A DI-LEUCINE MOTIF AND AN UPSTREAM SERINE IN THE INTERLEUKIN-6 (IL-6)SIGNAL TRANSDUCER GP130 MEDIATE LIGAND-INDUCED ENDOCYTOSIS AND DOWN-REGULATION OF THE IL-6 RECEPTOR

The interleukin-6 (IL-6) receptor complex is composed of two different subunits, the IL-6 binding protein (IL-6R, gp80) and the signal trans ducing component gp130. Our previous studies revealed that the 10-amin o acid sequence TQPLLDSEER within the intracellular domain of gp130 is crucial for the efficient internalization of IL-6. Since this sequenc e contains a putative dileucine internalization motif, we further anal yzed this region by constructing two additional deletions and a series of point mutants. Analyses of these mutants showed that the di-leucin e pair (Leu-145 and Leu-146) is essential for ligand internalization, with leucine 145 being less resilient to exchanges. Furthermore, when a chimeric protein (Tac-STQPLL) composed of the Tac antigen fused to t he hexapeptide STQPLL of gp130 was studied, we found that this sequenc e is sufficient to mediate endocytosis and lysosomal targeting of the chimera. Mutational analysis of three serine residues upstream of the di-leucine motif revealed that mutation of serine 139 to an alanine re duces the initial internalization rate by 50%. This finding suggests t hat a serine phosphorylation may be important for rapid endocytosis.