SEGREGATION OF 2 SPECTRIN ISOFORMS - POLARIZED MEMBRANE-BINDING SITESDIRECT POLARIZED MEMBRANE SKELETON ASSEMBLY

Spectrin isoforms are often segregated within specialized plasma membr ane subdomains where they are thought to contribute to the development of cell surface polarity. It was previously shown that ankyrin and be ta spectrin are recruited to sites of cell-cell contact in Drosophila S2 cells expressing the hemophilic adhesion molecule neuroglian. Here, we show that neuroglian has no apparent effect on a second spectrin i soform (alpha beta(H)), which is constitutively associated with the pl asma membrane in S2 cells. Another membrane marker, the Na,K-ATPase, c odistributes with ankyrin and alpha beta spectrin at sites of neurogli an-mediated contact. The distributions of these markers in epithelial cells in vivo are consistent with the order of events observed in S2 c ells. Neuroglian, ankyrin, alpha beta spectrin, and the Na,K-ATPase co localize at the lateral domain of salivary gland cells. In contrast, a lpha beta(H), spectrin is sorted to the apical domain of salivary glan d and somatic follicle cells. Thus, the two spectrin isoforms respond independently to positional cues at the cell surface: in one case an a pically sorted receptor and in the other case a locally activated cell -cell adhesion molecule. The results support a model in which the memb rane skeleton behaves as a transducer of positional information within cells.