Three-dimensional structure of a membrane-microtubule complex

Keep in contact: multiple roles of endoplasmic reticulum-membrane contact sites and the organelle interaction network in plants

Functional regulation and structural maintenance of the different organelles in plants contribute directly to plant development, reproduction and stress responses. To ensure these activities take place effectively, cells have evolved an interconnected network amongst various subcellular compartments, regulating rapid signal transduction and the exchange of biomaterial. Many proteins that regulate membrane connections have recently been identified in plants, and this is the first step in elucidating both the mechanism and function of these connections. Amongst all organelles, the endoplasmic reticulum is the key structure, which likely links most of the different subcellular compartments through membrane contact sites (MCS) and the ER-PM contact sites (EPCS) have been the most intensely studied in plants. However, the molecular composition and function of plant MCS are being found to be different from other eukaryotic systems. In this article, we will summarise the most recent advances in this field and discuss the mechanism and biological relevance of these essential links in plants.

The connection of cytoskeletal network with plasma membrane and the cell wall

The cell wall provides external support of the plant cells, while the cytoskeletons including the microtubules and the actin filaments constitute an internal framework. The cytoskeletons contribute to the cell wall biosynthesis by spatially and temporarily regulating the transportation and deposition of cell wall components. This tight control is achieved by the dynamic behavior of the cytoskeletons, but also through the tethering of these structures to the plasma membrane. This tethering may also extend beyond the plasma membrane and impact on the cell wall, possibly in the form of a feedback loop. In this review, we discuss the linking components between the cytoskeletons and the plasma membrane, and/or the cell wall. We also discuss the prospective roles of these components in cell wall biosynthesis and modifications, and aim to provide a platform for further studies in this field.

The hemimastigophora (Hemimastix amphikineta nov. gen., nov. spec.), a new protistan phylum from gondwanian soils

The morphology, morphogenesis and ultrastructure of Hemimastix amphikineta nov. gen., nov. spec, are described. This species occurred in some Australian and in 1 Chilean soil, but was absent from more than 1000 soil samples from Laurasian localities. Thus, it has probably a restricted Gondwanian distribution. Hemimastix amphikineta is a small (14-20 × 7-10 μn), colourless organism that looks distinctly Ciliophora-like because of its posteriorly located contractile vacuole and its 2 longitudinal somatic kineties each composed of about 12 cilia-like flagella. These 2 kineties are interposed between 2 large plicated and microtubule-bearing pellicular plates which are arranged inversely mirror-image like ("diagonal symmetry"). Hemimastix amphikineta has saccular to tubular mitochondrial cristae and complex extrusomes. It has 2 microtubular systems and a membranous sac associated with each kinetid. The nucleolus persists throughout nuclear division. A permanent cytostome-cytopharyngeal complex, pharyngeal rods, striated fibres, mastigonemes, and a paraflagellar rod are absent. This unique combination of characters dictates a very separate position for H. amphikineta within the known protists. Thus, the phylum Hemimastigophora nov. phylum (Hemimastigea nov. cl. and Hemimastigida nov. ord.), is established to include H. amphikineta and possibly Spironema multiciliatum Klebs, 1892. The structure of the pellicle and the nuclear apparatus of H. amphikineta indicate some relationship with the Euglenophyta. However, clear evidence for a certain affinity is lacking. Thus, the Hemimastigophora are placed in an incertae sedis position within the kingdom Protista Haeckel, 1866.

The microtubule lattice--dynamic instability of concepts

In the February 1995 issue of trends in CELL BIOLOGY, Linda Amos presented her view of our current understanding of the lattice structure of microtubules, 20 years after publication of the original paper describing the A- and B-lattices for flagellar microtubules. However, the question of the lattices of flagellar and cytoplasmic microtubules remains a matter for debate. In this article, Eckhard Mandelkow, Young-Hwa Song and Eva-Maria Mandelkow argue that the B-lattice is predominant, implying structural asymmetry for most microtubules.

