Drosophila β Spectrin Functions Independently of α Spectrin to Polarize the Na,k Atpase in Epithelial Cells

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Abstract
Spectrin has been proposed to function as a sorting machine that concentrates interacting proteins such as the Na,K ATPase within specialized plasma membrane domains of polarized cells. However, little direct evidence to support this model has been obtained. Here we used a genetic approach to directly test the requirement for the β subunit of the αβ spectrin molecule in morphogenesis and function of epithelial cells in Drosophila. β Spectrin mutations were lethal during late embryonic/early larval development and they produced subtle defects in midgut morphology and stomach acid secretion. The polarized distributions of αβH spectrin and ankyrin were not significantly altered in β spectrin mutants, indicating that the two isoforms of Drosophila spectrin assemble independently of one another, and that ankyrin is upstream of αβ spectrin in the spectrin assembly pathway. In contrast, β spectrin mutations had a striking effect on the basolateral accumulation of the Na,K ATPase. The results establish a role for β spectrin in determining the subcellular distribution of the Na,K ATPase and, unexpectedly, this role is independent of α spectrin.