Membrane endomembranes

Reuzeau C, Doolittle KW, McNally JG, Pickard BG. Covisualisation in living onion cells of putative integrin, putative spectrin, actin, putative intermediate filaments, and other proteins at the cell membrane and in an endomembrane sheath. Protoplasma 1997 199 173-197. 

The major polymers that make up the wall are polysaccharides and lignin. These occur together with more minor but very important constituents such as protein and lipid. Water constitutes a major and very important material of young, primary walls (2). The lignin is transported in the form of its building units (these may be present as glucosides) and is polymerized within the wall. Those polysaccharides which make up the matrix of the wall (hemicelluloses and pectin material) are polymerized in the endomembrane system and are secreted in a preformed condition to the outside of the cell. Further modifications of the polysaccharides (such as acetylation) may occur within the wall after deposition. Cellulose is polymerized at the cell surface by a complex enzyme system transported to the plasma membrane (3). 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

The fatty acids could be carried by proteins by a process similar to the way in which serum albumin binds fatty acid in the bloodstream of mammals. Other types of lipid might be formed into complexes analogous to low-density lipoproteins of the type found in animal tissues, where the lipid core of the lipoprotein is surrounded by a hydrophilic cortex made up of protein, phospholipid, and cholesterol (87). This allows the lipid to be moved in an aqueous environment. The protein of the lipoprotein shell could also act as possible ligands for particular receptors at the membrane of the cell at which the export occurs. The lipoproteins, if they are present, would probably be formed within the endomembrane lumen and would receive the proteins at the endoplasmic reticulum. 

Unfortunately, our knowledge of the transport mechanism is currently insufficient for a detailed discussion. The primary mechanism can probably be explained in terms of the endomembrane theory (16,17), i.e., phenylpropanoid monomers are transported via vesicles to the plasma membrane, and are then released into the cell wall following membrane fusion. However, more extensive experimental evidence is needed to unambiguously establish the detailed mechanism(s) involved. 

Keenan, T. W., Franke, W. W. and Kartenbeck, J. 1974A. Concanavalin A binding by isolated plasma membranes and endomembranes from liver and mammary gland. Febs. Lett. 44, 274-278. 

Morr6, D. J., Kartenbeck, J. and Franke, W. W. 1979. Membrane flow and interconversions among endomembranes. Biochim. Biophys. Acta 559, 71-152. 

The association of proton movement with electron transport is not reflected in the fatty acyl desaturase system universal to endomembranes. In these enzymes the dehydrogenase, NADH cyt b5 reductase and the cytochrome b5 (a single heme cytochrome) are associated exclusively with the cytosolic side of the membrane by acyl groups and have no transmembrane segment. The cytochrome b5 oxidase associated with the desaturation of fatty acyl CoA may be transmembranous but has not been associated with proton movement. It is an iron-containing protein. The other type of endomembrane cytochromes are the P-450 group of cytochrome bs... 

The V-type (or vacuolar-type) H+-ATPase was first found in fungal vacuoles and plant tonoplasts,13 l4) and is now known to be widely distributed in mammalian endomembrane systems.15) Like F-type ATPase, the subunit structure of the V-type ATPase is complicated. The membrane extrinsic sector is composed of 73 kDa, 58 kDa, 38 kDa, and 34 kDa subunits, while the membrane intrinsic sector is composed of 40 kDa 20 kDa and 16 kDa subunits. 

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

Figure 1 Ras GTPases function as regulated GDP/GTP molecular switches. Diverse extracellular signals, for example those received by membrane-bound receptors such as G-protein coupled receptors and receptor tyrosine kinases, can cause Ras GTPase activation at the plasma membrane and endomembranes. Receptor-mediated activation of Ras most commonly involves the activation of RasGEFs, which then cause transient activation of Ras. Activated Ras-GTP adopts a conformation that enhances its affinity for transient binding to and activation of downstream effectors (E).A The activated effectors then regulate distinct cytoplasmic signaling networks that control cellular proliferation, differentiation, and survival. Ras signaling is terminated by RasGAP-mediated stimulation of hydrolysis of bound GTP to GDP, which precludes further Ras-effector interaction. Tumor-associated Ras mutant proteins are insensitive to GAP stimulation.

The structure of a prokaryote is very much simpler than that of a eukaryote. There are no endomembranes, endosymbionts, nucleus or cytoskeleton. The DNA is carried on the genophore, a circular chromosome, in a ill defined area of the cytosol called the nucleoid. The chromosome is attached to the cell membrane during cell division (fission), frequently at a point called the mesosome. 

The nucleotide sugar compounds are hydrophilic and it is necessary to transfer them to the sites of the synthases within the endomembrane system. Their transport across the membrane is an obvious control point. Not only is the rate of polysaccharide synthesis controlled by this mechanism but in addition the qualitative nature of the polymer found in the membrane compartment may be determined. Thus although the synthases may be present in a particular part of the membrane system e.g. endoplasmic reticulum, the assembly of the polysaccharide does not occur at this level if the nucleotide sugar cannot enter the lumen of the compartment (13). The main assembly of the polysaccharides occurs at the Golgi apparatus. Figure 1. 

Fig. 4.3 Pathways of AEA inactivation. Once taken up by a purported transporter on the plasma membrane (AMT), AEA is rapidly cleaved by endomembrane-bound FAAFI, realesing arachidonic acid (AA) and ethanolamine (EA)...

FAAH has been found mainly in microsomal and mitochondrial fractions of rat brain and liver (Deutsch and Chin, 1993 Desarnaud et al., 1995), and of porcine brain (Ueda et al., 1995). Recent studies performed with confocal microscopy, showed that FAAH is localized intracellularly as a vesicular-like staining, that has no association with the plasma membranes and is partially co-localized with the endoplasmic reticulum (Fig. 4.5). These morphological data were corroborated by biochemical assays of FAAH activity in subcellular fractions, showing that AEA hydrolysis was primarily confined to the endomembrane compartment (Oddi et al., 2005). Moreover, by means of reconstituted vesicles derived from purified membrane fractions, it was demonstrated that transport activity is retained by plasma membrane vesicles devoid of FAAH, thereby indicating that AEA hydrolase activity is not necessary for AEA membrane transport. Overall, by means of confocal microscopy, subcellular fractionation, and... 

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