Fig. 1. PTP-PEST is activated by integrins and localizes to the tips of membrane protrusions. (A) CHOK1 fibroblasts were held in suspension (S) or plated on fibronectin-coated (FN, 10 µg/ml) or poly-L-lysine-coated (PL, 10 µg/ml) plates for 1 hour in serum-free medium. PTP-PEST was immunoprecipitated from TX-100 lysates. PTPase activity was determined by incubating the immunoprecipitates with a ³²P-labeled peptide substrate (³²P polyglu-tyr) and measuring the amount of ³²P released. PTP-PEST activity is 2-3-fold higher in cells plated on FN compared with cells in suspension (S) or plated on a non-specific substrate (PL). The results represent three independent experiments. (Lower panel) Immunoprecipates of PTP-PEST from CHOK1 cells in suspension, plated on FN or PL, show that equal amounts of PTP-PEST were precipitated for the activity assay. (B) Swiss 3T3 fibroblasts were plated on FN-coated coverslips for 20 minutes and then immunostained for PTP-PEST (left) and actin (right). PTP-PEST transiently localizes at the tips of protrusions.