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