SIGNAL-TRANSDUCTION BY THE ALPHA(6)BETA(4) INTEGRIN - DISTINCT BETA(4) SUBUNIT SITES MEDIATE RECRUITMENT OF SHC/GRB2 AND ASSOCIATION WITH THE CYTOSKELETON OF HEMIDESMOSOMES

We have examined the mechanism of signal transduction by the hemidesmosomal integrin alpha(6) beta(4), a laminin receptor involved in morphogenesis and tumor progression. Immunoprecipitation and immune complex kinase assays indicated that antibody- or laminin-induced ligation of alpha(6) beta(4) causes tyrosine phosphorylation of the beta(4) subunit in intact cells and that this event is mediated by a protein kinase(s) physically associated with the integrin. Co-immunoprecipitation and GST fusion protein binding experiments showed that the adaptor protein Shc forms a complex with the tyrosine-phosphorylated beta(4) subunit. Shc is then phosphorylated on tyrosine residues and recruits the adaptor Grb2, thereby potentially linking alpha(6) beta(4) to the ras pathway. The beta(4) subunit was found to be phosphorylated at multiple tyrosine residues in vivo, including a tyrosine-based activation motif (TAM) resembling those found in T and B cell receptors. Phenylalanine substitutions at the beta(4) TAM disrupted association of alpha(6) beta(4) With hemidesmosomes, but did not interfere with tyrosine phosphorylation of She and recruitment of Grb2. These results indicate that signal transduction by the alpha(6) beta(4) integrin is mediated by an associated tyrosine kinase and that phosphorylation of distinct sites in the beta(4) tail mediates assembly of the hemidesmosomal cytoskeleton and recruitment of Shc/Grb2.