Drosophila beta spectrin functions independently of alpha spectrin to polarize the Na,K ATPase in epithelial cells

Spectrin has been proposed to function as a sorting machine that concentrat es interacting proteins such as the Na,K ATPase within specialized plasma m embrane domains of polarized cells. However, little direct evidence to supp ort this model has been obtained. Here we used a genetic approach to direct ly test the requirement for the beta subunit of the alpha beta spectrin mol ecule in morphogenesis and function of epithelial cells in Drosophila. beta Spectrin mutations were lethal during late embryonic/early larval developm ent and they produced subtle defects in midgut morphology and stomach acid secretion. The polarized distributions of alpha beta(H) spectrin and ankyri n were not significantly altered in beta spectrin mutants, indicating that the two isoforms of Drosophila spectrin assemble independently of one anoth er, and that ankyrin is upstream of alpha beta spectrin in the spectrin ass embly pathway. In contrast, beta spectrin mutations had a striking effect o n the basolateral accumulation of the Na,K ATPase. The results establish a role for beta spectrin in determining the subcellular distribution of the N a,K ATPase and, unexpectedly, this role is independent of alpha spectrin.