Kinetics of Gi protein ribosylation. Membrane-rich protein from Jurkat cells treated with PTX for 0, 2, or 20 hours was prepared as described. Nonribosylated Gi proteins in untreated cells incorporated P32 under the influence of an excess of PTX, as evidenced by a band migrating at 41 kDa (left). The presence of the 41-kDa band at 2 hours of PTX incubation evidences incomplete Gi protein ribosylation at this time point (middle). Gi protein ribosylation was complete at 20 hours (right).