The mammalian bombesin-like neuropeptides, gastrin releasing peptide (GRP) and neuromedin B (NMB), mediate a range of biological responses in normal cells, including promotion of paracrine or autocrine growth in some human small cell lung carcinoma (SCLC) cells. Recently we reported the cloning and characterization of three pharmacologically distinct rodent and human bombesin peptide receptors gastrin releasing peptide receptor (GRP-R), neuromedin-B receptor (NMB-R) and bombesin receptor subtype 3 (BRS-3), with distinct patterns of expression in normal and malignant cells. These receptors consist of seven putative membrane-spanning domains separated by short intracellular and extracellular loops, which are coupled to phospholipase C activation, inositol phosphate metabolism and calcium mobilization through pertussis toxin insensitive G-proteins. GRP-R binds GRP at high affinity, NMB-R binds NMB at high affinity and BRS-3 binds both peptides at relatively low affinity. Construction and characterization of GRP-R/NMB-R chimeric receptors indicates that transmembrane domain 5 is critical for high affinity binding of NMB by NMB-R, in particular Ile216. Site directed mutagenesis studies highlight the importance of one or more serines and threonines in the C-terminal tail domain of the GRP-R for rapid and efficient receptor internalization. These residues appear to be rapidly phosphorylated when the receptor becomes coupled to its cognate heterotrimeric GTP-binding protein. Similar receptor mutagenesis analysis show that 15 amino acids immediately C-terminal to transmembrane domain 7, form a structural motif essential for the effector coupling, in conjunction with specific conserved amino acids in the second (Arginine139) and third (Lysine263) intracellular loop domains of the GRP-R.
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