Involved in regulation of planar cell polarity in the compound eye where it is required for correct specification of the R3 and R4 photoreceptor cells by regulating Fz activity in the R3/R4 precursor cells (PubMed:11893338). This is likely to occur through creation of an ft gradient so that the equatorial R3/R4 precursor cell has a higher level of ft function than its polar neighbor (PubMed:15548581). Also required for planar cell polarity of wing hairs (PubMed:12540853, PubMed:15240556). Mediates heterophilic cell adhesion in vitro and is required to stabilize ds on the cell surface (PubMed:15240556). Involved in regulation of eye imaginal disk size (PubMed:23667559). Upstream component of the Hippo pathway where it is likely to act as a cell surface receptor involved in regulation of tissue size and is required for the localization and stability of ex (PubMed:16996265). Probably acts as a cell surface receptor for ds (PubMed:20434335). Regulates mitochondrial electron transport chain integrity and promotes oxidative phosphorylation. Interacts with Fbxl7 (PubMed:25107277). Ft-mito interacts with NADH dehydrogenase subunit ND-24 and with ATP synthase subunit ATPsynC (PubMed:25215488). The extracellular domain is required for correct subcellular localization and for cell adhesion. The intracellular domain is sufficient for viability, growth control and planar cell polarity. Phosphorylated by fj on Ser/Thr of cadherin domains (PubMed:18635802). Phosphorylation by fj enhances binding to ds (PubMed:20434335). Phosphorylated in the cytoplasmic domain in a dco-dependent manner which is promoted by ds (PubMed:19574458). Proteolytically cleaved to yield stably associated N- and C-terminal fragments (PubMed:19574458). The C-terminal fragment is processed further to release a 68 kDa mitochondrial fragment, Ft-mito (PubMed:25215488). Severe overgrown imaginal disk derivatives and pupal death (PubMed:16996265). Overall larval growth is reduced (PubMed:25215488). Cells are round, swollen and have abnormal mitochondrial cristae due to defects in assembly of the mitochondrial electron chain complexes I and V (CI and CV) (PubMed:25215488). Loss of CI activity results in a switch to aerobic glycosis which increases lactate levels (PubMed:25215488). RNAi-mediated knockdown results in dorsal-ventral inversions in ommatidia planar cell polarity (PubMed:25215488). The name 'fat' originates from weak mutant alleles that exhibit a broadening of the abdomen.