Figure 8.

Native GPI domains contain negative regulators of the associated kinases. 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 effect of untreated GPI-enriched membranes was tested directly or at serial fourfold dilution on the kinase activity of the TX-100–treated GPI-enriched membranes by mixing both 10 min before the kinase reaction. All the assays were carried out in the same reaction volume containing 5 μCi of labeled ATP. Under the same assay conditions, the untreated GPI-enriched membranes did not contain demonstrable ATPase activity.