The role of the STAT1 SH2 domain in interferon-gamma signaling

The interferon-inducible transcription factor STAT1 (signal transducer and activator of transcription 1) plays an important role in the defense against viral, bacterial and par-asitic infections. Well studied is the phosphorylation of a signature tyrosine residue (Y701) in the carboxy-terminus of STAT1 upon stimulation of cells with cytokines, leading to the formation of transcriptionally active, parallel STAT1 dimers. In this the-sis, a residue in the STAT1 molecule conserved among the family members STAT1 to STAT4 was replaced with alanine by site-directed mutagenesis, and the corre-sponding point mutant (E585A) was functionally characterized in various experi-mental approaches. Other than the wild-type protein or an additionally generated point mutant (E524A), STAT1-E585A was hyperphosphorylated upon stimulation of cells with interferon (IFN). The exchange of the surface-exposed glutamic acid resi-due 585 in the SH2 domain of STAT1 unexpectedly resulted in a differential tran-scriptional behavior. While numerous known endogenous STAT1 target genes (irf1, gbp1, and mig1) were activated normally by the point mutant, real-time PCR meas-urements showed a significantly higher expression rate for the mcp1 gene. There-fore, STAT1-E585A characterized in this study can be regarded as a partial gain-of-function mutant, although the stat1 gene itself was not over-expressed. The molecu-lar basis underlying the observed hyperphosphorylation of the E585A mutant possi-bly results from a reduced rate of conformational exchange between a DNA-bound parallel and an anti-parallel dimer configuration, with the latter being required for ty-rosine dephosphorylation. In summary, my data demonstrate the role of a critical amino acid residue in the STAT1 SH2 domain in transcriptional responses in IFN signaling, thereby modifying the central impact of tyrosine phosphorylation on gene activation.2014-11-1