Identification of critical residues required for suppressor of cytokine signaling-specific regulation of interleukin-4 signaling

Suppressor of cytokine signaling (SOCS) family proteins were originally ide ntified as cytokine-induced negative regulators of cytokine signaling. We s how that SOCS-1 and SOCS-3 inhibit interleukin (IL)-4-dependent signal tran sducer and activator of transcription 6 (Stat6) activation of and subsequen t gene induction. By contrast, SOCS-2 and cytokine-inducible Src homology d omain 2 (SH2)-containing protein up-regulate these processes. IL-4 initiate s transmembrane signaling through two types of receptor complexes comprisin g the IL-4R alpha subunit and the associated Janus kinase 1 (Jak1) as commo n essential components. We demonstrate that both SOCS-1- and SOCS-3-mediate d down-regulation of IL-4 signaling is due to an inhibition of the receptor associated Jak1 activity. The SOCS proteins contain an amino-terminal regi on of variable length and primary structure, a central SH2 domain, and a ca rboxyl-terminal conserved motif termed SOCS-box. We show that the SH2 domai ns of SOCS-2, SOCS-3, and cytokine-inducible SH2-containing protein are fun ctionally redundant in regulating the IL-4-dependent Jak-Stat signaling. Th e Pre-SH2 domains of SOCS-2 and SOCS-3 confer the specificity of their regu latory function. Importantly, the Pre-SH2 domain of SOCS-3 alone can inhibi t IL-4 signaling. The SH2-proximal 25 amino acids of SOCS-3 are sufficient for this inhibition, and the Thr residue at position 24 and the Phe residue at position 25 are individually indispensable for its inhibitory function. Thus, the Thr-Phe motif in the Pre-SH2 domain plays a critical role in SOC S 3-mediated inhibition of the IL-4-dependent Jak-Stat signaling, likely by regulating the mode of SOCS-Jak interaction.