Multifunctional protein which functions as transmembrane receptor in the planar cell polarity (PCP) and is involved in the assembly of the proton-transporting vacuolar (V)-ATPase protein pump (PubMed:20579883, PubMed:20579879, PubMed:29127204, PubMed:29995586). As transmembrane receptor mediates fz/PCP signaling through interaction with fz and stabilizes asymmetric PCP domains through its interaction with stan (PubMed:20579883, PubMed:20579879, PubMed:23292348, PubMed:29995586). Also mediates Wnt/beta-cat signaling through interaction with fz/fz2 (PubMed:20579883, PubMed:20579879). Probably by controlling the assembly of the V-ATPase pump and thus the acidification of the endo-lysosomal system, plays a role in many neuronal processes including synapse morphology and synaptic transmission (PubMed:26376863). Stabilizes asymmetric Planar Cell Polarity (PCP) domains through its interaction with stan. Interacts with fz and fz2 (PubMed:20579883). Interacts (via N-terminus) with stan (PubMed:23292348). As an accessory component of the multisubunit proton-transporting vacuolar (V)-ATPase protein pump, might interacts with VhaAC45 (PubMed:29995586). Co-localizes at the apical junctions with stan. Localization to the planar cell polarity domains depends by stan. Expressed in whole larval extracts, fat body and larval brain (PubMed:29127204). At prepupal stages, expressed towards the wing margin; during the junctional remodeling phase relocalizes to intracellular compartments; before prehair formation, enriched at P-D membranes (at protein level) (PubMed:23292348). Proteolytically cleaved by a furin-like convertase in the trans-Golgi network to generate N- and C-terminal fragments (PubMed:23292348, PubMed:29127204). Cleavage is reduced in the fat body (PubMed:29127204). RNAi-mediated knockdown is lethal (PubMed:20579883). RNAi-mediated knockdown in the wings results in increased apoptosis, endoplasmic reticulum stress and lipid accumulation (PubMed:20579883, PubMed:20579879, PubMed:29127204, PubMed:29995586). This is accompanied by severe growth defects including venation defects, defective wing-hair polarity (as a result of defective fz and stan trafficking) and wg signaling (PubMed:20579883, PubMed:20579879, PubMed:29127204, PubMed:29995586). RNAi-mediated knockdown in the notum causes severe planar polarity defects affecting orientation morphology and number of sensory bristles or microchaetae (PubMed:20579879). RNAi-mediated knockdown in the eye results in defective phototaxis and presynaptic transmission in vacuolated photoreceptor neurons and pigment cells (PubMed:26376863). RNAi-mediated knockdown in neurons leads to defective autophagy and neurodegeneration, impaired synapse morphology, ultrastructural organization and axonal transport of the active zone component brp, ultimately resulting in lethality at different developmental stages (PubMed:26376863). The few adult survivors show strongly reduced spontaneous movements and poor climbing abilities (PubMed:26376863). RNAi-mediated knockdown in the mushroom body results in altered short- and long-term memory (PubMed:26376863). RNAi-mediated knockdown in the fat body results in increased autophagy (PubMed:29127204).