β-H-Spectrin is a key component of an apical-medial hub of proteins during cell wedging in tube morphogenesis.

Abstract

Coordinated cell shape changes are a major driver of tissue morphogenesis, with apical constriction of epithelial cells leading to tissue bending. During the tube budding of the salivary glands in the Drosophila embryo we previously identified a key interplay between the apical-medial actomyosin, driving apical constriction, with the underlying longitudinal microtubule array. At this microtubule-actomyosin-interface a hub of proteins accumulates: as shown before, the microtubule-actin-crosslinker Shot and the minus-end-binder Patronin, and now identified two actin-crosslinkers, β-H-Spectrin and Filamin, and the multi-PDZ-protein Big-bang. We show that tissue-specific-degradation of β-H-Spectrin led to reduction of apical-medial F-actin, Shot, Patronin and Big-bang and concomitant defects in apical constriction, but residual Patronin was still sufficient to assist microtubule reorganisation. Contrary to Patronin and Shot, neither β-H-Spectrin nor Big bang required microtubules for their localisation. β-H-Spectrin was instead recruited via binding to apical-medial phosphoinositides. Overexpression of β-H-33 containing the PH domain displaced endogenous β-H-Spectrin and led to strong morphogenetic defects. This protein hub therefore required the synergy and coincidence of membrane- and microtubule-associated components for its assembly and function in sustaining the apical constriction during tubulogenesis.