β_Havy-spectrin has a restricted tissue and subcellular distribution during Drosophila embryogenesis

The components of the membrane skeleton play an important role in maintaining membrane structure during the dynamic changes in cell shape that characterize development, β_Heavy-spectrin is a unique β-spectrin from Drosophila melanogaster that is closer in size (M_r=430×10³) to dystrophin than to other β-spectrin members of the spectrin/α-actinin/dystrophin gene super-family. Here we establish that both the subcellular localization of the β_Heavy-spectrin protein and the tissue distribution of β_Heavy-spectrin transcript accumulation change dramatically during embryonic development. Maternally loaded protein is uniformly distributed around the plasma membrane of the egg. During cellularization it is associated with the invaginating furrow canals and in a region of the lateral membranes at the apices of the forming cells (apicolateral). During gastrulation the apicolateral staining remains and is joined by a new apical cap, or plate, of β_Heavy-spectrin in areas where morphogenetic movements occur. These locations include the ventral and cephalic furrows and the posterior midgut invagination. Thus, dynamic rearrangement of the subcellular distribution of the protein is precisely coordinated with changes in cell shape. Zygotic message and protein accumulate after the germ band is fully extended, in the musculature, epidermis, hindgut, and trachea of the developing embryo, βHeavy-spectrin in the epidermis, hindgut, and trachea is apically localized, while the protein in the somatic and visceral musculature is not obviously polarized. The distribution of β_Heavy-spectrin suggests roles in establishing an apicolateral membrane domain that is known to be rich in intercellular junctions and in establishing a unique membrane domain associated with contractile processes.