Distinct activities of Scrib module proteins organize epithelial polarity

Abstract

A polarized architecture is central to both epithelial structure and function. In many cells, polarity involves mutual antagonism between the Par complex and the Scrib module. While molecular mechanisms underlying Par-mediated apical determination are well-understood, how Scrib module proteins specify the basolateral domain remains unknown. Here, we demonstrate dependent and independent activities of Scrib, Dlg and Lgl using the Drosophila follicle epithelium. Our data support a linear hierarchy for localization, but rule out previously proposed protein-protein interactions as essential for polarization. Membrane recruitment of Scrib does not require palmitoylation or polar phospholipid binding but instead an independent cortically-stabilizing activity of Dlg. Scrib and Dlg do not directly antagonize aPKC, but may instead restrict aPKC localization by enabling the aPKC-inhibiting activity of Lgl. Importantly, while Scrib, Dlg and Lgl are each required, all three together are not sufficient to antagonize the Par complex. Our data demonstrate previously unappreciated diversity of function within the Scrib module and begin to define the elusive molecular functions of Scrib and Dlg. SIGNIFICANCE STATEMENTTo enable their physiological functions, cells must polarize their plasma membrane. In many epithelia, polarity is regulated by balanced activity of the apical Par complex and basolateral Scribble module. While the former is understood in molecular detail, little is known about how the latter works. We identify distinct functions of the three Scribble module proteins, separating independent roles in a localization hierarchy from cooperative roles in apical polarity antagonism and showing that they are not together sufficient to specify basolateral identity. This work establishes an essential basis for a mechanistic understanding of this core polarity machinery that controls processes ranging from stem cell divisions to organ morphogenesis across animal species.