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Interleukin-6 (IL-6) exerts its action via a cell surface receptor complex consisting of two subunits, the IL-6 receptor and the signal transducer gp130. We have studied the role of both transmembrane proteins for IL-6 internalization and ligand-induced down-regulation of cell surface receptors. Co-expression of wild-type and mutant forms of both the IL-6 receptor and gp130 in transiently transfected COS-7 cells revealed that gp130 is essential for efficient endocytosis and receptor down-regulation. Whereas the cytoplasmic domain of the IL-6 receptor is not significantly involved in the internalization process, deletion of the corresponding domain of gp130 resulted in an almost complete loss of the ability to endocytose IL-6. Mutants with different truncations within the intracellular domain of gp130 revealed that a 10-amino acid sequence TQPLLDSEER is crucial for efficient internalization. Since this sequence contains a putative di-leucine internalization motif, we suggest that a di-leucine motif directs the receptor mediated endocytosis of the IL-6 receptor complex.
Dittrich et al. (Fri,) studied this question.