Extensive Membrane System Research Articles

Dynamic framework for large-scale modeling of membranes and peripheral proteins.

In this chapter, we present a novel computational framework to study the dynamic behavior of extensive membrane systems, potentially in interaction with peripheral proteins, as an alternative to conventional simulation methods. The framework effectively describes the complex dynamics in protein-membrane systems in a mesoscopic particle-based setup. Furthermore, leveraging the hydrodynamic coupling between the membrane and its surrounding solvent, the coarse-grained model grounds its dynamics in macroscopic kinetic properties such as viscosity and diffusion coefficients, marrying the advantages of continuum- and particle-based approaches. We introduce the theoretical background and the parameter-space optimization method in a step-by-step fashion, present the hydrodynamic coupling method in detail, and demonstrate the application of the model at each stage through illuminating examples. We believe this modeling framework to hold great potential for simulating membrane and protein systems at biological spatiotemporal scales, and offer substantial flexibility for further development and parametrization.

The Role of Chloroplast Membrane Lipid Metabolism in Plant Environmental Responses.

Plants are nonmotile life forms that are constantly exposed to changing environmental conditions during the course of their life cycle. Fluctuations in environmental conditions can be drastic during both day–night and seasonal cycles, as well as in the long term as the climate changes. Plants are naturally adapted to face these environmental challenges, and it has become increasingly apparent that membranes and their lipid composition are an important component of this adaptive response. Plants can remodel their membranes to change the abundance of different lipid classes, and they can release fatty acids that give rise to signaling compounds in response to environmental cues. Chloroplasts harbor the photosynthetic apparatus of plants embedded into one of the most extensive membrane systems found in nature. In part one of this review, we focus on changes in chloroplast membrane lipid class composition in response to environmental changes, and in part two, we will detail chloroplast lipid-derived signals.

Deficiency of platelet adhesion molecule CD226 causes megakaryocyte development and platelet hyperactivity.

This study used constitutive CD226 gene knockout (KO) mice as a model to investigate the functions and mechanisms of CD226 in megakaryocyte (MK) maturation and platelet activation. Although CD226 deficiency did not cause MK polyploidization or platelet granule abnormalities, increased MK counts were detected in the femora bone marrow (BM) and spleen of CD226 KO mice. Particularly, CD226 KO mice have a more extensive membrane system in MKs and platelets than wild-type (WT) mice. We also demonstrated that CD226 KO mice displayed increased platelet counts, shortened bleeding time, and enhanced platelet aggregation. CD226 KO platelets had an increased mature platelet ratio compared to the control platelets. In addition, the observed reduction in bleeding time may be due to decreased nitric oxide (NO) production in the platelets. Platelet-specific CD226-deficient mice showed similar increased MK counts, shortened bleeding time, enhanced platelet aggregation, and decreased NO production in platelets. Furthermore, we performed middle cerebral artery occlusion-reperfusion surgery on WT and CD226 KO mice to explore the potential effect of CD226 on acute ischemia-reperfusion injury; the results revealed that CD226 deficiency led to significantly increased infarct area. Thus, CD226 is a promising candidate for the treatment of thrombotic disorders.

Extensive membrane systems at the host-arbuscular mycorrhizal fungus interface.

During arbuscular mycorrhizal (AM) symbiosis, cells within the root cortex develop a matrix-filled apoplastic compartment in which differentiated AM fungal hyphae called arbuscules reside. Development of the compartment occurs rapidly, coincident with intracellular penetration and rapid branching of the fungal hypha, and it requires much of the plant cell's secretory machinery to generate the periarbuscular membrane that delimits the compartment. Despite recent advances, our understanding of the development of the periarbuscular membrane and the transfer of molecules across the symbiotic interface is limited. Here, using electron microscopy and tomography, we reveal that the periarbuscular matrix contains two types of membrane-bound compartments. We propose that one of these arises as a consequence of biogenesis of the periarbuscular membrane and may facilitate movement of molecules between symbiotic partners. Additionally, we show that the arbuscule contains massive arrays of membrane tubules located between the protoplast and the cell wall. We speculate that these tubules may provide the absorptive capacity needed for nutrient assimilation and possibly water absorption to enable rapid hyphal expansion.

