In vitro binding studies suggest a membrane-associated complex between erythroid p55, protein 4.1, and glycophorin C.

p55 is a palmitoylated peripheral membrane phosphoprotein of human erythrocytes. Primary structure of p55 includes a single copy of the SH3 motif, a COOH-terminal guanylate kinase domain, and an NH2-terminal domain of unknown function. Although the function of p55 is not known, interest in this component has been heightened by its similarity to the Drosophila tumor suppressor discs-large (dlg). In this report we provide evidence for the direct association of p55 with the NH2-terminal 30-kDa domain of protein 4.1, a key component of the erythroid membrane skeleton. In addition, p55 also binds to the cytoplasmic domain of glycophorin C, a transmembrane protein of red blood cells. We also provide evidence demonstrating the direct association of the 30-kDa domain of protein 4.1 with the cytoplasmic domain of glycophorin C. Taken together, these results suggest the existence of a novel ternary complex at the erythroid plasma membrane involving protein 4.1, p55, and glycophorin C. Since isoforms of protein 4.1, p55, and glycophorin C are present in many non-erythroid cells, the binding interactions may be prototypical of similar associations that modulate cytoskeletal-membrane linkage of broad significance.