Phosphorylation of Membrane-Located Proteins of Soybean In Vitro and Response to Auxin

Isolated membranes of soybean incorporate (32)P from gamma-[(32)P]ATP in vitro. The incorporation was rapid and did not require added calcium. When displayed on 10% sodium dodecyl sulfate-polyacrylamide gels, several protein bands were revealed. An apparent auxin (2,4-dichlorophenoxyacetic acid) stimulation of (32)P incorporation into material from membrane vesicles insoluble in trichloroacetic acid-perchloric acid may be reflected partly in enhanced incorporation into protein bands with apparent molecular weights of 45,000 and 50,000. Additionally, a low molecular weight component was sometimes observed where incorporation was stimulated 2- to 3-fold by auxin. However, protein-bound radioactivity represented only a small fraction of the total radioactivity of the acid-insoluble material. Other labeled constituents, not retained on the gels, may contribute to the apparent, rapid (10 s or less) auxin response of the isolated membranes. Stimulation of incorporation into the low molecular weight component was given by diglyceride plus calcium, constituents known to augment protein kinase activities in other systems.