Plasma membrane functions

Two of the cytoskeletal components, the actin filaments and the microtubules have been studied with molecular rotors. The main component of the actin filaments is the actin protein, a 44 kD molecule found in two forms within the cell the monomeric globulin form (G-actin) and the filament form (F-actin). Actin binds with ATP to form the microfilaments that are responsible for cell shape and motility. The rate of polymerization from the monomeric form plays a vital role in cell movement and signaling. Actin filaments form the cortical mesh that is the basis of the cytoskeleton. The cytoskeleton has an active relationship with the plasma membrane. Functional proteins found in both structures... 

Atrophy of the thymus is a consistent finding in mammals poisoned by 2,3,7,8-TCDD, and suppression of thymus-dependent cellular immunity, particularly in young animals, may contribute to their death. Although the mechanisms of 2,3,7,8-TCDD toxicity are unclear, research areas include the role of thyroid hormones (Rozman et al. 1984) interference with plasma membrane functions (Matsumura 1983) alterations in ligand receptors (Vickers et al. 1985) the causes of hypophagia (reduced desire for food) and subsequent attempts to alter or reverse the pattern of weight loss (Courtney et al. 1978 Seefeld et al. 1984 Seefeld and Peterson 1984) and excretion kinetics of biotransformed metabolites (Koshakji et al. 1984). 

Zaidi M, Shankar VS, Tunwell R, Adebanjo OA, Mackrill. 1. Pazianas M, O Connell D, Simon BJ, Rifkin BR, Venkitaraman AR, Huang, CL-H, Lai FA. 1995. A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing. J Clin Invest 96 1582-90. 

Two main mechanisms are generally proposed for copper induced changes to the plasma membrane function direct effects on sulphydryl groups of membrane constituents and direct or indirect free radical mediated lipid peroxidation. 

Switching Mediated by Guanine Nucleotide-Binding Proteins Another group of Intracellular switch proteins constitutes the GTPase superfamily. These proteins include monomeric Ras protein (see Figure 3-5) and the G subunit of the trimeric G proteins. Both Ras and G , are bound to the plasma membrane, function in cell signaling, and play a key role in cell proliferation and differentiation. Other members... 

Plasma membrane function Cell wall synthesis... 

Bacitracin is a mixture of similar peptides produced by fermentation of the bacterium Bacillus subtilis. The A-type component predominates. Its mode of action is to inhibit both peptidoglycan biosynthesis at a late stage (probably at the dephosphorylation of the phospholipid carrier step) and disruptions of plasma membrane function. It is predominantly active against Gram-positive microorganisms, and parenteral use is limited to IM injection for infants with pneumonia and empyema caused by staphylococci resistant to other agents. It is rather neuro- and nephrotoxic and, therefore, is used in this manner with caution. Bacitracin also is widely employed topically to prevent infection in minor cuts, scrapes, and burns. 

O Dell, B. L. (2000). Role of zinc in plasma membrane function. /. Nutr. 130,1432S-1436S. 

Fig. 1. The GP Ib-IX-V complex. The complex consists of seven transmembrane polypeptides denoted GP Iba (mol wt 145,000), GP IbP (mol wt 24,000), GPIX (mol wt 17,000) and GP V (mol wt 82,000), in a stoichiometry of 2 2 2 1. The hatched region represents the plasma membrane. The area above the hatched region represents the extracellular space that below represents the cytoplasm. The complex is a major attachment site between the plasma membrane and the cytoskeleton. Two molecules associated with the cytoplasmic domain are depicted a 14-3-3 dimer, which may mediate intracellular signaling, and actin-binding protein, which connects the complex to the cortical cytoskeleton and fixes its position and influences its function.

Biochemically, most quaternary ammonium compounds function as receptor-specific mediators. Because of their hydrophilic nature, small molecule quaternaries caimot penetrate the alkyl region of bdayer membranes and must activate receptors located at the cell surface. Quaternary ammonium compounds also function biochemically as messengers, which are generated at the inner surface of a plasma membrane or in a cytoplasm in response to a signal. They may also be transferred through the membrane by an active transport system. 

Adenosine, 6-amino-9-P-ribofuranosyl-9-ff-purine (see Table 1), is an endogenous nucleoside found in all ceUs of the body. Its ubiquitousness suggests that adenosine functions as an autocoid and that its actions are mediated by specific receptors on the plasma membranes of all ceUs. 

In addition to binding to sialic acid residues of the carbohydrate side chains of cellular proteins that the virus exploits as receptors, hemagglutinin has a second function in the infection of host cells. Viruses, bound to the plasma membrane via their membrane receptors, are taken into the cells by endocytosis. Proton pumps in the membrane of endocytic vesicles that now contain the bound viruses cause an accumulation of protons and a consequent lowering of the pH inside the vesicles. The acidic pH (below pH 6) allows hemagglutinin to fulfill its second role, namely, to act as a membrane fusogen by inducing the fusion of the viral envelope membrane with the membrane of the endosome. This expels the viral RNA into the cytoplasm, where it can begin to replicate. 

LIPOPROTEINS. Blood plasma lipoproteins are prominent examples of the class of proteins conjugated with lipid. The plasma lipoproteins function primarily in the transport of lipids to sites of active membrane synthesis. Serum levels of low density lipoproteins (LDLs) are often used as a clinical index of susceptibility to vascular disease. 

Cells make use of many different types of membranes. All cells have a cytoplasmic membrane, or plasma membrane, that functions (in part) to separate the cytoplasm from the surroundings. In the early days of biochemistry, the plasma membrane was not accorded many functions other than this one of partition. We now know that the plasma membrane is also responsible for (1) the exclusion of certain toxic ions and molecules from the cell, (2) the accumulation of cell nutrients, and (3) energy transduction. It functions in (4) cell locomotion, (5) reproduction, (6) signal transduction processes, and (7) interactions with molecules or other cells in the vicinity. 

FIGURE 9.32 Proteoglycans serve a variety of functions on the cytoplasmic and extracellular surfaces of the plasma membrane. Many of these functions appear to involve the binding of specific proteins to the glycosaminoglycan groups. 

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