Identification of a nuclear localization signal in suppressor of cytokine signaling 1.
Suppressor of cytokine signaling (SOCS) proteins are inducible feedback inhibitors of janus kinase and signal transducer and activators of transcription signaling pathways. In addition, SOCS1 has been identified to regulate stability of nuclear NF-kappaB subunits. However, details about the regulation of the nuclear pool of SOCS1 are unknown. Using different experimental approaches, we observed that SOCS1 but no further SOCS family members localized to the nucleus when expressed in various cell lines. Nuclear transport was confirmed for endogenous SOCS1 in macrophages stimulated with IFN-gamma. Sequence analysis revealed a bipartite nuclear localization signal (NLS) located between the src-homology 2 (SH2) domain and the SOCS box of SOCS1. Deletion of this region, introduction of a series of R/A point mutations, or substitution of this sequence with the respective region of SOCS3 resulted in loss of nuclear localization. Fusion of the SOCS1-NLS to cytokine-inducible SH2 region containing protein (CIS) resulted in nuclear localization of this otherwise cytoplasmic protein. SOCS1 mutants with loss of nuclear localization were still effective in suppressing IFN-alpha-mediated STAT1 tyrosine phosphorylation. However, they showed decreased inhibition of IFN-gamma-mediated induction of CD54. The results identify a hitherto unknown transport of SOCS1 into the nucleus which extends the spectrum of SOCS1 inhibitory activity.
Baetz
Andrea
A
Department of Hygiene and Medical Microbiology, Hygiene-Institute, University Heidelberg, Im Neuenheimer Feld 324, 69120 Heidelberg, Germany.
Koelsche
Christian
C
Strebovsky
Julia
J
Heeg
Klaus
K
Dalpke
Alexander H
AH
D000595:Amino Acid Sequence; D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D018799:Intercellular Adhesion Molecule-1 / Q000502:physiology; D007372:Interferons / Q000502:physiology; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation; D041681:NIH 3T3 Cells; D019913:Nuclear Localization Signals / Q000032:analysis* / Q000235:genetics; D021381:Protein Transport; D050794:STAT1 Transcription Factor / Q000378:metabolism; D015398:Signal Transduction; D000071222:Suppressor of Cytokine Signaling 1 Protein; D050826:Suppressor of Cytokine Signaling Proteins / Q000096:biosynthesis / Q000737:chemistry*
D019913:Nuclear Localization Signals; C108944:SOCS1 protein, human; D050794:STAT1 Transcription Factor; C491590:Socs1 protein, mouse; D000071222:Suppressor of Cytokine Signaling 1 Protein; D050826:Suppressor of Cytokine Signaling Proteins; D018799:Intercellular Adhesion Molecule-1; D007372:Interferons