNA, suggesting that G protein was synthesized on the outer nuclear membrane rather than redistributing to nuclear membranes from endoplasmic reticulum before or during cell fractionation. These results suggest a mechanism for incorporation of membrane glycoproteins into the nuclear envelope and suggest that in some cell types the nuclear envelope is a major source of newly synthesized membrane glycoproteins.     

THE ULTRASTRUCTURE OF CRUCIFORM NUCLEAR DIVISION IN SOROSPHAERA VERONICAE (PLASMODIOPHOROMYCETE)

Vegetative nuclear divisions in cystosoral Plasmodia from the shoot system of Sorosphaera veronicae Schroeter were studied with standard transmission electron microscopy. Each metaphase nucleus forms a cruciform configuration as the persistent nucleolus elongates perpendicularly to chromatin aligned on the equatorial plate. The nuclear envelope remains intact during metaphase and anaphase. Each spindle pole consists of a fenestrated nuclear envelope with an exteriorly situated centriole and closely associated endoplasmic reticulum. Intranuclear membranous vesicles occur within metaphase and anaphase nuclei and are closely associated with chromatin and nuclear envelope. Microtubules pass from centrioles into the nucleus and are also attached to chromatin at kinetochores.     

Effects of brefeldin A on the Golgi apparatus, the nuclear envelope, and the endoplasmic reticulum in a green alga,Scenedesmus acutus

Summary The effects of brefeldin A (BFA) on the structure of the Golgi apparatus, the nuclear envelope, and the endoplasmic reticulum (ER), and on the thiamine pyrophosphatase (TPPase) activity in these organelles were examined in a green alga,Scenedesmus acutus, to obtain evidence for the existence of a retrograde transport from the Golgi apparatus to the ER via the nuclear envelope. InScenedesmus, Golgi bodies are situated close to the nuclear envelope throughout the cell cycle and receive the transition vesicles not directly from the ER, but from the nuclear envelope. BFA induced the disassembly of Golgi bodies and an increase in the ER cisternae at the trans-side of decomposed Golgi bodies in interphase cells and multinuclear cells before septum formation. The accumulated ER cisternae connected to the nuclear envelope at one part. TPPase activity was detected in all cisternae of Golgi bodies, but not in the nuclear envelope or the ER in nontreated cells. On the contrary, in BFA-treated cells, TPPase activity was detected in the nuclear envelope and the ER in addition to the decomposed Golgi bodies. When septum-forming cells were treated with BFA, the disassembly of Golgi bodies was less than that in interphase cells, and TPPase activity was detected in the Golgi cisternae but not in the nuclear envelope or the ER. These results suggest mat BFA blocks the anterograde transport from the nuclear envelope to the Golgi bodies but does not block the retrograde transport from the Golgi bodies to the nuclear envelope in interphase and multinuclear cells.Abbreviations BFA brefeldin A - ER endoplasmic reticulum - TPPase thiamine pyrophosphatase     

Cytochemical characterization of the endomembranous system during oocyte secondary growth in Serrasalmus spilopleura (Teleostei, Characiformes, Characidae)

The endomembranous system of Serrasalmus spilopleura oocyte secondary growth was analysed using structural and ultrastructural cytochemical techniques. In vitellogenic oocytes, the endoplasmic reticulum components, the nuclear envelope intermembranous space, some Golgi dictiossomes, lysosomes, yolk granules, regions of the egg envelope and sites of the follicle cells react to acid phosphatase detection (AcPase). The cortical alveoli, some heterogeneous cytoplasmic structures, regions of the egg envelope, and sites of the follicle cells are strongly contrasted by osmium tetroxide and zinc iodide impregnation (ZIO). The endoplasmic reticulum components, some vesicles, and sites of the follicle cells also react to osmium tetroxide and potassium iodide impregnation (KI). The biosynthetic pathway of lysosomal proteins, such as acid phosphatase, required for vitellogenesis, involves the endoplasmic reticulum, Golgi complex, vesicles with inactive hydrolytic enzymes, and, finally, lysosomes. In S. spilopleura oocytes at secondary growth, the endomembranous system takes part in the production of the enzymes needed for vitellogenesis, and in the metabolism of yolk exogenous components (AcPase detection). The endomembranous system compartments also show reduction capacity (KI reaction) and are involved in the metabolism of proteins rich in SH‐groups (ZIO reaction).     

