Plasma membrane properties

Plasma membrane properties involved in the photodynamic efficacy of merocyanine 540 and tetrasuMbnated aluminum phthalocyanine, Photochem. Photobioh, 71, 341,2000. 

Cholesterol is a principal component of animal cell plasma membranes, and much smaller amounts of cholesterol are found in the membranes of intracellular organelles. The relatively rigid fused ring system of cholesterol and the weakly polar alcohol group at the C-3 position have important consequences for the properties of plasma membranes. Cholesterol is also a component of lipoprotein complexes in the blood, and it is one of the constituents oiplaques that form on arterial walls in atherosclerosis. 

Cardiac glycosides (CG) are potent and highly specific inhibitors of the intrinsic plasma membrane Na+/K+-ATPase, also known as the sodium pump. They modulate electrophysiological properties of the heart and its contractile functions. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

The number of different proteins in a membrane varies from less than a dozen in the sarcoplasmic reticulum to over 100 in the plasma membrane. Most membrane proteins can be separated from one another using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), a technique that has revolutionized their study. In the absence of SDS, few membrane proteins would remain soluble during electrophoresis. Proteins are the major functional molecules of membranes and consist of enzymes, pumps and channels, structural components, antigens (eg, for histocompatibility), and receptors for various molecules. Because every membrane possesses a different complement of proteins, there is no such thing as a typical membrane structure. The enzymatic properties of several different membranes are shown in Table 41-2. 

Primarily using isolated plasma membrane vesicles as an experimental preparation, the functional properties of Na /H exchangers have been elucidated. The important kinetic properties include (1) stoichiometry (one-for-one) (2) reversibility (3) substrate specificity (monovalent cations Na, H, Li, NH4, but not K, Rb, Cs, choline) (4) modes of operation (Na -for-H, Na -for-Na Li " -for-Na, Na -for-NH4 ) (5) existence of an internal site for allosteric activation by (6) reversible inhibition by amiloride (Af-amidino-5-amino-6-chloropyr-azine carboxamide) and its 5-amino-substituted analogs and (7) competitive nature... 

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Gudheti, M. V., Mlodzianoski, ML, and Ffess, S. T. (2007) Imaging and shape analysis of giant unilamellar vesicles (GUVs) as model plasma membranes Effect of fnms-DOPC (dielaidoyl phosphatidylcholine) on membrane properties. Biophys. J. 10.1529/biophysj.106.103374. 

There are multiple isoforms of the a and p subunits of soluble GC that exhibit distinct tissue and cellular distributions. Most of the isoforms are expressed in brain, including 0Ci 3 and and P3. The p2 subunit is found primarily in lung. This isoform also contains a consensus sequence at its carboxy terminus for isoprenylation or carboxymethyl-ation, which could serve to anchor the protein to the plasma membrane. It has been proposed that specific isoforms of the a and P subunits of GC may form heterodimers with distinct functional and regulatory properties, although this remains to be established with certainty. 

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