Fig. 3. Tyrosine phosphorylation triggered by clustering the various ß cytoplasmic domain mutants. Normal human fibroblasts expressing tac chimeras containing the various ß cytoplasmic domain mutations (A-C, ß1 mutants; E, ß3 mutants) were incubated in clustering assays for 40 minutes. Lysates (10 µg/sample) were separated by SDS-PAGE, blotted for phosphotyrosine (upper panels) and reprobed for p130CAS (lower panels). The ß1 and ß3 mutants were loaded as indicated above each lane. (D) A quantitative comparison of the ability of different tac-ß tail mutants to trigger tyrosine phosphorylation. The data from three separate experiments is presented as the mean ± s.e.m. A quantitative comparison of the ability of the ß3 mutants to activate tyrosine phosphorylation has already been published (Tahiliani et al., 1997).