Transmembrane channels formed

Blair, A., Ngo, L., Park, J., Paulsen, I. T. and Saier, M. H. Jr (1996). Phylogenetic analyses of the homologous transmembrane channel-forming proteins of the FoFi-ATPases of bacteria, chloroplasts and mitochondria, Microbiology, 142, 17-32. 

In nature, polypeptides with amphiphilic structures are known to form transmembrane channels formed by an assembly of several helices, so as to present their polar faces inward and their apolar faces outward. In view of such behavior, the photochromic amphiphilic polypeptide was incorporated into a cationic bilayer membrane composed of dipalmitoyl phosphatidyl choline.11201 Fluorescence and microscopic measurements provided evidence that the polypeptide was able to form bundles of helical molecules analogous to their natural counterparts, which acted as transmembrane channels for K+ ions. Irradiation, and the consequent transacts isomerization of the azobenzene link, caused a bending of the molecular structure and a destabilization of the transmembrane bundles. Therefore, formation of ion permeable channels would be favored or inhibited depending on whether the azo moiety... 

Fig. 2.4 An example of protein aggregation the K+-specific transmembrane channel formed by KcsA [4]...

Fig. 5.13 Hexameric transmembrane channels formed by synthetic peptides [42]...

Fig. 5.15 Transmembrane channels formed by crown ethers a single filter chundle approach [45] (left) and multifilter hydraphile [46] (right)...

FIGURE 10.33 The structure of the heptameric channel formed by o -hemolysin. Each of the seven subunits contributes a /3-sheet hairpin to the transmembrane channel. 

Recently, a variety of natural peptides that form transmembrane channels have been identified and characterized. Melittin (Figure 10.35) is a bee venom toxin peptide of 26 residues. The cecropins are peptides induced in Hyalophora cecropia (Figure 10.36) and other related silkworms when challenged by bacterial infections. These peptides are thought to form m-helical aggregates in mem-... 

The intramembranous part of the molecule may consist of 8-10 hydrophobic transmembrane helices (Mi-Mio) that anchor the Ca -ATPase to the lipid bilayer and form the transmembrane channel for the passage of (Fig. 2). 

If a substance that can form a transmembrane channel exists in several conformations with different dipole moments, and only one of these forms is permeable for ions, then this form can be favoured by applying an electric potential difference across the membrane. The conductivity of the membrane then suddenly increases. Such a dependence of the conductivity of the membrane on the membrane potential is characteristic for the membranes of excitable cells. 

Sympathetic parasympathetic ganglia ACh Nicotine Trimethaphan Ganglionic type (o3p4) Nicotinic (N) cholinoceptor ligandgated cation channel formed by five transmembrane subunits muscular type (al2PlY5)... 

Figure 5. Possible mechanisms of supramolecular organization of amphiphilic ion pair 1 to form the transmembrane channel and its open-closed transitions. The amphiphilic units are incorporated into bilayer lipid membrane and organized to the ion channel through aggregation and interconnection between two lipid layers. Ions can pass through the pore only when the aggregates are large enough, and interconnected between layers, and conformationally open.

The transmembrane domains have different functions, according to the type of receptor. For ligand-controlled receptors, the function of the transmembrane domain is to pass the signal on to the cytosohc domain of the receptor. For hgand-controlled ion channels, the transmembrane portion forms an ion pore that allows selective passage of ions (see Chapter 16). 

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