抗寒锻炼中不同抗寒性小麦细胞膜糖蛋白的细胞化学研究

本研究根据植物细胞的特点,修改了在动物和人体细胞方面立的酶标Con A的电镜细胞化学方法,成功地展现了2个不同抗寒性冬小麦品种幼苗在抗寒锻炼和脱锻炼过程中细胞膜系统上糖蛋白的分布动态,显示与Con A连接的标志酶-辣根过氧化物酶活性的反应产物呈颗粒状分散分布在质膜、内质网、核膜及液泡膜的一些部位上,揭示糖蛋白在冬小麦细胞膜系统上的分布似有其特定的位点。经抗寒锻炼后,强抗寒性品种燕大1817细胞内的糖蛋白在内质网和核膜上的分布量明显地增加;同时,几乎所有的胞间连丝通道中都有糖蛋白的分布。脱锻炼后,内质网和核膜上的糖蛋白分布量又减少,胞间连丝通道中的糖蛋白也消失,基本上回复到抗寒锻炼前的分布状态。抗寒性弱的冬小麦品种郑州39-1幼苗在同样的抗寒锻炼和脱锻炼过程中不产生这些明显的变化。这些结果说明,抗寒锻炼中内质网和核膜上糖蛋白分布量的增加,以及糖蛋白输入胞间连丝的动态变化是与植物抗寒力的提高和保持稳定密切相关的。     

The nuclear reticulum in placental cells ofLilium regale is a part of the endomembrane system

Summary Placental cells line the ovarian transmitting tract inLilium regale and produce exudates for secretion. Sections through the highly lobed nuclei of these cells reveal the presence of membrane profiles which form vesicles with varying dimensions in cross section. Computer reconstruction of the nucleus reveals that the vesicle profiles form a complex reticulum of tubular cisternae, which spans the whole nucleus, enclosing a maze of continuous lumen space. Connections between the vesicles and the inner nuclear envelope are visible at various points along the nuclear envelope. This complex network of tubules which constitutes the reticulum arises from the inner nuclear membrane. The nuclear reticulum dramatically increases the inner-envelope surface area, comprising 82% of the total membrane perimeter of inner nuclear envelope and nuclear reticulum. The inner nuclear envelope invaginates into the nucleus forming the nuclear reticulum and the outer nuclear envelope evaginates into the endoplasmic reticulum (ER), indicating that there is a continuity between the lumens of the nuclear reticulum and the ER. The nuclear reticulum is labelled with zinc iodide-osmium tetroxide, a staining pattern identical to that seen in the ER. Positive reaction to the zinc iodide-osmium tetroxide indicates that the nuclear reticulum is a site for Ca²⁺ deposition. The nuclear reticulum forms an extension of the endomembrane system which reaches deep into the nucleoplasm. The lumenal continuity of this system means that there is a channel for communication from the cytoplasm into the nucleoplasm, and that this channel sequesters calcium.Abbreviations ER endoplasmic reticulum - TEM transmission electron microscope - ZIO zinc iodide-osmium tetroxide     

ZOOSPORE STRUCTURE OF THE MYCOPARASITIC CHYTRID CAULOCHYTRIUM PROTOSTELIOIDES OLIVE

The subcellular organization of zoospores released from sessile, parasitic sporangia of Caulochytrium protostelioides was studied with light and electron microscopy. A single flagellum is posteriorly directed but laterally inserted into the cylindrical motile zoospore. A striated rhizoplast attaches the proximal end of the kinetosome to a specialized region of the nuclear envelope. A system of rough endoplasmic reticulum, smooth endoplasmic reticulum, dictyosomes and bristle-coated vesicles are associated with the one to several pulsating vacuoles typically located near the flagellar apparatus. The microbody-lipid globule complex (MLC) comprises one to many lipid globules. An extensive microbody branches around each lipid globule and encloses a portion of the rhizoplast. A reticulum of smooth surfaced cisternae interdigitates among the branches of the complex microbody, and cisternae are opposed to the surface of lipid globules opposite the microbodies. Mitochondria with predominantly circular profiles are scattered throughout the zoospore body, but several are always adjacent to the microbody, and hence, are also part of the MLC. Ribosomes are uniformly distributed throughout the zoospore, and one to several cisternae of rough endoplasmic reticulum are adjacent to the nuclear envelope. Zoospores of C. protostelioides are similar to several other chytrid zoospores, which also have the same type of microbody-lipid globule complex, but yet are structurally distinct from any other chytrid zoospore.     

