Scaffolding protein that facilitates the localization of proteins to the cell membrane (PubMed:11927608, PubMed:16678097, PubMed:22006950). Required for the correct formation of tight junctions and epithelial apico-basal polarity (PubMed:11927608, PubMed:16678097). Positively regulates epithelial cell microtubule elongation and cell migration, possibly via facilitating localization of PRKCI/aPKC and PAR3D/PAR3 at the leading edge of migrating cells (By similarity). Plays a role in the correct reorientation of the microtubule-organizing center during epithelial migration (By similarity). May regulate the surface expression and/or function of ASIC3 in sensory neurons (By similarity). May recruit ARHGEF18 to apical cell-cell boundaries (PubMed:22006950). Forms a ternary complex with PALS1, CRB1 and CRB3 (PubMed:11927608, PubMed:11964389). Component of a complex whose core is composed of ARHGAP17, AMOT, PALS1, INADL/PATJ and PARD3/PAR3 (PubMed:16678097). Forms a heterotrimeric complex composed of MMP5, LIN7B and PATJ; the N-terminal L27 domain of PALS1 interacts with the L27 domain of PATJ and the C-terminal L27 domain of PALS1 interacts with the L27 domain of LIN7B (By similarity). Component of a complex composed of CRB3, PALS1 and PATJ (By similarity). Interacts (via N-terminus) with PALS1/PALS (via PDZ domain) (PubMed:11927608, PubMed:12527193). Interacts with TJP3/ZO-3 and CLDN1/claudin-1 (PubMed:12021270). Interacts with ASIC3, KCNJ10, KCNJ15, GRIN2A, GRIN2B, GRIN2C, GRIN2D, NLGN2, HTR2A and SLC6A4 (By similarity). Interacts with MPP7 (PubMed:12021270). Directly interacts with HTR4 (By similarity). Interacts (via PDZ domain 8) with WWC1 (via the ADDV motif) (PubMed:18596123). Interacts with SLC6A4 (By similarity). Interacts (via C-terminus) with ARHGEF18 (PubMed:22006950). Interacts with NPHP1 (PubMed:19755384). Interacts with PARD3/PAR3 (By similarity). Localizes to the apical region at the start of epithelial cell polarization then locates to tight junctions as polarization is completed (PubMed:11964389). Localized in the paranodal region of myelinating Schwann cells (By similarity). Localized to the leading edge of the actin cortex of migrating epithelia cells (By similarity). Expressed in renal tubules (at protein level) (PubMed:19755384). Expressed in bladder, testis, ovary, small intestine, colon, heart, skeletal muscle, pancreas and cerebellum in the brain. The L27 domain (also called Maguk recruitment domain) is required for interaction with PALS1 and CRB3, and PALS1 localization to tight junctions. The PDZ domain 6 mediates interaction with the C-terminus of TJP3 and is crucial for localization to the tight junctions (PubMed:12021270). The PDZ domain 8 interacts with CLDN1 but is not required for proper localization (PubMed:12021270).