CELL-SURFACE PROTEOLYSIS AND DOWN-REGULATION OF THE HEPATIC INSULIN-RECEPTOR - EVIDENCE FOR SELECTIVE SORTING OF INTACT AND DEGRADED RECEPTORS AFTER INTERNALIZATION

Insulin binding to rat liver plasma membranes promotes proteolysis of the Mr 135,000 .alpha. subunit of the insulin receptor to a fragment of Mr 120,000 (Lipson, K. E., Yamada, K., Kolhatkar, A. A., and Donner, D. B.(1986) J. Biol. Chem. 261, 10833-10838). The enzyme that catalyzes this degradation copurifies with plasma membranes and cannot be identified in any other cellular organelle or in cytosol. The proteinase has optimal activity above pH 7 and is an integral protein based upon its resistance to extraction with 2 M NaCl. After affinity labeling, degraded insulin receptors were identified in plasma membranes isolated from a liver perfused with 1 nM 125I-insulin for 10 min at 37.degree. C, indicating that proteolysis occurs in the hepatocyte cell membrane under physiological conditions. Microsomes do not contain the receptor degrading activity or a detectable amount of degraded receptors under basal conditions. After perfusion of a liver with 125I-insulin, Mr 135,000 and Mr 120,000 complexes were detected in microsomes, suggesting that both intact and degraded receptors can be internalized. The initial absence of degraded receptors in plasma membranes suggests that, following internalization, such sites do not recycle. Thus, hormone-induced proteolysis of the insulin receptor begins at the surface of the rat hepatocyte and can lead to loss of receptors from the plasma membrane.