Localization of adenylate cyclase activity in the tissues of an intact planarian Dugesia lugubris (O. Schmidt)

Summary Adenylate cyclase was localized in tissues of an intact planarian Dugesia lugubris (O. Schmidt) by ultracytochemical methods. The enzyme was found in epithelium, muscles, secretory cells (mucous), and rhabdites forming cells and neoblasts. Adenylate cyclase occurred on the external side of cell membranes in cisterns of the endoplasmic reticulum, nuclear envelope and mitochondria. The problems of ultracytochemical localization of AC are discussed.     

Post-fertilization functions of annulate lamellae in the periphery of the egg of Spirorbis borealis (Daudin) (Serpulidae=Annelida)

Summary Post-fertilization changes in the periphery of the egg of Spirorbis borealis are described. The nuclear envelope and cell membrane are formed by the coalescence of short strands of membrane perhaps produced by the Golgi complex. Annulate lamellae are abundant in the forming cells giving rise to endoplasmic reticulum and apparently contributing to genesis of mitochondria. This possibility is discussed.We are grateful to Professor E. W. Knight-Jones in whose department the work was carried out.     

Plant nuclei can contain extensive grooves and invaginations

Plant cells can exhibit highly complex nuclear organization. Through dye-labeling experiments in untransformed onion epidermal and tobacco culture cells and through the expression of green fluorescent protein targeted to either the nucleus or the lumen of the endoplasmic reticulum/nuclear envelope in these cells, we have visualized deep grooves and invaginations into the large nuclei of these cells. In onion, these structures, which are similar to invaginations seen in some animal cells, form tubular or planelike infoldings of the nuclear envelope. Both grooves and invaginations are stable structures, and both have cytoplasmic cores containing actin bundles that can support cytoplasmic streaming. In dividing tobacco cells, invaginations seem to form during cell division, possibly from strands of the endoplasmic reticulum trapped in the reforming nucleus. The substantial increase in nuclear surface area resulting from these grooves and invaginations, their apparent preference for association with nucleoli, and the presence in them of actin bundles that support vesicle motility suggest that the structures might function both in mRNA export from the nucleus and in protein import from the cytoplasm to the nucleus.     

THE FINE STRUCTURE OF THE NUCLEUS AND NUCLEAR ASSOCIATIONS OF DEVELOPING CARPOSPORANGIA IN POLYSIPHONIA NOVAE-ANGLIAE (RHODOPHYTA)1

A high degree of activity of the nuclei in the developing carposporangia of the red alga Polysiphonia novae-angliae Taylor is described. Profiles of the nucleus are greatly convoluted, resulting in a much increased surface area. Regions where endoplasmic reticulum substitutes for the nuclear envelope are frequently observed. Various cytoplasmic reserves are associated with the nucleus during carposporangium maturation. Lipid bodies, fibrillar bodies, and striated vesicles (or cylindrical bodies) may occur within the nucleoplasm beneath areas of the nuclear envelope substituted by ER. Granules of Floridean starch are observed in proximity to the outer surface of these same areas as well as the nuclear membrane. The homologous nature of the nuclear envelope with the ER is stressed. The role of the nucleus as being actively involved in the synthesis of materials is suggested.     

Annulate lamellae in plant cells: Formation during microsporogenesis and pollen development in Canna generalis Bailey

Summary The occurrence of stacked annulate lamellae is documented for a plant cell system, namely for pollen mother cells and developing pollen grains of Canna generalis. Their structural subarchitecture and relationship to endoplasmic reticulum (ER) and nuclear envelope cisternae is described in detail. The results demonstrate structural homology between plant and animal annulate lamellae and are compatible with, though do not prove, the view that annulate lamellar cisternae may originate as a degenerative form of endoplasmic reticulum.     

