Figure 7.

Segregation of spectrin isoforms in response to positional cues at the cell surface. (A) The uniform distribution of αβ_H spectrin (submembrane shading) at the plasma membrane of S2 cells (left) implies the presence of a constitutive β_H-specific receptor that is uniformly distributed at the cell surface. In contrast, ankyrin and αβ spectrin are found in the cytoplasm of control S2 cells (*, left), but they are recruited to sites of cell–cell contact in response to neuroglian-mediated cell adhesion (right). The Na,K-ATPase also becomes concentrated at cell contacts in response to neuroglian expression and ankyrin/αβ spectrin assembly. (B) Neuroglian, ankyrin, αβ spectrin, and Na,K-ATPase are concentrated at sites of cell–cell contact in the larval salivary gland epithelium, in agreement with the order of events observed in S2 cells. However, the putative αβ_H spectrin receptor is sorted apically in these polarized cells. Despite their common features (including an identical α subunit), the two spectrin isoforms respond independently to their respective membrane receptors to form segregated membrane skeleton domains.