Fig. 1.

A cascade of integrin signalling establishes polarity. (A) Integrin-binding to extracellular collagen initiates a Rac1-dependent pathway that stimulates the production of a basal actin cortex through IRSp53. (B) Laminin is assembled into a basement membrane in a Rac1-depdendent mechanism. In the Drosophila egg chamber, the ER exit site factor Tango1 is required for basal secretion, while Rab10 and the GEF Crag restrict basement membrane secretion and assembly basally. (C) Integrin binding to extracellular laminin that is organised into a basement membrane induces polarity signalling through the scaffolding factor ILK. The Par3 complex forms independently, but basal ECM–integrin positioning is needed to orientate the apical surface.