Segregation of Two Spectrin Isoforms: Polarized Membrane-binding Sites Direct Polarized Membrane Skeleton Assembly

Abstract

Spectrin isoforms are often segregated within specialized plasma membrane subdomains where they are thought to contribute to the development of cell surface polarity. It was previously shown that ankyrin and ␤ spectrin are recruited to sites of cell-cell contact in Drosophila S2 cells expressing the homophilic adhesion molecule neuroglian. Here, we show that neuroglian has no apparent effect on a second spectrin isoform (␣␤ H ), which is constitutively associated with the plasma membrane in S2 cells. Another membrane marker, the Na,K-ATPase, codistributes with ankyrin and ␣␤ spectrin at sites of neuroglian-mediated contact. The distributions of these markers in epithelial cells in vivo are consistent with the order of events observed in S2 cells. Neuroglian, ankyrin, ␣␤ spectrin, and the Na,K-ATPase colocalize at the lateral domain of salivary gland cells. In contrast, ␣␤ H spectrin is sorted to the apical domain of salivary gland and somatic follicle cells. Thus, the two spectrin isoforms respond independently to positional cues at the cell surface: in one case an apically sorted receptor and in the other case a locally activated cell-cell adhesion molecule. The results support a model in which the membrane skeleton behaves as a transducer of positional information within cells.

Genetic Studies of Spectrin in the Larval Fat Body ofDrosophila melanogaster: Evidence for a Novel Lipid Uptake Apparatus

“…Guinea pig anti-lipophorin was a gift from Suzanne Eaton at the Max Planck Institute of Molecular Cell Biology and Genetics, Dresden, Germany (Eugster et al 2007). Affinity-purified anti-b H -spectrin was described previously (Dubreuil et al 1997). Western blots were performed with alkaline phosphatase-coupled secondary antibodies (Zymed) and stained with bromochloroindolyl phosphate as previously described (Dubreuil and Yu 1994).…”

“…Affinity-purified anti-b H -spectrin was described previously (Dubreuil et al 1997). Western blots were performed with alkaline phosphatase-coupled secondary antibodies (Zymed) and stained with bromochloroindolyl phosphate as previously described (Dubreuil and Yu 1994).…”

Mating induces an immune response and developmental switch in theDrosophilaoviduct

“…Different Spec isoforms localize to different domains of the epithelial cell plasma membrane: ␣␤-Spec resides at the basolateral surface, whereas ␣␤ H -Spec is restricted to the apical domain (24). ␣␤-Spec contributes to polarized membrane organization by binding membrane proteins at the cell surface.…”

The cell adhesion molecule Roughest depends on βHeavy-spectrin during eye morphogenesis in Drosophila

“…Roughest belongs to a different subfamily that includes Nephrin and has a completely divergent cytoplasmic domain that lacks the conventional Ankryin binding site (Strunkelnberg et al, 2003). Similarly,  H lacks the canonical Ankyrin binding domain (Thomas et al, 1997) and does not colocalize with Ankyrin in vivo (Dubreuil et al, 1997). This raises an interesting evolutionary question as to whether a protospectrin was bound to a common proto-Ig-CAM and that divergence of the spectrins and Ig-CAMs was accompanied by the acquisition or loss of Ankyrin as an adaptor in the conventional or heavy -spectrin lineage, or whether the spectrin-Ig-CAM association came later and this is an example of convergence.…”

“…However, neither Roughest nor  H contains a recognizable Ankyrin binding site (Strunkelnberg et al, 2003;Thomas et al, 1997). Moreover,  H does not colocalize with Ankyrin by immunofluorescence (Dubreuil et al, 1997). However, there remains the possibility of a  H -Roughest interaction mediated via another mechanism.…”