p.p1 channels respond to change in membrane

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0px 0.0px 0.0px; font: 12.0px Helvetica}span.s1 {font: 8.0px Helvetica}Membrane proteins that behave as signal transducers can be broadly categorisedinto three groups: Ion channels, G-protein linked receptors, (GPCR)and enzyme-linked receptors. Ion channels are usually multimeric and forman aqueous pore in the plasma membrane that, upon specific perturbation,allow movement of inorganic ions in and out of the cells 9. Voltage-gatedion channels respond to change in membrane potential whereas transmittergated channels respond to binding of neurotransmitters 10, 11.

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The secondclass of signal transducers, GPCRs, are one of the most diverse receptorsthat can transmit diverse extracellular signals ranging from peptides, lipids,neurotransmitters and nucleotides to light, Ca+ and odorants 12, 13, 14, 15.There are approximately 800 identified members in the GPCR superfamilyin the human genome. Despite the large diversity of GPCRs, they interactwith a relatively small number of heterotrimeric (composed of ?, ? and ? subunits)G proteins to initiate intracellular signalling cascades 16. GPCRs areusually characterised by the presence of seven-transmembrane regions andthis includes a class of approximately 140 GPCRs called ‘orphan GPCRs’whose sequence is known but the endogenous ligands to which they bind arestill largely unknown 17. The third class of signal transducers are enzymelinked receptors which, unlike the GPCRs, are usually single transmembraneproteins that, instead of recruiting a G-protein for signalling activity, usuallycontain intrinsic enzymatic activity in their cytoplasmic domain or associatedirectly with enzymes with signalling capabilities.

Six different classes of enzymelinked receptors have been described: (1) receptor guanylyl cyclases,(2) receptor tyrosine phosphatases, (3) receptor serine/threonine kinases, (4)


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