[View Larger Version of this Image (158K JPEG file)]
Fig. 1.
Schematic presentation of the general
structure of GPCRs and receptor-ligand interactions. A,
general structure and terminology. Several distinct modes have been
observed for ligand binding and signal generation at exclusively the TM
core for photon, biogenic amines, nucleosides, eicosanoids, and
moieties (lysophosphatidic acid and sphingosine 1-phosphate) of lipids
(B), and the core, exoloops, and N-terminal segment for
peptides of 
40 amino acids (C). D, protease
ligands like thrombin bind to and cleave the N-terminal segment. The
resulting shorter N-terminal segment interacts with exoloops to
generate a signal, whereas the released peptide binds to platelet to
stimulate platelet aggregation. E, glycoprotein hormones,
LH, FSH, human CG, and TSH, bind to the ~350-amino acid N-terminal
segment and the liganded N-terminal segment interacts with exoloops to
generate a signal. F, small neurotransmitters,
Ca2+, glutamate, and GABA, bind to the ~600-amino acid
N-terminal segment, and the liganded N-terminal segment interacts with
the membrane-associated domain, thus generating a signal.