Molecular characterization of a gene encoding a 29-kDa cytoplasmic protein of Babesia gibsoni and evaluation of its diagnostic potentiality

A cDNA expression library prepared from Babesia gibsoni merozoite mRNA was screened with B. gibsoni-infected dog serum. cDNA encoding 29-kDa protein was cloned and designated as the P29 gene. The complete nucleotide sequence of the P29 gene was 792 bp. Computer analysis suggested that the sequence of the P29 gene contained an open reading frame of 597 bp with a coding capacity of approximately 23.4 kDa and a single intron of 250 bp. The P29 protein had homology to Toxoplasma gondii cytoskeletal protein IMC1. Southern blot analysis indicated that the P29 gene was present as a single copy in the B. gibsoni genome. The native P29 protein of B. gibsoni with a molecular mass of 29 kDa was identified by Western blotting with anti-recombinant P29 mouse serum. Confocal laser microscopic analysis showed that the P29 protein was located on the cytoplasma of B. gibsoni merozoites. The recombinant P29 protein expressed in E. coli was used as an antigen in an enzyme-linked immunosorbent assay (ELISA). The ELISA was able to differentiate between B. gibsoni-infected dog serum and B. canis subspecies-infected dog serum or normal dog serum. Furthermore, the antibody response against the P29 protein was maintained during the chronic stage of infection in an experimentally infected dog, indicating that the recombinant P29 protein might be a useful diagnostic reagent for the detection of antibodies to B. gibsoni in dogs.

Subpellicular microtubules associate with an intramembranous particle lattice in the protozoan parasite Toxoplasma gondii

Application of Fourier analysis techniques to images of isolated, frozen-hydrated subpellicular microtubules from the protozoan parasite Toxoplasma gondii demonstrates a distinctive 32 nm periodicity along the length of the microtubules. A 32 nm longitudinal repeat is also observed in the double rows of intramembranous particles seen in freeze-fracture images of the parasite's pellicle; these rows are thought to overlie the subpellicular microtubules. Remarkably, the 32 nm intramembranous particle periodicity is carried over laterally to the single rows of particles that lie between the microtubule-associated double rows. This creates a two-dimensional particle lattice, with the second dimension at an angle of approximately 75 degrees to the longitudinal rows (depending on position along the length of the parasite). Drugs that disrupt known cytoskeletal components fail to destroy the integrity of the particle lattice. This intramembranous particle organization suggests the existence of multiple cytoskeletal filaments of unknown identity. Filaments associated with the particle lattice provide a possible mechanism for motility and shape change in Toxoplasma: distortion of the lattice may mediate the twirling motility seen upon host-cell lysis, and morphological changes observed during invasion.

Electrodynamics of microtubular motors: the building blocks of a new model

Microtubules are ubiquitous components of the cytoskeleton. They participate in many motility processes ranging from intracellular transport or chromosome movement during mitosis to ciliary and flagellar beating. The biophysical mechanism inherent in the generation and control of movement in all these motility phenomena has not yet been entirely elucidated. The authors propose a new model based on a charge transfer mechanism capable of shedding a new light on the molecular foundations of all motility processes. Electron transfer along the microtubular lattice is responsible for activation and control of all microtubule-associated ATPases (i.e. force generating enzymes). Microtubules are thus shown to be the basic motors of cell dynamics. The model is first applied to intracellular transport and ciliary and flagellar beating. Through two additional examples, the authors show the heuristic capabilities of the suggested hypothesis. The application of charge transfer control to the Protozoan Euglena gracilis leads to a plausible model capable of accounting for its phototactic response mechanism. Furthermore, the model allows a new interpretation of the electrophysiological response in vertebrate photoreceptors.

Cortical structure and function in euglenoids with reference to trypanosomes, ciliates, and dinoflagellates

The membrane skeletal complex (cortex) of euglenoids generates and maintains cell form. In this review we summarize structural, biochemical, physiological, and molecular studies on the euglenoid membrane skeleton, focusing specifically on four principal components: the plasma membrane, a submembrane layer (epiplasm), cisternae of the endoplasmic reticulum, and microtubules. The data from euglenoids are compared with findings from representative organisms of three other protist groups: the trypanosomes, ciliates, and dinoflagellates. Although there are significant differences in cell form and phylogenetic affinities among these groups, there are also many similarities in the organization and possibly the function of their cortical components. For example, an epiplasmic (membrane skeletal) layer is widely used for adding strength and rigidity to the cell surface. The ER/alveolus/amphiesmal vesicle may function in calcium storage and regulation, and in mediating assembly of surface plates. GPI-linked variable surface antigens are characteristic of both ciliates and the unrelated trypanosomatids. Microtubules are ubiquitous, and cortices in trypanosomes may relay exclusively on microtubules and microtubule-associated proteins for maintaining cell form. Also, in agreement with previous suggestions, there is an apparent preservation of many cortical structures during cell duplication. In three of the four groups there is convincing evidence that part or all of the parental cortex persists during cytokinesis, thereby producing mosaics or chimeras consisting of both inherited and newly synthesized cortical components.

