Subcellular tube guidance requires anchoring of the apical and basal actin cortices through late endosomes

The actin cytoskeleton participates in a range of cellular processes. It supports cell shape changes by propagating forces within cells and between cells and their environment. Terminal cells of the Drosophila respiratory system form a subcellular tube by invaginating their apical plasma membrane; cortical actin networks at the basal and apical plasma membranes are critical for proper morphogenesis. Basal actin affects apical membrane morphogenesis, and it is not known how the two separate actin pools communicate. We report here that actin assembles around vesicles of the late endocytic pathway, which are present in the growth cone of the cell, between the tip of the subcellular tube and the leading filopodia of the basal membrane. Actin organized at late endosomes extends towards both 20 membrane compartments. Preventing proper actin nucleation at late endosomes disturbs the directionality of tube growth, uncoupling it from the direction of cell elongation. Severing actin in this area affects tube integrity. These findings demonstrate a role for the late endocytic pathway in organizing actin for proper cell morphogenesis, in addition to its known role in membrane and protein trafficking. Introduction Cell shape changes require coordination between the actin cytoskeleton which determines shape, and the plasma membrane which has to be delivered to and removed from the appropriate domains to allow the formation of new structures. Therefore, membrane30 cytoskeleton interactions are important in shape transitions, since they transfer the forces generated by the cytoskeleton to the membrane at the cell surface, which will ultimately define the new shape (Sitarska and Diz-Muñoz, 2020). Shape changes also require subcellular regulation; migrating neural crest cells extend filopodia in the direction of migration to increase traction, while forming actomyosin contractions in the rear, propelling the cell forward (Piacentino et al., 2020). Cells undergoing apical constriction restrict an actomyosin network to the apical cell cortex and retrieve apical plasma membrane, while