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Vol. 58, Issue 2, 407-412, August 2000
1 Coexpression and Fusion
Protein Studies
Department of Biochemistry, Merck, Sharp & Dohme Research
Laboratories, Neuroscience Research Centre, Terlings Park, Harlow,
Essex, United Kingdom
Recombinant receptor cell lines are widely used in G-protein-coupled
receptor selectivity studies. To unequivocally interpret the results of
such studies, it is essential that the host cell line does not
endogenously express the receptor of interest and in addition is
unresponsive to the receptor's natural ligand. Here we describe an
approach to overcome such difficulties associated with orphan receptors
or, as in the present case, receptors whose endogenous ligand
ubiquitously affects mammalian cells. The functional heterologous assay
system described is for the hEdg2 receptor, which uses lysophosphatidic
acid as its endogenous ligand. Once activated, this receptor mediates
its effects via multiple secondary messenger pathways, including a
Gi-coupled pathway. We have transiently expressed a pertussis
toxin-insensitive hEdg2 receptor-ratGi
1 fusion protein into human
embryonic kidney cells and have monitored the ability of compounds to
stimulate [35S]GTP
S binding in membranes prepared from
these cells after pretreatment with toxin. Because the assay conditions
used favor Gi-mediated responses and because endogenous Gi
subunits
are rendered inactive, the response measured is, by definition, fusion
protein-mediated. Consequently, we have developed an assay that
monitors definitively Edg2 receptor-mediated responses in a mammalian
cell line. A limited structure activity relationship study suggests
that the lysophospholipid carbon chain has a role in receptor
activation and in addition indicates that certain modifications to the
phosphate group are tolerated.
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