During tubular morphogenesis, synchronised narrowing of apical cell surfaces transforms cells from a columnar to a wedge-like shape, resulting in bending of the epithelial sheet. This apical constriction is driven by a pool of apical-medial actomyosin interacting with the underlying longitudinal microtubule (MT) array, and both cytoskeletal systems and their interaction are necessary for the apical constriction. In this study (Gillard and Röper, 2024), the authors describe a dynamic apical-medial protein hub essential for apical constriction-driven tube budding of Drosophila salivary glands. They find that two actin crosslinkers, β-H-Spectrin and Filamin A, are enriched at this protein hub together with the MT-binding proteins Shot, Patronin and the multi-PDZ-protein Big bang. Here, β-H-Spectrin degradation results in the loss of F-actin and a reduction in the levels of hub proteins, indicating the need for β-H-Spectrin for hub assembly. Furthermore, when MTs are depleted by overexpressing the MT-severing protein Spastin, β-H-Spectrin and Big bang remain localised in apical-medial regions in contrast to what is seen for other hub components. These results suggest the existence of two groups of hub proteins: Patronin and Shot, which are dependent on the MT cytoskeleton, and β-H-Spectrin and Big bang, which are independent of it. Instead, β-H-Spectrin is recruited to the hub via phosphoinositide interactions, highlighting the importance of local phospholipid control. This study identifies a dynamic role for β-H-Spectrin in the apical-medial hub and offers insights into the mechanics of apical constriction regulation.
