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 hemidesmo somal integrin alpha(6) beta(4), a laminin receptor involved in morpho genesis 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) subuni t 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 protei n Shc forms a complex with the tyrosine-phosphorylated beta(4) subunit . Shc is then phosphorylated on tyrosine residues and recruits the ada ptor Grb2, thereby potentially linking alpha(6) beta(4) to the ras pat hway. The beta(4) subunit was found to be phosphorylated at multiple t yrosine 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) bet a(4) With hemidesmosomes, but did not interfere with tyrosine phosphor ylation of She and recruitment of Grb2. These results indicate that si gnal transduction by the alpha(6) beta(4) integrin is mediated by an a ssociated tyrosine kinase and that phosphorylation of distinct sites i n the beta(4) tail mediates assembly of the hemidesmosomal cytoskeleto n and recruitment of Shc/Grb2.