Protein interaction with the membrane

Peripheral membrane proteins interact with the membrane loosely and often reversibly (Figure 4-4). 

Proteins interact with the membrane (support) by hydrophobic and charge-transfer forces and hydrogen bridges. The extent of these interactions depends on the accessibility of respective area of a protein. The accessibility is influenced, among other things, by the composition of the surrounding buffer, e.g., pH, ionic strength and/or chaotropic additives. 

Thus, protein secretion in E. coli is dramatically affected by the physical state of the membrane lipids. Both increased and decreased fluidity inhibit secretion. It is possible that this effect arises because the signal sequence and/or the secreted protein interact with the membrane lipids, and these interactions are perturbed when the lipid fluidity is changed. However, changes in lipid fluidity also affect other membrane functions such as active transport (DiRienzo and Inouye, 1979). Thus the effect of membrane fluidity on protein secretion may be due to altered activity of a membrane-bound part of the secretory apparatus, and may not be an indication of signal sequence-membrane interaction. 

Structural information about the oxygenases provided limited insight into the mechanism (Schmidt et al. 2006). The crystallized enzyme from Synechocystis sp. PCC6803 is membrane associated and the interaction with the membrane is believed to be mediated by a nonpolar patch on the surface of the enzyme. This hydrophobic patch is thought to provide the necessary access of the protein to the membrane-bound carotenoids. Following withdrawal from the membrane, the substrate moves through the hydrophobic tunnel toward the metal center. The substrate orients the... 

R-groups, which act much like droplets of oil that coalesce in an aqueous environment. The nonpolar R-groups thus fill up the inte rior of the folded protein and help give it its three-dimensional shape. [Note In proteins that are located in a hydrophobic envi ronment, such as a membrane, the nonpolar R-groups are found on the outside surface of the protein, interacting with the lipid environment (see Figure 1.4).] The importance of these hydrophobic interactions in stabilizing protein structure is dis cussed on p. 19. 

The hydrophobic amino acid side chains on the exterior of the integral membrane protein interact with the hydrophobic lipid of the membrane exterior and are stable in the nonaqueous environment. These residues pack in the interior, hydrophobic environment of globular proteins. 

Unique mechanism of action of T-20 involves a specific protein-protein interaction with the extracellular portion of the transmembrane glycoprotein gp41 of the HIV virus, thus blocking viral fusion with the T cell membrane... 

Phosducin is a soluble, 33 kDa protein. It was originally discovered in the retina and in the pineal gland, a tissue related developmentally to the retina. In the rod outer segment membrane of the retina, as much phosducin as transducin is present. It is now established that phosducin and phosducin-like proteins (PhlPs) are evolutionarily conserved proteins, expressed in many species and tissues, where they may control G-protein signalling in other tissues than the eye or where they may have other, not yet known, functions. Whereas RGS proteins interact with the cc-subunits of G proteins, phosducins bind to both cc- and Py-subunits, but with pronounced preference for Py-subunits (Plate 12). 

Protein-membrane association via a post-translational modification introduces the notion of dynamic association and partitioning of proteins between the membrane phase of the cells and the aqueous phase (cytosol or inner phase of organelles). Consequently, such proteins can be found both as membrane-associated and membrane-free, which is not the case with intrinsic membrane proteins which are strictly membrane embedded. Another type of association to membrane is mediated by protein-protein interactions with other membrane proteins. A typical example of this situation is provided by the respiratory complexes. In the case of ubiquinol-cytochrome c oxidoreductase, core proteins 1 and 2 does not show any interaction with the lipid membrane, but only with the protein subunits spanning the membrane (e.g. cytochrome b) (Iwata et al. 1998). 

From the mere fact that CF, can be released from the membrane by EDTA treatment and the enzyme stays in solution without detergents, it is apparent that the catalytic sector has minimal, if any, direct interaction with the lipids of the chloroplast membrane. It is a globular protein that is held to the surface of the membrane via interaction with the membrane sector. Recently it was shown that the y subunit is in immediate contact with the membrane sector and the 8 and e subunits may induce proper binding for catalysis [17,18], The enzyme contains a few well-defined sites that were used for localization experiments by the method of fluorescent energy transfer [19,56-61], These studies revealed the position of those sites and helped to localize the various subunits of CF, in space relative to the chloroplast membranes (for a model of CF, see Refs. 61 and 62). These experiments are awaiting analysis of the amino acid sequence of the y subunit that is now under investigation in Herrmann s laboratory [148], Definite structural analysis could be obtained only after good crystals of the enzyme become available. 

Gag has three consituent proteins, linked by spacer peptides. These are the matrix protein associated with the membrane, the capsid protein forming the internal capsid of the virus, and the nucleocapsid protein binding the viral RNA. A classic electron microscopy study of the interaction of actin with HIV revealed the radial organization of the proteins in the immature capsid, where it binds the nucleocapsid domain of the Gag structural polyprotein (Wilk et al, 1999). A further... 

Figure 12.17. Integral and Peripheral Membrane Proteins. Integral membrane proteins (a, b, and c) interact extensively with the hydrocarbon region of the bilayer. Nearly all known integral membrane proteins traverse the lipid bilayer. Peripheral membrane proteins d and e) bind to the surfaces of integral proteins. Some peripheral membrane proteins interact with the polar head groups of the lipids (not shown).

Most proteins would be expected to interact with the membrane to some extent. The intrinsic sieving coefficient of an interacting solute can be determined using Equation 18.5 [12] ... 

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