The cytoplasmic domain of  4 integrin contains two pairs of fibronectin-like repeats separated by a connecting segment. The connecting segment harbors a putative tyrosine activation motif in which tyrosines 1422 and 1440 are phosphorylated in response to ␣ 6  4 binding to laminin-5. Primary  4 -null keratinocytes, obtained from a newborn suffering from lethal junctional epidermolysis bullosa, were stably transduced with retroviruses carrying a full-length  4 cDNA or a  4 cDNA with phenylalanine substitutions at Tyr-1422 and Tyr-1440. Hemidesmosome assembly was evaluated on organotypic skin cultures.  4 -corrected keratinocytes were indistinguishable from normal cells in terms of ␣ 6  4 expression, the localization of hemidesmosome components, and hemidesmosome structure and density, suggesting full genetic and functional correction of  4 -null keratinocytes. In cultures generated from  4
Y1422F/Y1440Fkeratinocytes,  4 mutants as well as ␣ 6 integrin, HD1/ plectin, and BP180 were not concentrated at the dermalepidermal junction. Furthermore, the number of hemidesmosomes was strikingly reduced as compared with  4 -corrected keratinocytes. The rare hemidesmosomes detected in  4 Y1422F/Y1440F cells were devoid of subbasal dense plates and of inner cytoplasmic plaques with keratin filament insertion. Collectively, our data demonstrate that the  4 tyrosine activation motif is not required for the localization of ␣ 6  4 at the keratinocyte plasma membrane but is essential for optimal assembly of bona fide hemidesmosomes.Human epidermis consists of a stratified squamous epithelium composed of keratinocytes organized in distinct cellular layers. Keratinocytes forming the basal layer firmly adhere to the basement membrane by means of hemidesmosomes (HDs), 1 multiprotein complexes linking the epithelial intermediate filament network to the dermal anchoring fibrils (see Refs. 1 and 2 for review). HDs are formed by the clustering of several cytoplasmic and trans-membrane proteins (2). The cytoplasmic HD plaque components, which include HD1/plectin (3) and the bullous pemphigoid antigen 1 (BP230) (4), act as linkers for elements of the cytoskeleton at the cytoplasmic surface of plasma membrane. The trans-membrane constituents of HDs, which include the ␣ 6  4 integrin (5, 6) and the bullous pemphigoid antigen 2 (BP180) (7), serve as cell receptors connecting the cell interior to extracellular matrix proteins.In particular, the ␣ 6  4 integrin is a receptor for the basement membrane component laminin-5, a heterotrimeric protein composed of three distinct polypeptides, ␣3, 3, and ␥2, which are encoded by three different genes known as LAMA3, LAMB3, and LAMC2, respectively (see Ref. 8 for review). Laminin-5 binds to the basal keratinocyte cell surface through the ␣ 6  4 integrin and tightens the dermal-epidermal junction by binding also to the N-terminal NC-1 domain of type VII collagen (9). The crucial importance of the interaction between laminin-5 and its ␣ 6  4 receptor in maintaining the integrity of the i...