Microtubule organization by cross-linking and bundling proteins

To understand microtubule function the factors regulating their spatial organization and their interaction with cellular organelles, including other microtubules, must be elucidated. Many proteins are implicated in these organizational events and the known consequences of their actions within the cell are increasing. For example, the function of microtubule bundles at the surfaces of polarized cells has recently received attention, as has the action in cortical rotation of a transient arrangement of microtubules found beneath the vegetal surface of fertilized frog eggs. The in vivo association of microtubules during early Xenopus oogenesis has added interest as microtubules bundled in cell-free extracts are protected against the action of a severing protein found in this animal. A 52 kDa F-actin bundling protein purified from Physarum polycephalum organizes microtubules and causes the cobundling of microtubules and microfilaments. These observations, in concert with others that are presented, emphasize the diversity within the family of microtubule cross-linking proteins. The challenge is to determine which proteins are relevant from a physiological perspective, to ascertain their molecular mechanisms of action and to describe how they affect cytoplasmic organization and cell function. To realize this objective, the proteins which cross-link and bundle microtubules must be investigated by techniques which reveal different but related aspects of their properties. Cloning and sequencing of genes for cross-linking proteins, their subcellular localization especially as microtubule-related changes in cell morphology are occurring and the application of genetic studies are necessary. Study of the neural MAP provides the best example of just how powerful current experimental approaches are and at the same time shows their limits. The neural MAP have long been noted for their enhancement of tubulin assembly and microtubule stability. Their spatial distribution has been studied during the morphogenesis of neural cells. Sequencing of cloned genes has revealed the functional domains of neural MAP including carboxy-terminal microtubule-binding sites. Similarities to microtubule binding proteins from other cell types stimulate interest in the neural MAP and further suggest their importance in microtubule organization. For example, MAP4 enjoys a wide cellular distribution and has microtubule-binding sequences very similar to those in the neural MAP. Moreover, the nontubulin proteins of marginal bands are immunologically related to neural MAP, indicating shared structural/functional domains. Even with these findings the mechanism by which neural MAP cross-link microtubules remains uncertain. Indeed, some researchers express doubt that microtubule cross-linking is actually a function of neural MAP in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Properties and topography of the major integral plasma membrane protein of a unicellular organism

The cellular distribution, membrane orientation, and biochemical properties of the two major NaOH-insoluble (integral) plasma membrane proteins of Euglena are detailed. We present evidence which suggests that these two polypeptides (Mr 68 and 39 kD) are dimer and monomer of the same protein: (a) Antibodies directed against either the 68- or the 39-kD polypeptide bind to both 68- and 39-kD bands in Western blots. (b) Trypsin digests of the 68- and 39-kD polypeptides yield similar peptide fragments. (c) The 68- and 39-kD polypeptides interconvert during successive electrophoresis runs in the presence of SDS and beta-mercaptoethanol. (d) The 39-kD band is the only major integral membrane protein evident after isoelectric focusing in acrylamide gels. The apparent shift from 68 to 39 kD in focusing gels has been duplicated in denaturing SDS gels by adding ampholyte solutions directly to the protein samples. The membrane orientation of the 39-kD protein and its 68-kD dimer has been assessed by radioiodination in situ using intact cells or purified plasma membranes. Putative monomers and dimers are labeled only when the cytoplasmic side of the membrane is exposed. These results together with trypsin digestion data suggest that the 39-kD protein and its dimer have an asymmetric membrane orientation with a substantial cytoplasmic domain but with no detectable extracellular region. Immunolabeling of sectioned cells indicates that the plasma membrane is the only cellular membrane with significant amounts of 39-kD protein. No major 68- or 39-kD polypeptide bands are evident in SDS acrylamide gels or immunoblots of electrophoresed whole flagella or preparations enriched in flagellar membrane vesicles, nor is there a detectable shift in any flagellar polypeptide in the presence of ampholyte solutions. These findings are considered with respect to the well-known internal crystalline organization of the euglenoid plasma membrane and to the potential for these proteins to serve as anchors for membrane skeletal proteins.

