Two different mutations in the cytoplasmic domain of the integrin β4 subunit in nonlethal forms of epidermolysis bullosa prevent interaction of β4 with plectin

The integrin alpha6 beta4 plays a crucial role in the assembly and maintenance of hemidesmosomes. Previous work has shown that the recruitment of plectin into hemidesmosomes is dependent on beta4 and involves a region of the beta4 cytoplasmic domain, which contains the first two fibronectin (FNIII) repeats and a short region of the connecting segment. Two missense mutations (R1225H and R1281W) in beta4 that are responsible for nonlethal forms of epidermolysis bullosa are located in the second FNIII repeat. One of them is confined to a loop region that connects two beta strands (EC') whereas the other is located at the N-terminal end of the second FNIII repeat. We here report that these mutations render beta4 unable to interact with plectin and prevent the localization of plectin in hemidesmosomes. Substitution of a lysine residue (K1279W) that forms part of the same loop as R1281 had no effect on the ability of beta4 to recruit plectin. Furthermore, we show that an extended loop structure in beta4, composed of the amino acids DDN (1262-1264), which resembles the RGD integrin-binding loop in fibronectin, is not involved in the binding to plectin. These results further demonstrate that binding of beta4 to plectin is essential for the proper formation and function of hemidesmosomes and that loss of the interaction between beta4 and plectin is associated with a mild form of epidermolysis bullosa.