Efficient internalization of IL-2 depends on the distal portion of the cytoplasmic tail of the IL-2R common gamma-chain and a lymphoid cell environment

The common gamma -chain (gammac), a subunit of the IL-2R, is essential for high affinity ligand binding and signal transduction due to Jak3 associatio n to gammac, Another consequence of IL-2/IL-2R interaction is rapid recepto r-mediated endocytosis of the receptor-ligand complex. In the present study , we establish that this rapid endocytosis of IL-2 in a T cell tumor line i s dependent upon the cytoplasmic tail of gammac, Deletion mutants of the cy toplasmic tail mapped this activity to 9 aa of gammac, 45-54 aa distal to t he transmembrane region. In contrast, ligand-independent constitutive endoc ytosis of gammac occurred more slowly and was dependent upon a PEST sequenc e in a more membrane-proximal region of the cytoplasmic tail of gammac, Thu s, this receptor subunit may use distinct sorting signals for its constitut ive regulation and ligand-induced endocytosis. Rapid endocytosis of IL-2 wa s inhibited by the tyrosine kinase inhibitor genistein, implicating a role for a signal transduction pathway in IL-2 internalization. However, one T c ell line bearing a mutant gammac exhibited impaired endocytosis of IL-2, de spite normal IL-2-induced Jak/STAT activation, Furthermore, inefficient end ocytosis of IL-2 was noted after transfection of the COS7 epithelial cell l ine with the IL-2R, and further reconstitution of these cells with Jak/STAT proteins did not enhance this internalization. Collectively, these latter findings indicate that rapid endocytosis of IL-2 is dependent upon cellular signaling in lymphoid cell environment that is not solely a consequence of the presence of the Jak/STAT pathway.