Channel-forming proteins

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. 

Unwin PNT, Ennis PD Two configurations of a channel forming protein. Nature 1984 307 609-613. 

Cell membranes are impermeable to most ions. Only a small number of ions can enter cells readily and these usually do so with the assistance of protein channels or pores. The principal anion of plasma (Box 5A) is Cl-, which passes through membranes readily by virtue of the presence of channel-forming proteins. Chloride ions are often distributed across membranes passively according to Eq. 8-5, which describes the Donnan equilibrium.167,479,480... 

Pawlak, M., Meseth, U., Dhanapal. B Mutter, M., and Vogel, H. (1994) Template-assembled melittin Structural and functional characterization of a designed, synthetic channel-forming protein. Protein Sci. 3,1788-1805. 

The simulation of ion channels and other pore-forming peptides and proteins at atomic detail is nowadays also possible. With the increase in computational power, these complex systems have attracted much more interest, and several simulations have been reported. Very often, only the transmembrane segments of the channel-forming proteins are included in the simulation to reduce the size and complexity of the system. The simulated systems range from synthetic model ion channels to a bacterial porine protein. 

Channel-forming proteins exhibit a number of structural motifs the influenza virus M2 proton channel and voltage-gated channels for K+, Na+ and Ca2+are all composed of four identical subunits that aggregate to form a central pore as shown in Fig. 5.7 an acid sensing Na+ ion channel has a similar structure but with threefold symmetry Ca2+ release channels, the divalent metal ion transporter CorA, the... 

Fig. 5.11 An acid sensitive transmembrane ion channel-forming protein [28]...

Outer membrane. Because it contains a large channel-forming protein (called porin), the outer membrane is permeable to all molecules 5000 daltons or less. Other proteins in this membrane include enzymes involved in mitochondrial lipid synthesis and enzymes that convert lipid substrates into forms that are subsequently metabolized in the matrix. 

This technique essentially comprises two aqueous compartments containing electrolyte connected by a pinhole across which a bilayer of phospholipid molecules is spread. An electrical potential is applied across the bilayer and in the presence of an ionophore the resultant current flowing between the two chambers is measured as a function of time.12 The method is often used to assess the activity of synthetic peptides, and has been used to measure the channel activity of several non-peptide channels. Of particular significance is its use in allowing the measurement of the single channel properties of isolated channel-forming proteins that have been reintroduced back into planar bilayer membranes.13... 

Giaume, C., Fromaget, C., Aoumari, A. E., Cordier, J., Glowinski, J., and Gros, D., Gap junctions in cultured astrocytes. Single-channel currents and characterisation of channel forming protein, Neuron, 6, 133, 1991. 

Some tissues contain proteins that increase the permeability of membranes to water. Each water-channel-forming protein, termed an aqua-porin, consists of six membrane-spanning a helices and a central channel lined with hydrophilic residues that allow water molecules to pass in single file. Aquaporins do not transport protons. 

One of the strategies described in Sect. 3 for BLM stabilization, mixing water-insoluble, nonlipid monomers with nonpolymerizable lipids, has also been applied to liposomes. Meier and coworkers [21] created stabilized, nanoscale bioreactors (Fig. 20) by incorporating OmpF, a channel-forming protein, into POPC vesicles to provide for passive transmembrane transport of low molecular weight compounds. p-Lactamase was entrapped during liposome formation, followed by addition of... 

Fig. 5.4 The OmpF porin from Eschericia coli is an integral membrane channel-forming protein which spans the outer membrane in Cram-negative bacteria. The structure of a monomer of the OmpF porin is shown. In total, 16 /3-bands are configured in the form of a cylinder and form the walls of a pore through which selective passage of ions takes place.

This chapter reviews some of the molecular biophysical questions that are raised by the properties of one class of channel-forming proteins, con-nexin, and that may be addressed through the study of connexin in reconstituted membrane systems. The first section introduces issues of biophysical interest and provides background information about connexin. The second section discusses the prospects for utilizing reconstituted systems to study the key questions, followed by a brief review of data from our laboratory. The final sections evaluate the findings and discuss future studies. 

The studies outlined in this section describe the ways we have addressed the foregoing problems of connexin reconstitution by utilizing connexin-32, the predominant form of connexin in rat liver. Our goals were to establish unambiguously that connexin-32 formed channels in liposome membranes, to identify connexin channels in planar bilayers, and to study their properties. Two methods were used to identify reconstituted channels formed by connexin-32. In one method, protein was solubilized from preparations of junctional membrane and incorporated into unilamellar liposomes. Connexin-32 was identified as a channel-forming protein by its specific enrichment in liposomes that were permeable to sucrose. In the other method, connexin-32 was affinity-purified (with a monoclonal antibody directed specifically against connexin-32) directly from octylglucoside-solubilized plasma membranes. Liposomes formed with such material were permeable to sucrose and Lucifer Yellow. Sucrose-permeable liposomes from each method were fused with planar bilayers to study the properties of connexin channels. 

Probes into the secondary structures of the three channel-forming proteins reveal a high degree of conservation. The nature of the channel and its location within the membrane greatly limit the possible secondary structures. A cylinder formed primarily of a beta-sheet wall was proposed (6, 15) and... 

Ivnitski et al. [61] reporteda new ion-channel biosensor based on a s-BLM for direct and fast detection of Campylobacter species. The sensing element is composed of a stainless-steel WE, which is covered by a self-assembled BLM. Antibodies to bacteria embedded in the BLM are used as channel-forming proteins. The biosensor has a strong signal amplification effect, which is defined as the total number of ions transported across the BLM. The total number of (univalent) ions flowing through the channels is 10 ° ions s. The biosensor showed a very good sensitivity and selectivity to Campylobacter species. 

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