Endoplasmic reticulum lipid composition

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct 26 Cholesterol transport and regulation in the central nervous system is distinct from that of peripheral tissues 26 In adult brain most cholesterol synthesis occurs in astrocytes 26 The astrocytic cholesterol supply to neurons is important for neuronal development and remodeling 27 The structure and roles of membrane microdomains (rafts) in cell membranes are under intensive study but many aspects are still unresolved 28... 

The lipid compositions of plasma membranes, endoplasmic reticulum and Golgi membranes are distinct. All... 

Keenan, T. W. and Huang, C. M. 1972B. Membranes of mammary gland. VI. Lipid and protein composition of Golgi apparatus and rough endoplasmic reticulum from bovine mammary gland. J. Dairy Sci. 55, 1586-1596. 

The acquisition of a lipid bilayer from the infected host cell is the defining feature of enveloped viruses. However, the origin and composition of the bilayer can differ from virus to virus. Viruses are known to acquire a membrane, or bud, from various cellular compartments that include the nucleus, the endoplasmic reticulum (ER), the Golgi, and the plasma membrane. As the Upid contents of those membranes differ, so do the contents of the viral membranes. In some, if not all, cases the composition of the lipid bUayer is an important aspect of the viral assembly pathway. 

Most of the reactions involved in lipid biosynthesis appear to be located in the smooth endoplasmic reticulum (SER), although several enzyme activities have also been detected in the Golgi complex. Because each enzyme is a membrane-associated protein with its active site facing the cytoplasm, the biosynthesis of phospholipid occurs at the interface of ER membrane and cytoplasm. The fatty acid composition of phospholipids changes somewhat after their synthesis. (Typically, unsaturated fatty acids replace the original saturated fatty acids incorporated during synthesis.) Most of this remodeling is accomplished by certain phospholipases and acyl transferases. Presumably, this process allows a cell to adjust the fluidity of its membranes. 

The existence of such an abnormality would not necessarily imply that the sterol concentration of the cancer cells must be different from that of the normal cells of origin. However, a decrease in the cholesterol/phospholipid (C/PL) ratio of leukaemic cells as compared to normal cells has been reported [194-197]. Gottfried suggested that the altered lipid patterns found in leukaemic cells reflected cell immaturity [195]. All these lipid analyses were performed in whole cell extracts it is uncertain whether the difference in C/PL ratio was due to alteration in the lipid composition of cell surface membranes or to differences in the relative content of various intracellular membranes between normal and leukaemic cells. Membranes from nuclei, mitochondria, endoplasmic reticulum, and other intracellular membranes contain phospholipid, but insignificant amounts of cholesterol, or none at all. Ideally, analysis of the lipids of isolated cell membranes from normal and tumour cells is necessary if firm conclusions concerning the nature of the changes in lipid composition are to be made. 

Lipid bilayers are the basis of the membranes that surround the cytoplasm (the plasma membrane) and subcel-lular stmctures such as the nucleus, mitochondria, and the endoplasmic reticulum. Membranes are made up of lipids (most commonly phosphogylcerides, phosphosphingolipid, and, in the plasma membranes of animals, cholesterol) and proteins. Although small amounts of carbohydrate are also present, this is usually present as a glycolipid or glycoprotein. The composition of membranes varies depending on the type of membrane. 

An example is given for the composition of the liver cell membrane [2], There is also a considerable difference in the proportion of phospholipids in different cell types and different species. Figure 11-1 shows the variation of sphingomyelin (S) lo phosphatidylcholine (PC) content in the total lipid fraction of erythrocytes from various species and of ox endoplasmic reticulum in various organs [2], The lateral mobility of lipids in membranes is another essential aspect [7],... 

Recently [3] we described a mutant (actl) which was almost completely deficient in activity of the chloroplast glycerol-3-phosphate acyltransferase the first enzyme of the prokaryotic pathway. Analysis of the fatty acid composition of leaf lipids and C-labelling experiments indicated that PG was essentially the only lipid produced by the prokaryotic pathway in this mutant. However, this blockage in the prokaryotic pathway is compensated for by increased flux through the eukaryotic pathway and furthermore the proportions of the different chloroplast lipids made by the eukaryotic pathway are altered so that there is only a small alteration in lipid composition in the actl mutant compared with wild type [3]. Thus in the actl mutant (Fig. lb) 950 of every 1000 fatty acids enter the eukaryotic pathway and 650 of these are subsequently returned to the chloroplast. The mutant is essentially an 18 3-plant and clearly demonstrates the existence of controls which regulate reactions in the chloroplast and endoplasmic reticulum so as to provide the complement of lipids required for correct membrane synthesis in leaf cells. 

The chloroplast is surrounded by two envelope membranes. Techniques for separating these membranes from each other have allowed their analysis. These studies revealed that, while the inner envelope has a lipid composition similar to that of thylakoids, the outer envelope contains little glycosylgly-ceride and is similar to endoplasmic reticulum. ... 

Mitochondria contain an outer membrane which has PC and PE as its main components while the inner membrane is the unique site of diphos-phatidylglycerol (DPG, cardiolipin) in plant cells. " Microbodies, such as those of potato or the glyoxysomes of castor bean, have a rather simple lipid pattern with PC, PE, and PG usually predominating. On the other hand, microsomal fractions, which are assumed to contain mainly endoplasmic reticulum, have a mixture of PC, PE, PG, and PI, with PC predominating. Some examples of the phospholipid compositions of different plant membranes are given in Table 3.3. 

Phospholipids may be transported within cells by binding to specific phospholipid exchange proteins (PLEP) whose function may be to introduce lipids into cell membranes and organelles remote from their sites of synthesis. For example, mitochondria do not possess enzymes for the complete biosynthesis of phospholipids de novo and may acquire their characteristic lipid composition by exchange from endoplasmic reticulum via the PLEP. Carrier proteins also exist for sterols, including vitamin D, and for retinol. These are referred to in the appropriate sections. 

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