Fine structure of the aggregate silk nodules in the orb-web spider Nephila clavata

ABSTRACTAmong the triad spinning units for capture thread producing system in the orb-web spiders, aqueous gluey substances are produced from two pairs of aggregate silk glands (ASG). Although biochemical analysis of glue substance is produced by way of their passage through the ASG, its structural modifications in the nodular area have been nearly neglected till now. This paper focused on identifying the fine structural characteristics of the aggregate nodules in the golden orb-web spider Nephila clavata using both of light and transmission electron microscopes. The ASG in N. clavata is composed of a multi-lobed secretory region and a thick excretory duct surrounded by large irregular nodules. Histological analysis of the nodules demonstrates that the nodule forming cells have extensive membrane-bound tubular system that is continuous with the surface membrane. In particular, the nodule forming cells contain numerous mitochondria and glycogen particles within their cytoplasms, and they are surrounded by the same sheath of thin connectives. As previously described, each gluey droplet is formed of a central glycoprotein mass surrounded by an aqueous covering components, the nodular organization in N. clavata indicates that the extensive membrane system is thought to have a function for gluey silk production in spider. The results of this study also strengthen the premise that spider glues are made of glycoproteins, and the aggregate nodule functions as a key component for the spider web glue production.

Long-term acclimation of barley photosynthetic apparatus to narrow-band red and blue light

Chloroplasts of barley plants grown under red light (RL, 660 nm) dramatically differed from the chloroplasts of plants raised under blue light (BL, 450 nm) or control plants (white light). The chloroplasts under RL had an extensive membrane system with high stacking degree and disordered irregular shaped stacks (shaggy-formed grana). After 5 h in darkness, dynamic rearrangements of chloroplast architecture in RL- and especially BL-grown plants were restricted compared with control plants. The light spectral quality affected the content and proportions of photosynthetic pigments. The leaves of RL-grown plants had the increased ratio of low-temperature fluorescence bands, F741/F683, corresponding to emission of PSI and PSII, respectively. This increase can be related to specific architecture of chloroplasts in RL-treated plants, providing close spacing between the two photosystems, which enhances energy transfer from PSII to PSI and facilitates the movement of LHCII toward PSI.

New Insights on Thylakoid Biogenesis in Plant Cells.

Photosynthetic membranes, or thylakoids, are the most extensive membrane system found in the biosphere. They form flattened membrane cisternae in the cytosol of cyanobacteria and in the stroma of chloroplasts. The efficiency of light energy capture and conversion, critical for primary production in ecosystems, relies on the rapid expansion of thylakoids and their versatile reorganization in response to light changes. Thylakoid biogenesis results from the assembly of a lipid matrix combined with the incorporation of protein components. Four lipid classes are conserved from cyanobacteria to chloroplasts: mono- and digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol, and phosphatidyldiacylglycerol. This review focuses on the production and biophysical properties of galactolipids, making them determinant factors for the nonvesicular/nonlamellar biogenesis and for the three-dimensional architecture of nascent thylakoids. The regulation of MGD1, the committing enzyme of galactolipid biosynthesis in Arabidopsis, via feedback regulatory loops and control of protein binding to membranes, is also detailed.

Evaluation of the 17-kDa Prenyl-binding Protein as a Regulatory Protein for Phototransduction in Retinal Photoreceptors

The mammalian rod photoreceptor phosphodiesterase (PDE6) holoenzyme is isolated in both a membrane-associated and a soluble form. Membrane binding is a consequence of prenylation of PDE6 catalytic subunits, whereas soluble PDE6 is purified with a 17-kDa prenyl-binding protein (PDEdelta) tightly bound. This protein, here termed PrBP/delta, has been hypothesized to reduce activation of PDE6 by transducin, thereby desensitizing the photoresponse. To test the potential role of PrBP/delta in regulating phototransduction, we examined the abundance, localization, and potential binding partners of PrBP/delta in retina and in purified rod outer segment (ROS) suspensions whose physiological and biochemical properties are well characterized. The amphibian homologue of PrBP/delta was cloned and sequenced and found to have 82% amino acid sequence identity with mammalian PrBP/delta. In contrast to bovine ROS, all of the PDE6 in purified frog ROS is membrane-associated. However, addition of recombinant frog PrBP/delta can solubilize PDE6 and prevent its activation by transducin. PrBP/delta also binds other prenylated photoreceptor proteins in vitro, including opsin kinase (GRK1/GRK7) and rab8. Quantitative immunoblot analysis of the PrBP/delta content of purified ROS reveals insufficient amounts of PrBP/delta (<0.1 PrBP/delta per PDE6) to serve as a subunit of PDE6 in either mammalian or amphibian photoreceptors. The immunolocalization of PrBP/delta in frog and bovine retina shows greatest PrBP/delta immunolabeling outside the photoreceptor cell layer. Within photoreceptors, only the inner segments of frog double cones are strongly labeled, whereas bovine photoreceptors reveal more PrBP/delta labeling near the junction of the inner and outer segments (connecting cilium) of photoreceptors. Together, these results rule out PrBP/delta as a PDE6 subunit and implicate PrBP/delta in the transport and membrane targeting of prenylated proteins (including PDE6) from their site of synthesis in the inner segment to their final destination in the outer segment of rods and cones.

Membrane distribution in dividing endosperm cells of Haemanthus.

