Proteins membrane rigidity

Generally extracellular proteins are rigid and are often cross-linked by S-S bridges. Intracellular proteins are more mobile especially if they are required to equilibrate between bound and unbound states involving DNA, RNA, or membrane surfaces. 

Lipid peroxidation is one of the major sources of free-radical mediated injury that directly damages membranes and generates a number of secondary products. In particular, markers of lipid peroxidation have been found to be elevated in brain tissues and body fluids in several neurodegenerative diseases, and the role of lipid peroxidation has been extensively discussed in the context of their pathogenesis. Peroxidation of membrane lipids can have numerous effects, including increased membrane rigidity, decreased activity of membrane-bound enzymes (e.g., sodium pumps), altered activity of membrane receptors, and altered permeability [Anzai et al., 1999 Yehuda et al., 2002], In addition to effects on phospholipids, lipid-initiated radicals can also directly attack membrane proteins and induce lipid-lipid, lipid-protein, and protein-protein cross-linking, all of which obviously have effects on membrane function. 

Intriguing as they may be, available results do not even partially answer one of the major questions about the endoplasmic reticulum s function. The membranes of the endoplasmic reticulum provide a sort of scaffold on which protein biosynthesis takes place. The template and a prefabricated assembly line (the ribosome) come from the nucleus. The energy is derived from ATP generated in mitochondria and the cytosol. Can any part of the scaffold be used for the biosynthesis of any protein, or are specific templates directed toward a specific portion of the cytoplasm Is the specificity of the interaction between assembly line and membrane rigidly restrictive, such as the hole that restricts introduction and turning of the key, or are the conditions of the interaction much more permissive and simply conditioned—for example, by availability of empty membrane surface, precursor gradients, and sources of ATP What directs the movement of template and ribosome away from the nucleus ... 

Related systems are inclusions in membranes, which could model large proteins or rigid gel-like domains. Even if these inclusions are flat and symmetric they can affect the conformation and the fluctuations of the surrounding membrane via boundary conditions. This induces effective interactions between them [39,40]. Although most of these problems have been studied mainly for planar membranes, a recent study addresses inclusions on a spherical vesicle [41]. 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

Phosphatidylcholine is an important component of cell membranes but cell mem branes are more than simply lipid bilayers Although their composition varies with their source a typical membrane contains about equal amounts of lipid and protein and the amount of cholesterol m the lipid fraction can approximate that of phosphatidylcholine The lipid fraction is responsible for the structure of the membrane Phosphatidyl choline provides the bilayer that is the barrier between what is inside the cell and what IS outside Cholesterol intermingles with the phosphatidylcholine to confer an extra measure of rigidity to the membrane... 

Cell wall Peptidoglycan a rigid framework of polysaccharide cross-linked by short peptide chains. Some bacteria possess a lipopolysaccharide- and protein-rich outer membrane. Mechanical support, shape, and protection against swelling in hypotonic media. The cell wall is a porous nonselective barrier that allows most small molecules to pass. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

A state of fluidity and thus of translational mobitity in a membrane may be confined to certain regions of membranes under certain conditions. For example, protein-protein interactions may take place within the plane of the membrane, such that the integral proteins form a rigid matrix—in contrast to the more usual situation, where the hpid acts as the matrix. Such regions of rigid protein matrix can exist side by side in the same membrane with the usual lipid matrix. Gap junctions and tight junctions are clear examples of such side-by-side coexistence of different matrices. 

---