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G protein-coupled receptors (GPCRs) constitute a vast protein family that
encompasses a wide range of functions (including various autocrine, para-
crine and endocrine processes). They show considerable diversity at the
sequence level, on the basis of which they can be separated into distinct
groups. We use the term clan to describe the GPCRs, as they embrace a group
of families for which there are indications of evolutionary relationship,
but between which there is no statistically significant similarity in
sequence . The currently known clan members include the rhodopsin-like
GPCRs, the secretin-like GPCRs, the cAMP receptors, the fungal mating
pheromone receptors, and the metabotropic glutamate receptor family.
The rhodopsin-like GPCRs themselves represent a widespread protein family
that includes hormone, neurotransmitter and light receptors, all of
which transduce extracellular signals through interaction with guanine
nucleotide-binding (G) proteins. Although their activating ligands vary
widely in structure and character, the amino acid sequences of the
receptors are very similar and are believed to adopt a common structural
framework comprising 7 transmembrane (TM) helices [3-5].
Lysophospholipids (LPs), such as lysophosphatidic acid (LPA), sphingosine
1-phosphate (S1P) and sphingosylphosphorylcholine (SPC), have long been
known to act as signalling molecules in addition to their roles as
intermediates in membrane biosynthesis . They have roles in the
regulation of cell growth, differentiation, apoptosis and development, and
have been implicated in a wide range of pathophysiological conditions,
including: blood clotting, corneal wounding, subarachinoid haemorrhage,
inflammation and colitis . A number of G protein-coupled receptors bind
members of the lysophopholipid family - these include: the cannabinoid
receptors; platelet activating factor receptor; OGR1, an SPC receptor
identified in ovarian cancer cell lines; PSP24, an orphan receptor that has
been proposed to bind LPA; and at least 8 closely related receptors, the EDG
family, that bind LPA and S1P .
S1P is released from activated platelets and is also produced by a number
of other cell types in response to growth factors and cytokines . It is
proposed to act both as an extracellular mediator and as an intracellular
second messenger. The cellular effects of S1P include growth related
effects, such as proliferation, differentiation, cell survival and
apoptosis, and cytoskeletal effects, such as chemotaxis, aggregation,
adhesion, morphological change and secretion. The molecule has been
implicated in control of angiogenesis, inflammation, heart-rate and tumour
progression, and may play an important role in a number of disease states,
such as atherosclerosis, and breast and ovarian cancer . Recently, 5
G protein-coupled receptors have been identified that act as high affinity
receptors for S1P, and also as low affinity receptors for the related
lysophospholipid, SPC . EDG-1, EDG-3, EDG-5 and EDG-8 share a high degree
of similarity, and are also referred to as lpB1, lpB3, lpB2 and lpB4,
respectively. EDG-6 is referred to as lpC1, reflecting its more distant
relationship to the other S1P receptors .
EDG-1 was the first member of the family to be cloned (from phorbol-ester
differentiated human endothelial cells); its ligand, however, was unknown,
so it was named endothelial differentiation gene (EDG) 1, reflecting its
potential function . EDG-1 is expressed widely, with highest levels in
the brain, heart, lung, liver and spleen. Moderate levels are also found in
the thymus, kidney and muscle . Within these regions, EDG-1 is expressed
in endothelial cells, vascular smooth muscle, fibroblasts, melanocytes and
cells of epithelioid origin . Upon binding of S1P, the receptor can
couple to Gi1, Gi2, Gi3, Go and Gz type G proteins, leading to inhibition of
adenylyl cylase, phospholipase C activation and MAP kinase activation [6,8].
EDG1RECEPTOR is a 9-element fingerprint that provides a signature for the
EDG-1 sphingosine 1-phosphate receptor. The fingerprint was derived from an
initial alignment of 5 sequences: the motifs were drawn from conserved
sections within N- and C-terminal and loop regions, focusing on those
areas of the alignment that characterise the EDG-1 receptors but distinguish
them from the rest of the S1P receptor family - motifs 1 and 2 lie at the
N-terminus; motif 3 spans the first cytoplasmic loop; motif 4 is located in
the first external loop; motif 5 encodes the second cytoplasmic loop; motif
6 spans the third cytoplasmic loop; and motifs 7-9 reside at the C-terminus.
A single iteration on SPTR39_15f was required to reach convergence, no
further sequences being identified beyond the starting set.