Orphan receptor. Has spontaneous activity for beta-arrestin recruitment (PubMed:28827538). Shows a reciprocal regulatory interaction with the melatonin receptor MTNR1B most likely through receptor heteromerization (PubMed:28827538). Interacts with MTNR1B (PubMed:28827538). Interacts with ARRB1 and ARRB2 in a spontaneous and agonist-independent manner; leading to the internalization of GPR135 in the endosomal compartment (PubMed:28827538). Colocalizes with ARRB2/beta-arrestin-2 in the endosome. Belongs to the G-protein coupled receptor 1 family.