Internalization of MHC class I molecules is a prerequisite for endocytosis of endorphin by lymphocytes0

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Abstract
SUMMARY: The nature of the inleraction between γ-type endorphins and the HLA class I molecules was studied by immunoelectronmicroscopy. The HLA molecules were not involved in the actual binding of endorphin to the cell. In contrast, for the endocytosis of γ-endorphin, co-internalization of the HLA class I molecules is essential. The internalization process starts with clustering of γ-endorphin and HLA class I molecules in coated pits. Cells that do not carry HLA class I molecules (Daudi) or do not internalize HLA class I molecules (EBV-transformed B cells) bind but do not internalize γ-endorphin. On the basis of these observations, we suggest that the MHC class I molecules may function as transport molecules. Whether it is a general phenomenon that non-immunological ligands use the HLA class I molecules to get into the cell and immunological ligands (viral proteins) to reach the cell surface, remains to be established.