A role for the immunoglobulin-like domain of the human IL-6 receptor. Intracellular protein transport and shedding.

Interleukin (IL)-6, IL-11 and cililary neurotrophic factor (CNTF) belong to the same family of hematopoietic and neurotrophic cytokines. Their receptor complexes contain a cytokine-binding α receptor and the common glycoprotein (gp)130 subunit for signal transduction. The extracellular parts of the α-receptor subunits consist of a membrane-proximal cytokine-binding domain and an N-terminal immunoglobulin (Ig)-like domain with unknown function. We examined the role of the Ig-like domain of IL-6R by constructing deletion mutants lacking the Ig domain (IL-6RΔIg and soluble IL-6RΔIg). IL-6RΔIg was shed as effectively as wild-type IL-6R from transfected COS-7 cells upon 4β-phorbol 12-myristate 13-acetate (PMA) treatment, whereas nonstimulated shedding of IL-6RΔIg was not observed. The shed sIL-6RΔIg from PMA-treated cells, as well as the transmembrane IL-6RΔIg, had the same biological activity as wild-type sIL-6R, as measured by the induction of haptoglobin secretion in HepG2-IL-6 cells and IL-6-dependent proliferation of IL-6RΔIg transfected BAF/gp130 cells. In COS-7 cells transfected with IL-6RΔIg or soluble IL-6RΔIg cDNA, transport of the deletion mutants through the secretory pathway appeared to be delayed because a sizeable proportion of the mutants was detected as an endo-β-N-acetylglucosaminidase-sensitive intermediate, suggesting that transport and processing of the ΔIg mutants on the secretory pathway were impaired. These experiments suggest that the Ig-like domain of the IL-6R is important for intracellular transport of IL-6R through the secretory pathway. Furthermore, the Ig-like domain is necessary for noninduced shedding of the IL-6R, whereas it has no function in PKC-dependent shedding of the IL-6R.