Figure 3.

Intermolecular phosphorylation of recombinant SRK proteins. Microsomes from Sf21 cells expressing either SRK₃HA (HA) or kinase-defective ^mSRK₃His (^mHis) or from Sf21 cells coexpressing ^mSRK₃His and SRK₃HA (^mHis + HA) were radiolabeled, and proteins carrying the hexahistidine tag were purified on Ni-NTA agarose beads. For the sample shown in lane 4, extracts of unlabeled microsomes from cells expressing ^mSRK₃His were added before purification on Ni-NTA agarose beads. Proteins eluted from Ni-NTA agarose beads were separated by SDS/PAGE and analyzed by autoradiography. Truncated recombinant SRK₃ and contaminating polypeptides are indicated by gray arrowheads, and phosphorylated SRK is indicated by an arrow.