The ubiquitous intracellular Ca2+ sensor calmodulin (CaM) regulates numerou
s proteins involved in cellular signaling of G protein-coupled receptors, b
ut most known interactions between GPCRs and CaM occur downstream of the re
ceptor. Using a sequence-based motif search, we have identified the third i
ntracellular loop of the opioid receptor family as a possible direct contac
t point for interaction with CaM, in addition to its established role in G
protein activation. Peptides derived from the third intracellular loop of t
he mu-opioid (OP3) receptor strongly bound CaM and were able to reduce bind
ing interactions observed between CaM and immunopurified OP3 receptor. Func
tionally, CaM reduced basal and agonist-stimulated S-35-labeled guanosine 5
'-3-O-(thio)triphosphate incorporation, a measure of G protein activation,
in membranes containing recombinant OP3 receptor. Changes in CaM membrane l
evels as a result of overexpression or antisense CaM suppression inversely
affected basal and agonist-induced G protein activation. The ability of CaM
to abolish high affinity binding sites of an agonist at OP3 further suppor
ts the hypothesis of a direct interaction between CaM and opioid receptors,
An OP3 receptor mutant with a Lys(273) --> Ala substitution (K273A-OP3), a
n amino acid predicted to play a critical role in CaM binding based on moti
f structure, was found to be unaffected by changes in CaM levels but couple
d more efficiently to G proteins than the wild-type receptor, Stimulation o
f both the OP1 (delta-opioid) and OP3 wild-type receptors, but not the K273
A-OP3 mutant, induced release of CaM from the plasma membrane. These result
s suggest that CaM directly competes with G proteins for binding to opioid
receptors and that CaM may itself serve as an independent second messenger
molecule that is released upon receptor stimulation.