The apical transmembrane protein Crumbs is a determinant of apical-basal cell polarity. The short cytoplasmic domain of Crumbs, containing two highly conserved motifs, a PSD95/Discs-large/ZO1 (PDZ) binding motif (PBM) and a 4.1/ezrin/radixin/moesin (FERM)-binding motif (FBM), plays an essential role for Crumbs’ function in definition of the apical boundary during development. It has been widely accepted that Crumbs forms a critical polarity complex with a membrane-associated guanylate kinase (MAGUK) family protein Pals1 and a PDZ-containing protein PATJ via the PBM/Pals1 interaction. However, the roles of FBM in Crumbs are poorly understood. Recently, Médina et al reported that Crumbs interacts with a FERM domain-containing protein Moesin via its FBM. As Moesin connects to cyto...[ Read more ]

The apical transmembrane protein Crumbs is a determinant of apical-basal cell polarity. The short cytoplasmic domain of Crumbs, containing two highly conserved motifs, a PSD95/Discs-large/ZO1 (PDZ) binding motif (PBM) and a 4.1/ezrin/radixin/moesin (FERM)-binding motif (FBM), plays an essential role for Crumbs’ function in definition of the apical boundary during development. It has been widely accepted that Crumbs forms a critical polarity complex with a membrane-associated guanylate kinase (MAGUK) family protein Pals1 and a PDZ-containing protein PATJ via the PBM/Pals1 interaction. However, the roles of FBM in Crumbs are poorly understood. Recently, Médina et al reported that Crumbs interacts with a FERM domain-containing protein Moesin via its FBM. As Moesin connects to cytoskeleton, the Crumbs/Moesin interaction provides a potential mechanism to stabilization of the apical domain.

Here, we biochemically and structurally characterized the interaction between the cytoplasmic domain of Crumbs and the FERM domain of Moesin. We confirmed that the FBM is required for the Crumbs/Moesin interaction. Surprisingly, we further identified that the PBM also contributes to the interaction. We determined the crystal structure of the Crumbs/Moesin complex at 1.5 Å resolution. Consistent with our biochemical data, the complex structure shows that both the FBM and PBM are involved in the binding of Crumbs to the FERM domain of Moesin. Together, our findings indicate that the two motifs of Crumbs not only act as binding sites but also regulate the interactions between Crumbs and their binding partners. This finding provides a direct linkage between the establishment of the apical-basal polarity and the stabilization of the apical domain together. Additionally, the PBM in the Crumbs/Moesin complex occupies the InsP₃ binding site in the Moesin FERM domain, suggesting that Crumbs may compete the binding of Moesin with plasma membrane.