Polypeptide Hormone Receptor Phosphorylation: Is There a Role in Receptor–Mediated Endocytosis of Human Growth Hormone?

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Abstract
To determine whether receptor phosphorylation is a critical step in the internalization of polypeptide hormones and their receptors, model system wherein insulin stimulates phosphorylation of its receptor and is also internalized was studied. Using insulin as a positive control, it was found that it stimulated a partially purified plasma membrane preparation of IM-9 lymphocytes to autophosphorylate its receptor and to catalyze the phosphorylation of a tyrosine-containing substrate. The human GH [growth hormone] (hGH) receptor of the IM-9 lymphocytes, when coupled to [125I]iodo-hGH, migrated as a 140,000-dalton protein on polyacrylamide gel electrophoresis. This protein, in contrast to the insulin receptor, was not phosphorylated by the addition of hGH, nor did hGH stimulate this preparation to phosphorylate the tyrosine-containing substrate poly-(GluNa,Tyr)4:1, casein, or histone f2b under a variety of conditions. Apparently, receptor phosphorylation is not a critical intermediate in the receptor-mediated endocytosis of hGH and probably other polypeptide hormones and growth factors.