Scribble is a highly conserved cell polarity regulator that plays major roles in development and tumour suppression in Drosophila and mammals. Indeed, loss of function Scribble mutations in the mouse and human population leads to severe neural tube defects and neonatal lethality, and we have shown in mouse and Drosophila models that its loss of function leads to loss of cell polarity and neoplasia in multiple epithelial tissues in vivo.
Scribble is part of a genetic module that includes Discs-large (Dlg) and Lethal-giant-larvae (Lgl) proteins, and acts as a highly localized signalling adaptor. Scribble regulates cell polarity as well as other critical cellular processes through binding to a large number of signalling proteins including RhoGEFS, kinases, phosphatases and signalling receptors. In addition, Scribble and Dlg proteins bind to, and are targeted for degradation by the oncogenic human HPV E6 proteins implicating them in the regulation of human cancer. How Scribble manages and discriminates between the large number of reported protein interactions is unclear but is key to understanding its fundamental role in development and cancer.
To dissect Scribble binding interactions, we have used a combination of structural, imaging, proteomics, and functional studies in Drosophila and mammalian cells. Scribble contains a 16 LRR domain, and 4 PDZ domains that alone account for the majority of reported interactions of Scribble to its ligands. Here, we report for the first time atomic scale protein structures for human and Drosophila Scribble PDZ domains bound to their key biological partners. Together with biochemical and proteomic approaches, our experiments reveal multiple modes of ligand recognition, including specific binding hierarchies for each ligand, structural features that provides ligand specificity, and emerging evidence for higher order structural regulation of ligand accessibility. These studies thus provide critical insight into how Scribble may regulate distinct signalling pathways in vivo, and pave the way for the development of chemical and genetic strategies to therapeutically target pathological alterations in the Scribble complex.