Figure 6.

Enzyme kinetics of PTK activity in GPI-enriched membranes isolated without or with TX-100. Untreated and TX-100–treated GPI-enriched membranes equivalent to 25 × 10⁶ cells and originating from the same cell homogenate were sedimented and tested for in vitro kinase activity as in Figure 2C. The reaction mix contained low-specific-activity ATP (3 Ci/mmol) to a final concentration of 100 μM and varying amounts of the exogenous PTK substrate enolase. Phosphorylated proteins were separated by SDS-PAGE and autoradiographed. The enolase bands were excised, and the incorporated radiolabel was quantitated by liquid scintillation counting as a measure of the velocity of PTK activities. Western blotting of aliquots of untreated and TX-100–treated GPI-enriched membranes showed comparable amounts of Lck and Fyn kinases as in Figure 2B. The data from one of the three separate experiments, which gave similar results, are shown.