Role of Binding of Plectin to the Integrin β4 Subunit in the Assembly of Hemidesmosomes

We have previously shown that plectin is recruited into hemidesmosomes through association of its actin-binding domain (ABD) with the first pair of fibronectin type III (FNIII) repeats and a small part of the connecting segment (residues 1328–1355) of the integrin β4 subunit. Here, we show that two proline residues (P1330 and P1333) in this region of the connecting segment are critical for supporting β4-mediated recruitment of plectin. Additional binding sites for the plakin domain of plectin on β4 were identified in biochemical and yeast two-hybrid assays. These sites are located at the end of the connecting segment (residues 1383–1436) and in the region containing the fourth FNIII repeat and the C-tail (residues 1570–1752). However, in cells, these additional binding sites cannot induce the assembly of hemidesmosomes without the interaction of the plectin-ABD with β4. Because the additional plectin binding sites overlap with sequences that mediate an intramolecular association of the β4 cytoplasmic domain, we propose that they are not accessible for binding and need to become exposed as the result of the binding of the plectin-ABD to β4. Furthermore, these additional binding sites might be necessary to position the β4 cytoplasmic domain for an optimal interaction with other hemidesmosomal components, thereby increasing the efficiency of hemidesmosome assembly

Dual Role of α6β4 Integrin in Epidermal Tumor Growth: Tumor-suppressive Versus Tumor-promoting Function

An increased expression of the integrin α6β4 is correlated with a poor prognosis in patients with squamous cell carcinomas. However, little is known about the role of α6β4 in the early stages of tumor development. We have isolated cells from mouse skin (mouse tumor-initiating cells [mTICs]) that are deficient in both p53 and Smad4 and carry conditional alleles of the β4 gene (Itgb4). The mTICs display many features of multipotent epidermal stem cells and produce well-differentiated tumors after subcutaneous injection into nude mice. Deletion of Itgb4 led to enhanced tumor growth, indicating that α6β4 mediates a tumor-suppressive effect. Reconstitution experiments with β4-chimeras showed that this effect is not dependent on ligation of α6β4 to laminin-5, but on the recruitment by this integrin of the cytoskeletal linker protein plectin to the plasma membrane. Depletion of plectin, like that of β4, led to increased tumor growth. In contrast, when mTICs had been further transformed with oncogenic Ras, α6β4 stimulated tumor growth, as previously observed in human squamous neoplasms. Expression of different effector-loop mutants of RasV12 suggests that this effect depends on a strong activation of the Erk pathway. Together, these data show that depending on the mutations involved, α6β4 can either mediate an adhesion-independent tumor-suppressive effect or act as a tumor promotor