Fig. 4. Cytochalasin D does not compromise proximal events in prolactin signalling or tyrosine phosphorylation of FAK. Cells were cultured as in Fig. 3. (A) Lysates were immunoprecipitated with antibodies to Jak2 or PrlR and then blotted with antibodies for phosphotyrosine (4G10) or the appropriate precipitating antibody. (B) Lysates were immunoprecipitated with antibodies to FAK and then blotted with antibodies for phosphotyrosine (PY20) or the appropriate precipitating antibody. In control experiments, primary mammary epithelial cells were removed to the non-adhesive substratum polyHEMA for 1 hour. Under these conditions, FAK becomes dephosphorylated, in contrast to the cells on basement membrane, in which FAK phosphorylation remains even after cytochalasin-D treatment.