Phosphorylation of the transferrin receptor in isolated sheep reticulocyte plasma membranes

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Abstract
The transferrin receptor of sheep reticulocyte plasma membranes undergoes phosphorylation with [γ-32P]ATP in isolated plasma membranes. The phosphorylation is stimulated by Mn2+, Co2+, and Mg2+, but not by Ca2+, Ba2+, Zn2+, Fe2+, or Cu2+. There is no detectable effect of cyclic nucleotides on the phosphorylation of the receptor. Transferrin and a monoclonal antibody against the transferrin receptor have no apparent effect on receptor phosphorylation in intact cells or isolated membranes. Immunoprecipitates of the receptor retain ability to phosphorylate the receptor. The phosphorylation appears to be at a serine residue which turns over with a half time of 20–30 min. ATP appears to be the best, but not the only substrate for receptor phosphorylation.