Updated project metadata. The microbiota generates diverse metabolites to modulate host physiology and disease, but their protein targets and mechanisms of action have not been fully elucidated. To address this challenge, we focused on microbiota-derived indole metabolites and develop photoaffinity chemical probes for proteomic studies. We identified many potential indoleinteracting proteins, including metabolic enzymes, transporters, immune sensors, and Gprotein coupled receptors. Notably, we discovered aromatic monoamines can activate - arrestin recruitment by the orphan receptor GPRC5A. Metabolomic and functional profiling of microbiota species revealed specific aromatic amino acid decarboxylaseexpressing bacteria producing monoamine agonists for GPRC5A. Structure-activity relationship studies of synthetic aromatic monoamines identified 7-fluorotryptamine as a more potent activator of -arrestin recruitment by GPRC5A. Our results highlight the utility of chemoproteomics to identify novel microbiota metabolite-interacting proteins and discover potential microbiota-derived small molecule agonists for orphan receptors.