Pores in membranes

Prince, R. C., 1990. At least one Bacillus thuringiensis toxin forms ion-selecdve pores in membranes. Trends in Biochemical Sciences 15 2-3. 

K Channels belong to a class of membrane proteins that form highly K-selective pores in membranes. All known K Channels are composed of several (usually four) pore forming alpha subunits and auxiliary beta subunits. K Channels play an essential role in cellular excitability, being involved in repolarization of Action Potentials and setting the cell resting potential as well as contributing to potassium homeostasis. 

It is well recognized that is an important regulatory element for many cellular processes, and that the major entry pathway for Ca in many cell types is via plasma membrane Ca channels. Ca channels are functional pores in membranes. They exist in plasma membranes, transverse tubule membranes and in intracellular membranes such as the sarcoplasmic and endoplasmic reticulum. Ca channels are normally closed when opened, Ca passively flows through the chan-... 

Tolpekina, T.V., den Otter, W.K., Briels, W.J. Simulations of stable pores in membranes system size dependence and line tension. J. Chem. Phys. 2004, 121, 8014-20. 

The pharmacokinetics of PVP is well understood as a result of this agent s experimental use to determine the properties of pores in biological membranes. PVP molecules can readily penetrate hydrophilic pores in membranes if they are small enough, and they are also taken up by pinocytotic vesicles. Apparently, PVP is not detectably bound to membrane surfaces and hence does not provide long-lasting viscosity enhancement beyond the normal residence time in the tears. 

Nunes, S.P., Recent advances in the controlled formation of pores in membranes. Trend Polym. Sci., 5, 187, 1997. 

Initial speculation on the existence of small aqueous pores in membranes was based on high membrane permeability of small polar molecules. For example, the permeability of water is 1000-fold more, and that of urea 10- to 100-fold more, than predicted. These types of observations led to the prediction of aqueous channels with radii of approximately 4 A. 

Figure 10. Corresponding orientations of columnar pores in membranes having two different configurations shown in Figure 9.

Thus, the nature of these membranes and the chemical and physical properties of the toxicant in question are important factors affecting uptake. The mechanisms by which chemical agents pass through the membranes include (1) filtration through spaces or pores in membranes (2) passive diffusion through the spaces or pores, or by dissolving in the lipid material of the membrane and (3) facilitated transport, whereby specialized transport systems carry water-soluble substances across the membrane by a lipid soluble "carrier" molecule, which complexes with the chemical. It can be seen then that, as far as the chemical properties are concerned, lipophilicity is the most important factor affecting absorption. 

As demonstrated above, the edge tension is a sensitive parameter, which effectively characterizes the stability of pores in membranes. Compiling a database for the effect of various types of membrane inclusions will be useful for understanding the lifetime of pores in membranes with more complex compositions, which is important for achieving control over medical applications for drug and gene delivery in cells. 

This category also includes a large number of families of toxic or apoptotic molecules that form pores in the membrane, typically killing the cell. One family of these pore-forming molecules is comprised of simple ceramides that can form pores in membranes at physiological concentrations. At low concentrations these molecules serve as second messengers... 

Organic molecules forming complexes with sodium ions in water, such as monensin, speed up ion transport by factors of about 10, molecules that form tunnels by factors of 10 ° and more. Scheme 2.7.1 depicts two typical structures of ion transport agents, namely monensin and filipin. Gramicidine, on the other hand, is thought to form a pore in membranes. The only clear-cut distinction between an ion pore and an ion transporter is that a pore can be closed and reopened by stopper molecules, whereas ion transport will always occur as long as complexation in membranes and decomplexation in water can occur. 

Bayley, H. Building Doors into Cells. Sd. Amer. 277 (3), 62-67 (1997). [Protein engineering can create artificial pores in membranes for drug delivery.]... 

Some papers have dealt with the dynamics of proton-containing cations within the pores in membranes of Nafion, a polymer with a highly hydro-phobic poly(tetrafluoroethylene) backbone and sulfonic acid side groups, used in fuel cells. Neves and co-workers reported fast-field cycling measurements of H relaxation in unmodified Nafion as well as in membrane modified by incorporation of RTILs. The data were interpreted using a... 

The pores in membranes of this type are formed by packing plate-shaped crystals in a parallel fashion. Slit-shaped pores where the slit width and plate thickness are about the same [8] are obtained in this way. 

---