Membranes in cell-wall-free dividing endosperm cells of Haemanthus were examined after postfixation with osmium tetroxide-potassium ferrocyanide. We found that preservation and staining of membranes in metaphase cells was highly variable. Even adjacent cells often showed different degrees of preservation of membrane. However, this method does reveal a much more extensive membrane system in the mitotic spindle of Haemanthus than has been revealed previously using glutaraldehyde-osmium fixation. At prometaphase a system of membranes becomes associated with the kinetochore bundles. By metaphase, membranes constitute a prominent feature of kinetochore bundles, terminating near the kinetichores. Minipoles, identified by converging microtubules and associated membranes, are distributed in a zone extending laterally across the polar regions of the cell. The microtubules appear to terminate at the minipoles, whereas the membrane system becomes oriented generally perpendicular to the spindle axis and interfaces distally with a region of amorphous electron-dense material, helical polyribosomes, and cell organelles. The role of this extensive membrane system, if any, in chromosome movement is unknown. However, its distribution is coincident with the distribution of calcium-rich membranes and kinetochore fibers at metaphase in these cells (Wolniak, S. M., P. K. Hepler, and W. T. Jackson, 1981, Eur. J. Cell Biol., 25:171-174). Thus, these membranes may function in creating calcium domains that, in turn, may play a regulatory role in chromosome movement.

Fine structure of sporogenesis and septum formation in Micromonospora globosa Kriss and M. fusca Jensen.

Sporogenesis in two species of Micromonospora M. globosa and M. fusca (Actinomycetes) was quite similar. As in fungi, spore formation began as a blowing-out of a hyphal tip with the subsequent centripetal invagination of the plasma membrane. Septal wall material was deposited in a typical three-layered pattern, i.e., two electron-opaque layers separated by an electron-transparent layer. A second electron-opaque wall layer was later formed within the spore and finally a third, less electron-opaque wall was produced. Spore dihiscence was facilitated by the fragmentation of the first-formed wall surrounding the spore. Sporogenesis in Micromonospora is blastic in nature producing terminal, thick-walled spores. In M. fusca, a sporulation process was observed which closely resembled sporangial formation. The process appeared similar to that described for the genus Actinoplanes. Swollen, multiseptate structures were also present. Also in M. fusca, perforate septa with flared pore margins were observed. These septa were similar in appearance to the dolipore septa of Basidiomycetes although they lack a parenthesome and pore plug. Although an extensive membrane system (mesosome) was associated with the finishing septum, its function in the process of septum formation was not determined.

Lipid turnover during morphogenesis in the water mold [formula omitted

The lipid content of Blastocladiella emersonii zoospores is 5 pg/cell or about 13% of dry weight. Within the first few minutes of germination 60–70% of total zoospore lipid is lost, with neutral lipid, glycolipid and phospholipid fractions decreasing to about the same extent. These changes in lipid content precede the breakdown during germination of the complex and extensive membrane system of zoospores. During growth, which immediately follows germination, net phospholipid synthesis resumes so that total lipid is maintained at 6% of dry weight, but net synthesis of neutral and glycolipid does not begin until induction of sporulation. During sporulation the phospholipid level decreases so that the distribution of lipid among the three fractions approaches that found in zoospores. These changes in lipid content suggest that zoospore membranes containing neutral and glycolipids are synthesized de novo during spore formation.

An electron microscope study of differentiation and growth in oocytes of Ophioderma panamensis

Electron microscope studies on oocytes of the ophiuroid Ophioderma panamensis indicate the numerous nucleoli to be active throughout much of oogenesis. The nucleoli appear to “spin out” coarse strands of granules and fibers from their periphery (nucleolonema), and become attached at certain points to the inner layer of the nuclear envelope. The nucleolonema appear to fragment into small granular clusters or granule-filament complexes, both of which are also observed in the cytoplasm. During the period of nucleolar activity, mitochondria illustrate a special relationship to the nuclear surface, being preferentially aligned adjacent to the outer layer of the nuclear envelope. Proteid yolk granules first become apparent within certain of the Golgi cisternae. The origin of cortical granules also appears to be associated with the Golgi complexes. During oocyte differentiation an extensive membrane system of vesicles and lamellae, part rough- and part smooth-surfaced, forms in the peripheral portion of the ooplasm. The morphological variations observed during oocyte growth indicate that this membrane system differentiates into (a) annulate lamellae, (b) endoplasmic reticulum which becomes organized into concentric whorls of membranes (within which lipid droplets appear), and (c) single lamellae of endoplasmic reticulum which partially surround each of the Golgi complexes in the cell. The specific arrangement and association between the Golgi complexes and endoplasmic reticulum strongly suggest that the two organelles play a role in the elaboration of proteid yolk granules and cortical granules.