Differentiation and Ultrastructure of the Protophloem Sieve Elements of Minor Veins in the Maize Leaves: Changes in the Protoplast

Protophloem sieve element differentiation in the minor veins of the maize ( Zea mays L. ) leaves was first evidenced as an increase of the wall thickness, which began in the comers of the cell and then extended to other parts of the wall, and the appearance of long rough endoplasmic reticulum cistemae distributed throughout the cytoplasm, and then the presence of characteristic crystalloid inclusions within the plastids. As differentiation progressed, long cisternae of rough endoplasmic reticulum appeared to transform into shorter forms and eventually aggregated into small stacks, losing their ribosomes during the process. The nuclei degenerated, although frequently persisted until very late in differentiation the stages of maturation, as darkly stained amorphous aggregates surrounded by double nuclear envelope or only inner membrane of nuclear envelope. Subsequently, the nuclear envelope collapsed and became discontinuous. At the beginning of nuclear degeneration the perinuclear spaces were partly dilated and sometimes the outer nuclear envelope in the dilated portions then ruptured, and was accompanied by the disappearance of the cytoplasmic portion near it. During the peried of nuclear degeneration, in addition to the endoplasmic reticulum, plastids and mitochondria underwent structural modification, while components such as ribosomes, cytoplasmic ground substances, vacuoles and dictyosomes disintegrated and disappeared. At maturity, the surviving protoplasmic components, including plasmalemma, mitochondria, small stacked smooth endoplasmic reticulum and P-type plastids with crystalloids, became parietal in position. As differentiation of adjacent metaphloem sieve elements proceeded, the protoplasmic components of the mature protophloem sieve elements progresively degenerated and finally obliterated.     

Ultrastructural characteristics of insect corpora allata in relation to larval diapause

Summary The ultrastructure of active and inactive corpora allata from last instar larvae of the southwestern corn borer, Diatraea grandiosella, was examined. Active glands were obtained from pre-, early, and mid-diapausing larvae; inactive ones from late and non-diapausing larvae. Each gland contains 13 to 18 cells which have the following common features: well developed smooth endoplasmic reticulum, scattered rough endoplasmic reticulum and free ribosomes, microtubules, vacuolated nucleoli, and interlocking plasma membranes. The gland contains intercellular deposits, and is supplied by regular and neurosecretory axons.Special ultrastructural features of the corpus allatum from the five groups of larvae examined were as follows: pre-diapause: extensive vesicular smooth endoplasmic reticulum, numerous cup-shaped mitochondria and Golgi bodies with stacked cisterns and vesicles, few small lipid droplets, large nuclei with dispersed chromatin, absence of lysosomes; early diapause: stacked, whorled, and vesicular smooth endoplasmic reticulum of equal abundance, numerous rod-shaped mitochondria, some Golgi bodies but without distinct stacks of cisterns, few lipid droplets and lysosomes, chromatin dispersed and also attached to the nuclear envelope; mid-diapause: similar to early diapause except for the presence of more stacked, smooth endoplasmic reticulum, chromatin in large chunks mostly attached to the nuclear envelope; late diapause: whorled smooth endoplasmic reticulum and rod-shaped mitochondria predominating, complicated Golgi bodies with stacks of cisterns and large empty sacs, few large lipid droplets, some lysosomes containing mainly whorled bodies, chromatin in large chunks attached to the nuclear envelope; non-diapause: similar to late diapause except for less extensive smooth endoplasmic reticulum, more abundant mitochondria, fewer intercellular deposits. Although these observations suggest that the smooth endoplasmic reticulum, and possibly mitochondria, and Golgi bodies are involved in juvenile hormone production, specific sites of synthesis or storage of the hormone were not revealed.Supported in part by grant no. PCM 74-18155 A01 from the National Science Foundation. Contribution from the Missouri Agricultural Experiment Station as journal series no. 8234. We thank Ms. L. Yin for her skillful assistance, and Dr. M.F. Brown of the College of Agriculture Electron Microscope Facility for his advice and the use of equipment.     

Membrane anchoring domain of herpes simplex virus glycoprotein gB is sufficient for nuclear envelope localization.

