Abstract
Background and Purpose: A large number of GPCRs are potentially valuable
drug targets but remain understudied. Many of these lack well-validated
activating ligands and are considered “orphan” receptors, and G
protein coupling profiles have not been defined for many orphan GPCRs.
Here we asked if constitutive receptor activity can be used to determine
G protein coupling profiles of orphan GPCRs. Experimental Approach: We
monitored GDP-sensitive interactions (coupling) between 48 understudied
orphan GPCRs and five G proteins (240 combinations) using
bioluminescence resonance energy transfer (BRET). No activating ligands
were used. Constitutive BRET between the same receptors and β-arrestins
was also measured. Key Results: We found sufficient GDP-sensitive BRET
to generate G protein coupling profiles for 22 of the 48 receptors we
studied. Altogether we identified 48 coupled receptor-G protein pairs,
many of which have not been described previously. Conclusion and
Implications: We conclude that receptor-G protein complexes that form
spontaneously in the absence of guanine nucleotides can be used to
profile G protein coupling of constitutively active GPCRs. This approach
may prove useful for studying G protein coupling of other GPCRs for
which activating ligands are not available.