A highly phosphorylated subpopulation of insulin-like growth factor II/mannose 6-phosphate receptors is concentrated in a clathrin-enriched plasma membrane fraction.

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Abstract
Insulin-like growth factor II (IGF-II)/mannose 6-phosphate (Man-6-P) receptors immunoprecipitated from purified plasma membranes of 32P-labeled rat adipocytes are markedly heterogenous in their phosphorylation state. Approximately 80% of the plasma membrane receptors are solubilized in 1% (vol/vol) Triton X-100 and are phosphorylated on serine residues at a stoichiometry of approximately 0.1-0.2 mol of phosphate per mol of receptor. In contrast, 15-20% of the receptors are Triton X-100-insoluble and are phosphorylated on serine and threonine residues at approximately 4 or 5 mol of phosphate per mol of receptor. This Triton X-100-insoluble membrane subfraction contains only 5% of the total plasma membrane protein and yet contains all of the clathrin heavy chain associated with plasma membrane, as detected by immunoblotting with a monoclonal antibody. Based on the relative yields of protein in the detergent-insoluble material, IGF-II/Man-6-P receptors are concentrated approximately equal to 3-fold in this clathrin-enriched subfraction. Insulin treatment of intact cells increased the total IGF-II/Man-6-P receptors in the Triton X-100-soluble fraction of the plasma membrane, whereas no change in receptor number in the detergent-insoluble fraction was seen. However, insulin markedly decreased the phosphorylation stoichiometry of the Triton X-100-insoluble receptors. Taken together, these results indicate that insulin decreases the phosphorylation state of a highly phosphorylated subpopulation of IGF-II/Man-6-P receptors on the plasma membrane. In addition, insulin action may prevent the concentration of these receptors in a clathrin-enriched membrane subfraction.