Structural and functional analysis of a nuclear localization signal in SOCS1

Suppressor of cytokine signaling 1 (SOCS1) belongs to a family of genes involved in inducible feedback inhibition of janus kinases (JAKs) and signal transducers and activators of transcription (STATs) signaling pathway. Recently, we were able to show that SOCS1 surprisingly translocates to the nucleus due to the presence of a functional nuclear localization signal (NLS). However, the precise nature of the NLS remained ill-defined. Here we investigated further details of the SOCS1 NLS and analyzed its functional importance. We show that nuclear transport of SOCS1 particularly depends on the second cluster of basic amino acid residues within the NLS. Neither the first nor a nearby identified third cluster of basic amino acids were sufficient for mediating nuclear localization of SOCS1. Altering the subcellular localization of SOCS1 by mutating clusters of arginine residues within the NLS did not affect the inhibition of interferon mediated STAT1 tyrosine-phosphorylation, but surprisingly led to impaired inhibitory activity of STAT mediated reporter gene induction and IFN-gamma induced CD54 regulation. A SOCS-box deletion mutant (E176X) also had reduced inhibitory activity. In contrast, nuclear factor kappaB (NFkappaB) signaling was not affected by SOCS1 wt or mutants. Thus, SOCS1 may accomplish its inhibitory function in the IFN-pathway in part through nuclear localization.