The generation and maintenance of correct lumen size and shape is essential for the function of tubular organs. Now, Monn Monn Myat and co-workers report that p21-activated kinase (Pak1) plays a novel role during lumen formation in Drosophila embryonic salivary glands (see p. 4177). The researchers show that Pak1 regulates the size and elongation of the apical domain of individual epithelial cells in the developing gland by decreasing and increasing E-cadherin levels at adherens junctions and basolateral membranes, respectively. Pak1 mediates these effects, they report, through Rab5- and Dynamin-dependent endocytosis of E-cadherin. Moreover, constitutively active Pak1 induces the formation of multiple intercellular lumens in the gland, an effect that is dependent on Rab5 and Dynamin, and on the Pak1 substrate Merlin. Together, these results identify a crucial role for Pak1 and E-cadherin endocytosis in lumen size and shape determination in fly salivary glands, and highlight a mechanism for multiple lumen formation, a process that occurs in pathological conditions such as breast ductal carcinoma in situ.

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