Polar redistribution of 125I-labelled insulin on the plasma membrane of cultured human lymphocytes.

Cultured human lymphocytes of the IM-9 line have specific binding sites for 125I-insulin that have been characterized in detail. When the hormone--cell interaction was studied by quantitative electron microscopic (EM) autoradiography, it was shown that initial binding was to the plasma membrane and that at 37 degrees C a small fraction of the ligand was internalized by the cell. The internalized ligand was found preferentially localized in lysosomes in the Golgi region of the cell. It is well known that di- or multivalent ligands redistribute in the plane of the membrane before internalization. We now report that following initial binding the univalent ligand insulin also undergoes a polar redistribution ('capping') before internalization.