We showed previously that peptides derived from the alpha 1 domain of the major histocompatibility complex class I protein (MHC-I) inhibit internalization of some receptors, thereby increasing the steady-state number of active receptors on the cell surface. In consequence, sensitivity to hormone (e.g., insulin) is enhanced, transport (e.g., of glucose by GLUT-4) is increased, and carrier proteins (e.g., transferrin) operate less efficiently. Now we report that a bioactive peptide (but not closely related inactive ones) binds to MHC-I on the cell surface, not in the groove but apparently to the alpha 1 helix. The binding is saturable, and the number of peptide binding sites on the cell surface approximately equals the number of MHC-I molecules. Antibodies to MHC-I inhibit peptide binding. Most significant, antibodies to MHC-I mimic the effect of a bioactive peptide, inhibiting receptor internalization. These results indicate that MHC-I participates in the regulation of cell surface receptor activity
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