Membrane-bound lipids, composition

K. Christiansen and P.K. Jensen, Membrane-bound lipid particles from beef heart chemical composition and structure, Biochim. Biophys. Acta, 1972, 260, 449-459. 

Abstract To understand how membrane-active peptides (MAPs) function in vivo, it is essential to obtain structural information about them in their membrane-bound state. Most biophysical approaches rely on the use of bilayers prepared from synthetic phospholipids, i.e. artificial model membranes. A particularly successful structural method is solid-state NMR, which makes use of macroscopically oriented lipid bilayers to study selectively isotope-labelled peptides. Native biomembranes, however, have a far more complex lipid composition and a significant non-lipidic content (protein and carbohydrate). Model membranes, therefore, are not really adequate to address questions concerning for example the selectivity of these membranolytic peptides against prokaryotic vs eukaryotic cells, their varying activities against different bacterial strains, or other related biological issues. 

Eukaryotic cells have evolved a complex, intracellular membrane organization. This organization is partially achieved by compartmentalization of cellular processes within specialized membrane-bounded organelles. Each organelle has a unique protein and lipid composition. This internal membrane system allows cells to perform two essential functions to sort and deliver fully processed membrane proteins, lipids and carbohydrates to specific intracellular compartments, the plasma membrane and the cell exterior, and to uptake macromolecules from the cell exterior (reviewed in [1,2]). Both processes are highly developed in cells of the nervous system, playing critical roles in the function and even survival of neurons and glia. 

An important question arises about the effects of phospholipid composition and the function of membrane-bound enzymes. The phospholipid composition and cholesterol content in cell membranes of cultured cells can be modified, either by supplementing the medium with specific lipids or by incubation with different types of liposomes. Direct effects of phospholipid structure have been observed on the activity of the Ca2+-ATPase (due to changes in the phosphorylation and nucleotide binding domains) [37]. Evidence of a relationship between lipid structure and membrane functions also comes from studies with the insulin receptor [38]. Lipid alteration had no influence on insulin binding, but modified the kinetics of receptor autophosphorylation. 

It is not clear why LA and none of the saturated fatty acids that were studied disrupted endothelial barrier function. The injurious effects of LA on cultured endothelial cells may be mediated, in part, by the induction of peroxisomes and, thus, by excessive hydrogen peroxide formation. In addition, enrichment of endothelial lipids with selective fatty acids can modify specific cellular lipid pools and alter the morphology of cultured cell monolayers. Such fatty acid-mediated compositional changes may be sufficient to alter membrane properties, e.g., fluidity and activities of membrane-bound enzymes. One may speculate from these and other data that high dietary intakes of certain unsaturated fatty acids, such as LA, might not be entirely safe. 

CTa contains a domain that is extensively phosphorylated and the state of phosphorylation can affect CT activity (S.L. Pelech, 1982). CT that is bound to membranes is less phosphorylated than soluble CT. Incubation of hepatocytes with oleic acid demonstrated that CT associates with membranes in an active, phosphorylated form and that is subsequently dephosphorylated (M. Houweling, 1994). Thus, a change in the lipid composition of membranes mediates the initial binding of CT to membranes and subsequently CT is... 

Electrochemical impedance spectroscopy provides a sensitive means for characterizing the structure and electrical properties of the surface-bound membranes. The results from impedance analysis are consistent with a single biomembrane-mimetic structure being assembled on metal and semiconductor electrode surfaces. The structures formed by detergent dialysis may consist of a hydrophobic alkyl layer as one leaflet of a bilayer and the lipid deposited by dialysis as the other. Proteins surrounded by a bound lipid layer may simultaneously incorporate into pores in the alkylsilane layer by hydrophobic interactions during deposition of the lipid layer. This model is further supported by the composition of the surface-bound membranes and by Fourier transform infrared analyses (9). 

The erythrocyte-bound malaria parasite acquires host lipids by diverse methods and transports to the area of celluar growth. There they are remodeled significantly to meet the parasite s needs, including phospholipid head group alteration, modification of the fatty acid molecular species composition of certain phospholipids, and transformation of the erythrocyte membrane lipid composition in both infected and uninfected cells. 

Vesicular transport occurs when a membrane completely surrounds a compound, particle, or cell and encloses it into a vesicle. When the vesicle fuses with another membrane system, the entrapped compounds are released. Endocytosis refers to vesicular transport into the cell, and exocytosis to transport out of the cell. Endocytosis is further classified as phagocytosis if the vesicle forms around particulate matter (such as whole bacterial cells or metals and dyes from a tattoo), and pinocy-tosis if the vesicle forms around fluid containing dispersed molecules. Receptor-mediated endocytosis is the name given to the formation of clathrin-coated vesicles that mediate the internalization of membrane-bound receptors in vesicles coated on the intracellular side with subunits of the protein clathrin (Eig. 10.14). Potocytosis is the name given to endocytosis that occurs via caveolae (small invaginations or caves ), which are regions of the cell membrane with a unique lipid and protein composition (including the protein caveolin-1). 

Lipids themselves are only the matrix of a biomembrane, the more functionally active part being membrane-bound proteins. Therefore, many publications have focused on the interaction of such proteins or hydrophobic peptides with lipids. Synthetic peptides are of special interest. The influence of their composition on the lipid phase can be studied in detail, and in addition when incorporated into the bilayer their structure can be deduced (see below). Two papers in which these aspects are addressed and in which many earlier publications are cited have appeared. 

Incubations with Cloned, Expressed Enzymes Individual UGT enzymes have been expressed in a wide variety of systems including insect cells (Supersomes or Baculosomes), Escherichia coli, yeast, and mammalian cells. Zakim and Dannenberg have demonstrated that the lipid membrane composition can influence activity (Zakim, 1992). There tends to be excellent protein expression insect cells transfected with baculovirus, but when activity is measured compared to mammalian cells systems, there appears to be significant amounts of inactive protein due to either poor membrane insertion or improper folding (lack of chaperones ). Bacteria do not have an ER, but alteration of the signal sequence results in active membrane bound preparations. Yeast and mammalian cells such as HEK293 or V79 cells have a more typical membrane environment and may be preferable for expression of ER proteins. 

Although it is known that the lipid composition of cell membranes determines membrane fluidity as well as the function and activity of membrane-bound proteins and enzymes (24), it is not known what effect changes in the relative proportions of specific fatty acids of phospholipids have on the capacitive properties of membranes. The maintenance of an electrical potential across membranes is essential to cell function and survival and may be altered in severe disease states (25). 

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