Binding of the alpha-fodrin SH3 domain to the leading lamellae of locomoting chicken fibroblasts.

Fodrin (nonerythroid spectrin) is a membrane skeletal protein that plays an important role in the establishment and maintenance of the cell shape and polarity. We have identified in alpha-fodrin an src homology 3 (SH3)-related region, a small domain that is present in a large number of proteins that are involved in signal transduction, cell polarization and membrane-cytoskeleton interactions. In this study we have explored the function of the alpha-fodrin SH3 by incubating fixed and permeabilized cultured chicken fibroblasts with the alpha-fodrin SH3 peptide, expressed in bacteria as a fusion protein with glutathione S-transferase. Immunofluorescence and immunoelectron microscopy showed that alpha-fodrin SH3 binds to the cytoplasmic face of the plasma membrane in the leading lamellae and the pseudopodial lobes of the spreading and locomoting cells. No, or only minimal, binding was seen in immotile cells, or in the stationary trailing ends of the locomoting cells. SH3 binding was also seen in cytochalasin-D-treated cells, suggesting that actin filaments are not responsible for the binding. These findings suggest that alpha-fodrin SH3 interacts with plasma membrane components that are present in the leading lamellae exclusively or are modulated in a manner specific to the leading lamellae.