Requirement of the C-terminus and the WSKWS motif of the sγc for cytokine-inhibitory activity and binding to cells.

(A) Two C-terminal deletions (244P, 235S), and a double-point mutant (GSKGS instead of WSKWS) were generated by site-directed mutagenesis and expressed in 293 EBNA cells. tm, transmembrane domain. (B) CTLL-2 cells were preincubated with different sγc variants at concentrations indicated or equally treated control supernatant 30 minutes before adding IL-2 (2 ng/mL). Cell proliferation was determined as described in the legend to Figure 6. (C) Affinity-purified ¹²⁵I-sγc (255N) and IL-4 were added to human TF-1 cells expressing murine IL-4R in the presence or absence of 200-fold excess amounts of unlabeled 255N-Stop-sγc, 244P-Stop-sγc, or GSKGS-255N-Stop-sγc, respectively. The percentages of cell-bound ¹²⁵I-sγc were determined as described in “Materials and methods.” Data given are the mean (± SEM) of triplicate determinations.