We have used the glycoprotein gB of herpes simplex virus type 1 (gB-1), which buds from the inner nuclear membrane, as a model protein to study localization of membrane proteins in the nuclear envelope. To determine whether specific domains of gB-1 glycoprotein are involved in localization in the nuclear envelope, we have used deletion mutants of gB-1 protein as well as chimeric proteins constructed by replacing the domains of the cell surface glycoprotein G of vesicular stomatitis virus with the corresponding domains of gB. Mutant and chimeric proteins expressed in COS cells were localized by immunoelectron microscopy. A chimeric protein (gB-G) containing the ectodomain of gB and the transmembrane and cytoplasmic domains of G did not localize in the nuclear envelope. When the ectodomain of G was fused to the transmembrane and cytoplasmic domains of gB, however, the resulting chimeric protein (G-gB) was localized in the nuclear envelope. Substitution of the transmembrane domain of G with the 69 hydrophobic amino acids containing the membrane anchoring domain of gB allowed the hybrid protein (G-tmgB) to be localized in the nuclear envelope, suggesting that residues 721 to 795 of gB can promote retention of proteins in the nuclear envelope. Deletion mutations in the hydrophobic region further showed that a transmembrane segment of 21 hydrophobic amino acids, residues 774 to 795 of gB, was sufficient for localization in the nuclear envelope. Since wild-type gB and the mutant and chimeric proteins that were localized in the nuclear envelope were also retained in the endoplasmic reticulum, the membrane spanning segment of gB could also influence retention in the endoplasmic reticulum.     

Characterization of a membrane-associated 3,3',5-triiodo-L-thyronine binding protein by use of monoclonal antibodies

Four mouse hybridoma cell lines have been isolated which secrete antibodies to the membrane-associated thyroid hormone binding protein (Mr 55,000) from human epidermoid carcinoma A431 cells. J6 is rat specific; J2 is human and monkey specific; J8 and J9 have a wider specificity and react with similar thyroid hormone binding proteins (p55) from human, monkey, rat, and hamster. None of these antibodies reacts with mouse cells. J2, J6, and J9 are of the IgG1k class, and J8 is an IgAk antibody. p55 was characterized by using these monoclonal antibodies. It is not posttranslationally processed by glycosylation, phosphorylation, or sulfation. It has a cellular degradation rate t1/2 approximately equal to 3.2 h. Using immunofluorescence and electron microscopic immunocytochemistry, p55 was found to be associated with the lumenal face of the endoplasmic reticulum and nuclear envelope. When cell homogenates were prepared, significant amounts of p55 were released into the 110000g supernatant, indicating that p55 is loosely associated with the endoplasmic reticulum and nuclear envelope.     

Retention and mobility of the mammalian lamin B receptor in the plant nuclear envelope

Background information. In a previous study, we showed that GFP (green fluorescent protein) fused to the N‐terminal 238 amino acids of the mammalian LBR (lamin B receptor) localized to the NE (nuclear envelope) when expressed in the plant Nicotiana tabacum. The protein was located in the NE during interphase and migrated with nuclear membranes during cell division. Targeting and retention of inner NE proteins requires several mechanisms: signals that direct movement through the nuclear pore complex, presence of a transmembrane domain or domains and retention by interaction with nuclear or nuclear‐membrane constituents. Results. Binding mutants of LBR—GFP were produced to investigate the mechanisms for the retention of LBR in the NE. FRAP (fluorescence recovery after photobleaching) analysis of mutant and wild‐type constructs was employed to examine the retention of LBR—GFP in the plant NE. wtLBR—GFP (wild‐type LBR—GFP) was shown to have significantly lower mobility in the NE than the lamin‐binding domain deletion mutant, which showed increased mobility in the NE and was also localized to the endoplasmic reticulum and punctate structures in some cells. Modification of the chromatin‐binding domain resulted in the localization of the protein in nuclear inclusions, in which it was immobile. Conclusions. As expression of truncated LBR—GFP in plant cells results in altered targeting and retention compared with wtLBR—GFP, we conclude that plant cells can recognize the INE (inner NE)‐targeting motif of LBR. The altered mobility of the truncated protein suggests that not only do plant cells recognize this signal, but also have nuclear proteins that interact weakly with LBR.