Selective anchoring in the specific plasma membrane domain: a role in epithelial cell polarity

Using FRAP, it has been shown that Band III is considerably restricted in its ability to diffuse in the plane of the membrane. The restriction depends on interactions with the ankyrin spectrin submembrane cytoskeleton, since the diffusion coefficient, very low in normal erythrocytes (D = 4.5 x 10 -tl) is increased 50-fold in spectrin-deficient spherocytes (68) and under conditions that favor spectrin dissociation (79) . The interaction between a subpopulation (~40%) of Band III molecules and spectrin is mediated by ankyrin (5), while another subpopulation shows free rotational diffusion (13, 51) , and seems to be confined within the spaces defined by the submembrane spectrin network (36, 79) . It is not known whether the basolateral fodrin cytoskeleton forms a network in epithelial cells. However, if the epithelial spectrin was organized like its erythroid counterpart, it could be predicted that basolateral membrane proteins should have a detergent-extractable and -diffusible fraction as well as a cytoskeleton-anchored fraction. In that case the high D's that we measured in FRAP experiments are compatible with a coarse or incomplete fodrin network with free diffusion spaces in the order of the size of our laser spot (~1 lan). In this model, TIF would correspond either to cells with a denser network or to spots located on nodes of the network. On the other hand, it has been shown that the Na÷-K + ATPase/ankyrin/spectrin complex can be formed in vitro, with molecules in solution (50) . Therefore, we cannot rule out the possibility that freely diffusive plasma membrane proteins located within the spaces of the fodrin network may bind to ankyrin/spectrin sites during the course of the Triton X-100 extraction. In that case, the detergent insolubility may be reporting more on the number of binding sites available than on the number of molecules bound to the cytoskeleton at any given time. Even under this model that explains FRAP and detergent extraction together, the data presented in this work indicates that epithelial cells have a specialized submembrane cytoskeleton capable of immobilizing subpopulations of plasma membrane proteins in a domain-specific fashion.