Cell membranes receptor function

Figure 4.3 A prototype signal protein and its receptor is illustrated here. The human body relies on hundreds of different signal proteins docking into very specific and selective cell membrane receptor proteins to control which cell proteins are made or broken down and how the cell functions by sending specific chemical signals to other parts of the cell. The signal proteins may be produced by a nearby cell or reach its target from a distant cell through the blood stream.

The GABAA receptor is now believed to be the major target site for anaesthetic action. The GABAA receptors exist as a family of subtypes with their pharmacology determined by their composition. GABAA receptors are pentameric and comprise of two a, two 3 (or 0), and one y (or s) subunits, which assemble to form a chloride-sensitive pore. When the receptor is activated, transmembrane chloride conductance increases, resulting in hyperpolarisation of the postsynaptic cell membrane and functional inhibition of the postsynaptic neurone. 

NPs have physical dimensions close to cell membrane receptors and other biomolecules. This opens a new scenario to be explored. For example, the small dimensions are responsible for an enhanced penetrability of the cell membrane, since endocytosis is favored. The interaction with proteins is also affected by dimensions differences in surface curvature influence the ability of proteins to interact with surface functionalities, and consequently possible differences in conformational modification may occur.13 This is particularly relevant in the case of enzymes, where activity may depend on conformation. 

Differences in Fc receptor y-chain transmembrane domains (particularly polar or charged residues among the C-terminal 11 amino acids) influence cell membrane receptor expression and function [91]. The charged residues might protect Fc RI and FcYRIIIa receptors during membrane-associated receptor recycling in proteasomes associated with the endoplasmic reticulum or other intracellular structures [91]. 

Proteins embedded in the lipid bilayer of membranes play an important role in membrane functions, involving transport across the bilayer, electron flow and energy conversion, cell recognition, receptor functions, etc. There is not much information available on structural features of these proteins due to difficulties in crystallisation, necessary for complete structure determination. 

Numerous applications have been reported in the literature on the use of cell membrane receptors which require the use of ligand bilayer or phospholipid vesicles to maintain functionality, but other immobilization methods were also successfully used. The biosensing of e.g. acetylcholine and cholinergies has been reported with different transducers (ISFETs, interdigitated electrodes with measurements of capacity changes and optical fiber optode with fluorescence detection). 

The visual transduction system of retinal membranes contains a plethora of prenylated proteins. One of the prenylated components is a protein called transducin. This protein is a member of a family of G proteins that contain three distinct subunits (a, P, and y) and function in mediating signal transduction from cell membrane receptors to downstream effector proteins. In the case of visual transduction, photoactivation of rhodopsin leads to a conformational change that is sensed by transducin. In the absence of photoactivation of rhodopsin, transducin exists in a resting state in which Gt)P is bound to its a-subunit. Photoactivated rhodopsin catalyzes the exchange of GDP bound to transducin with GTP. This GTP-bound activated transducin then stimulates the enzymatic activity of another membrane-bound protein termed cyclic GMP phosphodiesterase. The latter... 

Recently the relationships of complement components to cell membrane receptors, cellular immune responses, and antibody production have been stimulated by the knowledge that some complement component genes are located on the histocompatibility chromosome. It is generally felt that structural analysis may show important unsuspected relationships to other proteins coded for in this area. There is harmony in these immune functions, and the details should soon be known. 

Tumor cells need certain nutrients such as iron to support their fast growing rate. Transferrin is the protein that transports iron through the bloodstream. Therefore, certain types of tumor cells overexpress transferrin receptors to capture this important element Binding transferrin to MSNPs helps in achieving a selective internalization in pancreatic tumor cells (PANC-1) and pre-metastatic breast cancer cells (BT-549) [42]. Another example of the use of proteins can be found in the functionalization of MSNPs with the cell membrane receptor CD4 that binds the glycoprotein gpl20 present on the capsid of HIV. These particles could be used in HIV diagnosis or treatment [44]. 

It can be seen then that the metabolic state of the cell is an important factor influencing surface membrane functions. Where viral transformation causes cancer-like properties, metabolic control at the nucleic acid level is likely, although viral-host interactions seem more complex than first theorized (Altman and Katz, 1976). Receptors for enteroviruses have been reported and shown to be specific for various viral strains. Susceptibility to viral infection is correlated with the presence of receptor sites on intracellular membranes as well as on the cell surface. Chemically, virus receptors solubilized from plasma membranes have been determined to be lipoproteins, with the protein moiety being most important in determining receptor activity (McLaren et al., 1968). A review of cell membrane receptors for viruses, antigens and... 

Other auxin-like herbicides (2,48) include the chlorobenzoic acids, eg, dicamba and chloramben, and miscellaneous compounds such as picloram, a substituted picolinic acid, and naptalam (see Table 1). Naptalam is not halogenated and is reported to function as an antiauxin, competitively blocking lAA action (199). TIBA is an antiauxin used in receptor site and other plant growth studies at the molecular level (201). Diclofop-methyl and diclofop are also potent, rapid inhibitors of auxin-stimulated response in monocots (93,94). Diclofop is reported to act as a proton ionophore, dissipating cell membrane potential and perturbing membrane functions. 

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. 

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