The Fat4 intracellular domain controls turnover of Fat4/Dchs1 planar polarity membrane complexes

The Fat/Dachsous (Ft/Ds) pathway is a highly conserved pathway regulating planar cell polarity (PCP) across different animal species. Proteins from the Ft and Ds family are large transmembrane protocadherins that form heterophilic complexes on the boundaries between cells. Fat4 and Dchs1, the main mammalian homologues of this pathway, have been implicated in PCP in various epithelial tissues and were shown to form extremely stable complexes at the boundaries between cells. It is unclear however, what are the dynamics controlling such stable boundary complexes, and how the formation and turnover of these complexes is regulated. Here, we use quantitative live imaging to elucidate the role of the intracellular domains (ICD) of Fat4 and Dchs1 in regulating Fat4/Dchs1 complex dynamics. We show that removing the ICD of Fat4 results in a reduction of both Trans-endocytosis (TEC) of Dchs1 into the Fat4 cells and boundary accumulation, but does not affect the diffusion of the complexes at the boundary. We further show that the ICD of Fat4 controls the turnover rate of Fat4/Dchs1 complexes. Finally, we find that while actin polymerization is required for maintaining the boundary accumulation of Fat4/Dchs1 complexes, we do not identify correlations between Fat4/Dchs1 complexes and local actin accumulation. Overall, we suggest that the Fat4 ICD is important for the turnover and plasticity of the highly stable Fat4/Dchs1 complexes associated with PCP. Statement of SignificanceThe purpose of this study is to elucidate the dynamics leading to the formation and maintenance of Fat4/Dchs1 complexes at the cell boundary and how it is affected by the ICD of Fat4 in mammals. The insights from this work are important for obtaining a mechanistic molecular framework for understanding formation of planar cell polarity as well as for highlighting how cell boundary membrane complexes are regulated.