Endomembrane

Bruyant-Vannier et al [5] have shown that pectin methyltransferases solubilized from endomembranes of flax cells consisted of several polypeptides, the molecular relative mass of which varied from very low (Mr < 5,000) to very high (Mr ca 200,000) values. 

Morre DJ. Endomembrane system of plants and fungi, in Tip Growth in Plant and Fungal Systems (Heath B, ed.), Academic Press, San Diego, CA, 1990. 

Okita TW, Rogers JC. Compartmentation of proteins in the endomembrane system of plant cells. Ann Rev Plant Physiol Plant Mol Biol 1996 47 327-350. 

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. 

Danley JM, Staggers S, Varner A, Llewellyn GC, Dashek WV. A combined biochemical and ultrastructural analysis of aflatoxin action on the endomembrane system of plant cells. Actual Bot 1982 129 5-13. 

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 are synthesized in chloroplasts or proplastids and moved into the cytoplasm and the endomembrane system for further modification and synthesis of neutral fats, phospholipids, and other compounds... 

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. 

The incorporation of 3H from [3H]arabinose into Fer-Ara2 units showed a lag of about 4.2 min, after which it too became linear. This means that as little as 0.7 min after their incorporation into a (possibly still nascent) polysaccharide, [3H]arabinose residues were susceptible to feruloy-lation. The feruloylation reaction is thus likely to have been occurring in the endomembrane system, co-synthetically, as one of the highly regulated parts of the sophisticated polysaccharide-biosynthetic machinery. This can be compared to the co-translational modification known to occur in many proteins. 

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. 

Nurnberg B, Ahnert-Hilger G (1996) Potential roles of heterotrimeric G proteins of the endomembrane system. FEBS Lett 389 61-65... 

Liu, C.J. and Dixon, R.A., Elicitor-induced association of isofiavone 0-methyltransferase with endomembranes prevents the formation and 7-0-methylation of daidzein during isofiavonoid phytoalexin biosynthesis. Plant Cell, 13, 2643, 2001. 

Williams and Keeling 2002). We also suggest that this close association of the mitochondrion-like organelle and endomembrane system facilitates the assembly of cytosolic Rlil, the only known essential FeS protein for yeast viability (Iill et al. 2005 reviewed in Tachezy and Dolezal 2007). These data are consistent with previous hypotheses that the primary function of the mitochondrion-like organelle in C. parvum is the assembly of FeS clusters in order to provide mature FeS proteins to all cellular compartments, including the cytosol, mitochondrion, and nucleus. 

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. 

Mather, I. H. and Keenan, T. W. 1983. Function of endomembranes and the cell surface in the secretion of organic milk constituents. In Biochemistry of Lactation. T. B. Mepham (Editor). Elsevier/North-Holland, Amsterdam, pp. 231-283. 

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

Gelebart, P., Kovacs, T., Brouland, J. P., van Gorp, R., Grossmann, J., Rivard, N., Panis, Y., Martin, V., Bredoux, R., Enouf, J. and Papp, B., 2002, Expression of endomembrane calcium pumps in colon and gastric cancer cells. Induction of SERCA3 expression during differentiation. J Biol Chem 277, 26310-20. 

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