Organization of microtubules in centrosome-free cytoplasm

Many different cell types possess microtubule patterns which appear to be polarized and oriented, in part, by cytoplasmic factors not directly associated with a centrosome. Recently, we demonstrated that cytoplasmic extensions ("arms") of teleost melanophores will reorganize their microtubule population outward from their centers after surgical isolation (McNiven, M. A., M. Wang, and K. R. Porter. 1984. Cell. 37:753-765). In the study reported here, we examine microtubule dynamics within the centrosome-free fragments and find that, after severing, microtubule reorganization is initiated at the proximal (cut) end of an arm and migrates distally with the aggregated pigment mass until it becomes permanently positioned at the middle of the arm. Computer-aided image analysis demonstrates that this middle position is located at the arm centroid, implicating the action of a cytoplasmic gel in this process. Morphological studies of arms devoid of pigment reveal that microtubules do not emanate from a single site or structure within the centroid area, but from a more generalized region. Taken together, these findings suggest that factors distributed throughout cytoplasm participate in microtubule assembly and organization.

A 60-kDa cytoskeletal protein from Trypanosoma brucei brucei can interact with membranes and with microtubules

The cytoskeleton of eukaryotic cells is a major determinant of cellular architecture and of many cellular functions. In addition to or in place of the transcellular cytoskeleton, many eukaryotic cells also contain membrane-associated cytoskeletal structures (membrane skeletons), which are important for cellular structure and function. The membrane skeleton of the parasitic hemoflagellate Trypanosoma brucei consists of a dense array of singlet microtubules (subpellicular microtubules), which are tightly associated to the overlying cell membrane. This study reports the identification of a microtubule-associated protein from Trypanosoma brucei that constitutes a component of the link between this microtubular array and the cell membrane. The protein can bind in vitro both to microtubules and to membrane vesicles or liposomes. Furthermore, it can crosslink microtubules and membrane vesicles, suggesting that it exerts a similar function in the membrane skeleton.

Intermembrane linkage mediated by tubulin

Two membranes from brain lipids were formed in the presence of brain tubulin and their electrical potentials were simultaneously measured. When electrical pulses were applied across one of them, displacements of the potential of the other membrane were found even when the membranes were not in contact. This effect was observed only in the presence of polymerized tubulin. It was not found in the presence of depolymerized tubulin or in other control experiments. The findings suggest that the microtubule fiber networks may serve as an interconnecting system between membranes or membrane bounded compartments.

Chemical probes and possible targets for the induction of aneuploidy

There is an increasing interest in developing assay systems that are effective in detecting aneuploidy-producing agents. Because the current methodology for detecting aneuploidy is extremely varied, presently no comparisons of the validity and sensitivity of various assays can be made. This is due to a lack of sufficient data on the testing of the same compounds in multiple systems. Thus, there is an imminent need to select a few model compounds to be tested in all the available assays. This chapter discusses the rationale for the selection of model compounds. Approximately 30 compounds were identified as candidate compounds for various reasons. It is not our intention to discourage studies of compounds not discussed in this chapter. It is merely our effort to facilitate the final selection of a few model compounds to be used for comparative studies in diverse assays or for collaborative studies to determine interlaboratory variation of selected assays.

Disassembly and reconstitution of a membrane-microtubule complex

The cell membrane of the unicellular algae Distigma proteus is associated with arrays of parallel microtubules. Fragments of the membrane-microtubule complex have been isolated and partially purified. The microtubules were stable in vitro at room temperature as well as at 0 degree C, but were specifically and rapidly disassembled by Ca2+. After removal of all endogenous microtubules, the membrane-microtubule complex could be reassembled from brain microtubule protein and denuded Distigma membrane fragments. The readded microtubules bound in a fixed orientation, and only to those regions of membrane that are normally associated with microtubules in vivo.