Membrane channel

Membrane proteins comprise another important class of protein crystallized in 2D. These proteins perform important functions as membrane channels and recognition sites for cells. Unlike the streptavidin crystals, membrane proteins... 

Parker, M., Buckley, J., Postma, J., et al., 1994. Structure of the Aeromonas toxin proaerolysin in its water-soluble and membrane-channel states. Nature 367 292-295. 

At the level of a single channel, addition of ACh is followed by transient openings of the channel. The current i flowing through an open channel is 4 pA at a membrane potential Voi-l 00 mV. Since one ampere (A) represents the flow of 6.24-1018 charges per second, 2.5-107 Na+ ions per second flow through an open channel. The conductance g of a plasma membrane channel is the measure of the ease of flow of cuirent between the extracellular space and the cytosol or vice... 

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

Plasma membrane channels. The most common mechanism for the movement of into smooth muscle cells Ifom the extracellular space is the electrodiffusion of Ca " ions through highly selective channels. This movement can be significant in two quite different ways. First, Ca ions carry two positive charges and, in fact, most of the inward charge movement across the plasma membrane of smooth muscle myocytes is carried by Ca. Most smooth muscle action potentials are known to be Ca " action potentials. And second, the concentration of intracellular free calcium, the second messenger, is increased by inward calcium movement. 

Barcilon, V, Ion Flow Through Narrow Membrane Channels Part I, SIAM Journal of Applied Mathematics 52, 1391, 1992. 

STRICHARTZ CASTLE Marine Toxins and Membrane Channels... 

With respect to toxins which target specific sites, insight can be obtained from the anomolies that are observed. For example, both puffer fish and tetrodotoxin-containing crabs (96) are insensitive to tetrodotoxin. The investigation of such insensitivities can provide information about membrane channels and their toxin binding sites. 

In the light of such considerations, it is possible to discuss toxins which have already been analyzed in terms of their sites of action. Such a discussion is best conducted by categorizing the various possible cellular sites at which a toxin might act. The most obvious sites are the membrane channels for ions, receptors for neurotransmitters, membrane pumps, and the membrane itself. Invertebrate toxins acting on membrane channels include the conotoxins (10) and several of the sea anemone toxins (97). 

An alternative to most of these mechanisms is the existence of efficient efflux systems, so that toxic concentrations of the drug are not achieved. There are three major families of proton-dependent multidrug efflux systems (1) the major facilitator superfamily, (2) the small multidrug resistance family, and (3) the resistance/nodulation/cell division family (Paulsen et al. 1996). It should be emphasized that several of these systems are involved not with antibiotic efflux but with, for example, acriflavine, chlorhexidine, and crystal violet. An attempt is made only to outline a few salient features of the resistance/nodulation/cell division family that mediates antibiotic efflux, and these are given in Table 3.3 (Nikaido 1996). They consist of a transporter, a linker, and an outer membrane channel. 

Transporter Linker Outer Membrane Channel Organism Substrates... 

Some microbes are able to decrease the permeability of their membranes to prevent toxic metals from entering. If the toxic metals are not able to physically enter the cell, they will not be able to affect vital metal-sensitive structures, such as proteins. One way to prevent heavy metals from entering is by decreasing the production of membrane channel proteins.18 It is also possible for the metal-binding sites in the membrane and periplasm to be saturated with nontoxic metals.37 A third possibility is the formation of an extracellular polysaccharide coat, which binds and prevents metals from reaching the surface of the cell.24,38... 

The acetyl choline receptor is a ligand-gated ion channel that allows cations to flow out of the neuron to initiate an action potential during neurotransmission (Fig. 9-6). When the receptor binds acetylcholine, a conformational change of the receptor opens a membrane channel that conducts ions. 

The answers are 31-b, 32-a, 33-d (Katzung, pp 4—7.) The absorption, distribution, and elimination of drugs require that they cross various cellular membranes The descriptions that are given in the question define the various transport mechanisms. The most common method by which ionic compounds of low molecular weight (100 to 200) enter cells is via membrane channels. The degree to which such filtration occurs varies from cell type to cell type because their pore sizes differ. 

Concerning the nature and structure of such amyloid peptide or protein channels, oligomers with annular morphologies have in fact been observed by EM for a-synuclein (Lashuel et al., 2002) and equine lysozyme (Malisauskas et al., 2003) even in the absence of any lipids or membranes. Channel-like structures have also been reconstituted in liposomes and observed by SFM for A/ i 4o, A/ j 42, human amylin, a-synuclein, ABri, ADan, and serum amyloid A (Fig. 5A Lin et al., 2001 Quist et al., 2005). Doughnut-shaped structures with a diameter of 10-12 nm and a central hole size of 1-2 nm (Fig. 5B) were imaged on top of lipid membranes (Quist et al., 2005). However, the radius of curvature of the SFM tips meant that it is not possible to say whether the pores were really traversing the lipid bilayer. 

Ca2+ enters cells via a number of plasma membrane channels which belong to three families (Figure 11.2) (i) voltage-gated channels (ii) ligand-gated channels and (iii) capacitative, or store-operated channels (SOCs). 

Cellular Nanotubes Membrane Channels for Intercellular Communication... 

Tunneling nanotubes (TNTs) were first described in cultured rat pheochromocytoma PC 12 cells as thin continuous membranous channels that span the shortest distance between connected cells (Fig. 16.1) (Rustom et al., 2004). They have a diameter between 25 and 200 nm, a length up to several tens of micrometers, and they are extended above the substratum and not in contact with it (reviewed in Gerdes et al. 

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