Proteins mediator

Figure 13.3 G protein-mediated activation of adenylate cyclase by hormone binding. Hormone binding on the extracellular side of a receptor such as the P adrenergic receptor activates a G protein on the cytoplasmic ATP side. The activated form of the G protein...

Models also can assist in experimental design and the determination of the limits of experimental systems. For example, it is known that three proteins mediate the interaction of HIV with cells namely, the chemokine receptor CCR5, the cellular protein CD4, and the viral coat protein gpl20. An extremely useful experimental system to study this interaction is one in which radioactive CD4, prebound to soluble gpl20, is allowed to bind to cellular receptor CCR5. This system can be used to screen for... 

The first idea to consider is the effect of receptor density on sensitivity of a functional system to agonists. Clearly, if quanta of stimulus are delivered to the stimulus-response mechanism of a cell per activated receptor the amount of the total stimulus will be directly proportional to the number of receptors activated. Figure 5.8 shows Gi-protein-mediated responses of melanophores transiently transfected with cDNA for human neuropeptide Y-l receptors. As can be seen from this figure, increasing receptor expression (transfection with increasing concentrations of receptor cDNA) causes an increased potency and maximal response to the neuropeptide Y agonist PYY. 

Uptake of LCFAs across the lipid-bilayer of most mammalian cells occurs through both a passive diffusion of LCFAs and a protein-mediated LCFA uptake mechanism. At physiological LCFA concentrations (7.5 nM) the protein-mediated, saturable, substrate-specific, and hormonally regulated mechanism of fatty acids accounts for the majority (>90%) of fatty acid uptake by tissues with high LCFA metabolism and storage such as skeletal muscle, adipose tissue, liver,... 

Fatty Acid Transporters. Figure 2 Quencher-based real-time fatty acid uptake assay with a fluorescently labeled FFA analogue (C1-Bodipy-C12). Predominantly protein-mediated fatty acid uptake by 3T3-L1 adipocytes (diamonds) was compared with diffusion-driven uptake by fibroblasts (squares) using the QBT Fatty Acid Uptake reagent (Molecular Devices Corp., CA, USA), which contains C1-Bodipy-C12 as substrate in conjunction with a cell impermeable quencher. Uptake kinetics was recorded using a Gemini fluorescence plate reader. Error bars indicate the standard deviations from 12 independent wells. RFU relative fluorescence units. 

It should be noted that fast inactivation of receptor signaling not only involves the desensitization of the receptor but also the components of the downstream signaling cascade. The deactivation of active Ga subunits is controlled by the intramolecular hydrolysis of bound GTP, allowing it to reform the inactive heterotrimer. Termination of G protein-mediated signaling in vivo is 10- to 100-fold faster than the in vitro rate of GTP hydrolysis by Ga subunits, suggesting... 

Liu Y, Jones M, Hingtgen CM, Bu G, Laribee N, Tanzi RE, Moir RD, Nath A, He JJ (2000) Uptake of HIV-1 tat protein mediated by low-density lipoprotein receptor-related protein disrupts the neuronal metabolic balance of the receptor ligands. Nat Med 6 1380-1387... 

Neither L nor T channels appear susceptible to the form of G-protein-mediated inhibition characteristic of N or P/Q channels. However, as in the heart cells, L-type Ca +... 

Transition metal- or haem protein-mediated oxidative or reductive decomposition. 

G. V. Bloembcrg, E. Kamst, M. Hartevcid, K. M. G. M. van der Drift, J. Haverkamp, J, E. Thomas-Oates, B. J. J. Lugtenberg, and H. P. Spaink. A central domain of Rhizobium NodE protein mediates host specificity by determining the hydropho-bicity of fatty acyl moieties of nodulation factors. Mol. Microbiol. I6 123 (1995). 

Ou W, Silver J. Role of protein disulfide isomerase and other thiol-reactive proteins in HIV-1 envelope protein-mediated fusion. Virology 2006 350(2) 406-417. 

Viard M, Parolini I, Sargiacomo M, et al. Role of cholesterol in human immunodeficiency virus type 1 envelope protein-mediated fusion with host cells. J Virol 2002 76(22) 11584-11595. 

A G-protein-mediated effect has an absolute requirement for GTP. Reference has already been made to the requirement for GTP in reconstituting hormone-stimulated adenylate cyclase activity. A similar requirement can be demonstrated when the effector is an ion channel, such as the cardiac atrial inward-rectifier K+ channel which is activated following stimulation of the M2 muscarinic acetylcholine receptor. Thus, in the experiment illustrated in Figure 7.8, the channel recorded with a cell-... 

Effects mediated by G-protein coupled receptors (GPCRs) are very much slower than those mediated by, for example, ligand-gated ion channels, primarily because more steps are involved between activation of the receptor and the final response. For example, even in a simple, three-step, G-protein-mediated effect, such as the opening of atrial GIRK channels following the activation of M2 muscarinic receptors by acetylcholine, which follows the scheme ... 

FIGURE 7.14 Time-course of G-protein-mediated activation of GIRK potassium channels in rabbit sinoatrial node cells, (a). Outward current evoked by a 33-msec, 50-nA iontophoretic pulse of acetylcholine (between arrows), (b). Response of the unclamped cell to an iontophoretic pulse of acetylcholine (ACh). (Record (a) is adapted with permission from Trautwein et al., in Drug Receptors and Their Effectors, Birdsall, N. J. M., Ed., Macmillan, New York, 1980, pp. 5-22 record (b) is adapted with permission from Noma, in Electrophysiology of Single Cardiac Cells, Noble, D. and Powell, T., Eds., Academic Press, San Diego, CA, 1987, pp. 223-246.)... 

McGrath J, Solter D 1984 Inability of mouse blastomere nuclei transferred to enucleated zygotes to support development invitro. Science 226 1317-1319 Miyazaki S 1988 Inositol 1,4,5-trisphosphate-induced calcium release and guanine nucleotidebinding protein-mediated periodic calcium rises in golden hamster eggs. J Cell Biol 106 345-353... 

Soccio, R. E. and Breslow, J. L. 2003. StAR-related lipid transfer (START) proteins Mediators of intracellular lipid metabolism. J. Biol. Chem., 278(25) 22183-22186. 

Gu, W., Schneider, J.W., Condorelli, G., Kaushal, S., Mahdavi, V. and Nadal-Ginard, B. (1993) Interaction of myogenic factors and the retinoblastoma protein mediates muscle cell commitment and differentiation. Cell 72, 309-324. 

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