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BACKGROUND:Planar cell polarity (PCP) is a phenomenon in which epithelial cells are polarized along the plane of a tissue.PCP is critical for a variety of developmental processes and is regulated by a set of evolutionarily conserved PCP signaling proteins.Many of the PCP proteins adopt characteristic asymmetric localizations on the opposing cellular boundaries.Currently,the molecular mechanisms that establish and maintain this PCP asymmetry remain largely unclear.Newly synthesized integral PCP proteins are transported along the secretory transport pathway to the plasma membranes.Once delivered to the plasma membranes,PCP proteins undergo endocytosis.Recent studies reveal insights into the intracellular trafficking of PCP proteins,suggesting that intracellular trafficking of PCP proteins contributes to establishing the PCP asymmetry.OBJECTIVE:To understand the intracellular trafficking of planar cell polarity proteins in the secretory transport pathway and endocytic transport pathway.METHODS:This review summarizes our current understanding of the intracellular trafficking of PCP proteins.We highlights the molecular mechanisms that regulate sorting of PCP proteins into transport vesicles and how the intracellular trafficking process regulates the asymmetric localizations of PCP proteins.RESULTS:Current studies reveal novel insights into the molecular mechanisms mediating intracellular trafficking of PCP proteins.This process is critical for delivering newly synthesized PCP proteins to their specific destinations,removing the unstable or mislocalized PCP proteins from the plasma membranes and preserving tissue polarity during proliferation of mammalian skin cells.CONCLUSION:Understanding how PCP proteins are delivered in the secretory and endocytic transport pathway will provide mechanistic insights into how the asymmetric localizations of PCP proteins are established and maintained.