Significance
Epithelial cells have a spatially polarized organization. For example, one surface of an intestinal epithelial cell, called the apical side, faces the lumen of the gut and has a membrane composition distinct from those of the basolateral sides. Several proteins that control the development and maintenance of apical-basolateral polarity have been identified, but their molecular mechanisms are poorly understood. Lethal giant larvae (Lgl) is a basolateral polarity protein that is lost selectively from the apical membrane during development, due to its phosphorylation by atypical protein kinase C. Here, we describe the 3D structure of Lgl in both its unmodified and phosphorylated states, and show that phosphorylation of Lgl mediates a structural switch that controls its association with the plasma membrane.
Abstract
Metazoan cell polarity is controlled by a set of highly conserved proteins. Lethal giant larvae (Lgl) functions in apical-basal polarity through phosphorylation-dependent interactions with several other proteins as well as the plasma membrane. Phosphorylation of Lgl by atypical protein kinase C (aPKC), a component of the partitioning-defective (Par) complex in epithelial cells, excludes Lgl from the apical membrane, a crucial step in the establishment of epithelial cell polarity. We present the crystal structures of human Lgl2 in both its unphosphorylated and aPKC-phosphorylated states. Lgl2 adopts a double β-propeller structure that is unchanged by aPKC phosphorylation of an unstructured loop in its second β-propeller, ruling out models of phosphorylation-dependent conformational change. We demonstrate that phosphorylation controls the direct binding of purified Lgl2 to negative phospholipids in vitro. We also show that a coil–helix transition of this region that is promoted by phosphatidylinositol 4,5-bisphosphate (PIP2) is also phosphorylation-dependent, implying a highly effective phosphorylative switch for membrane association.
Common Design 