p.p1 to light, Ca+ and odorants [12, 13, 14,

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Membrane proteins that behave as signal transducers can be broadly categorised
into three groups: Ion channels, G-protein linked receptors, (GPCR)
and enzyme-linked receptors. Ion channels are usually multimeric and form
an aqueous pore in the plasma membrane that, upon specific perturbation,
allow movement of inorganic ions in and out of the cells 9. Voltage-gated
ion channels respond to change in membrane potential whereas transmitter
gated channels respond to binding of neurotransmitters 10, 11. The second
class of signal transducers, GPCRs, are one of the most diverse receptors
that 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 superfamily
in the human genome. Despite the large diversity of GPCRs, they interact
with a relatively small number of heterotrimeric (composed of ?, ? and ? subunits)
G proteins to initiate intracellular signalling cascades 16. GPCRs are
usually characterised by the presence of seven-transmembrane regions and
this includes a class of approximately 140 GPCRs called ‘orphan GPCRs’
whose sequence is known but the endogenous ligands to which they bind are
still largely unknown 17. The third class of signal transducers are enzyme
linked receptors which, unlike the GPCRs, are usually single transmembrane
proteins that, instead of recruiting a G-protein for signalling activity, usually
contain intrinsic enzymatic activity in their cytoplasmic domain or associate
directly with enzymes with signalling capabilities. Six different classes of enzyme
linked receptors have been described: (1) receptor guanylyl cyclases,
(2) receptor tyrosine phosphatases, (3) receptor serine/threonine kinases, (4)

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