The C-terminal domain of Drosophila _Heavy-spectrin exhibits autonomous membrane association and modulates membrane area

Current models of cell polarity invoke asymmetric cues that reorganize the secretory apparatus to induce polarized protein delivery. An important step in this process is the stabilization of the protein composition in each polarized membrane domain. The spectrin-based membrane skeleton is thought to contribute to such stabilization by increasing the half-life of many proteins at the cell surface. Genetic evidence is consistent with a negative role for Drosophila _Heavy-spectrin in endocytosis, but the inhibitory mechanism has not been elucidated. Here, we investigated the membrane binding properties of the C-terminal nonrepetitive domain of _Heavy-spectrin through its in vivo expression in transgenic flies. We found that this region is a membrane-association domain that requires a pleckstrin homology domain for full activity, and we showed for the first time that robust membrane binding by such a C-terminal domain requires additional contributions outside the pleckstrin homology. In addition, we showed that expression of the _Heavy-spectrin C-terminal domain has a potent effect on epithelial morphogenesis. This effect is associated with its ability to induce an expansion in plasma membrane surface area. The membrane expansions adopt a very specific bi-membrane structure that sequesters both the C-terminal domain and the endocytic protein dynamin. Our data provide supporting evidence for the inhibition of endocytosis by _Heavy-spectrin, and suggest that the C-terminal domain mediates this effect through interaction with the endocytic machinery. Spectrin may be an active partner in the stabilization of polarized membrane domains.