Figure 3.

Comparison of SOCS-3 affinity for phosphorylated vs. nonphosphorylated gp130 peptide and JAK-derived peptides. (A) Solution binding of phosphorylated and nonphosphorylated forms of gp130(750–764) peptide to SOCS-3 was measured in a competitive binding assay. The IC₅₀s for inhibition of SOCS-3 binding to immobilized ligand were 110 ± 2.7 nM for the phosphorylated peptide and 2.1 ± 0.2 mM for the unphosphorylated peptide. (B) Solution binding of JAK-derived peptides to SOCS-3. These phosphopeptides represent the activation loop sequences in JAK1, 2, and 3 and contain a phosphotyrosine residue corresponding to the autophosphorylation site. The amino acid sequences of these peptides were JAK1: IETDKE(pY)YTVKDDRD, JAK2: LPQDKE(pY)YKVKEPGE, JAK3: LPLGKD(pY)YVVREPGQ. The IC₅₀ values for these peptides were: JAK1, 230 ± 6.6 μM; JAK2, 1,200 ± 50 μM; JAK3, 140 ± 4.8 μM. The IC₅₀ value for the gp130(750–764) phosphopeptide, measured in the same experiment, was 110 ± 4.6 nM