What does autophosphorylation mean? When a receptor tyrosine kinase binds its ligand and forms a dimer, do the individual receptor molecules phosphorylate themselves or does one receptor cross-phosphorylate the other, and vice-versa? To investigate this question, you’ve constructed genes for three forms of a receptor tyrosine kinase: the normal form with an active kinase domain and three sites of phosphorylation; a large form that carries and inactivating point mutation in the kinase domain but retains the three phosphorylation sites; and a short version that has an active kinase domain but is lacking the sites of phosphorylation Fig. 15-13A). You express the genes singly and in combination in a cell line that lacks this receptor tyrosine kinase, allow them to take up radioactive ATP, then add ligand for the receptor. You purify the receptors and analyze them for expression levels by staining for protein (Fig. 15-13B) and for phosphorylation by autoradiography (Fig. 15-13C). A.What results would you expect on the autoradiograph if individual receptors phosphorylated themselves? (Draw another version of Fig. 15-13C) B.What would you expect if receptors cross-phosphorylated each other? C.Which model for autoophosphorylation do your data support?