A family-wide assessment of latent STAT transcription factor interactions reveals divergent dimer repertoires

The conversion of signal transducer and activator of tran-scription (STAT) proteins from latent to active transcription factors is central to cytokine signaling. Triggered by their signal-induced tyrosine phosphorylation, it is the assembly of a range of cytokine-specific STAT homo-and heterodimers that marks a key step in the transition of hitherto latent proteins to transcription activators. In contrast, the constitutive self -assembly of latent STATs and how it relates to the func-tioning of activated STATs is understood less well. To provide a more complete picture, we developed a co-localization-based assay and tested all 28 possible combinations of the seven unphosphorylated STAT (U-STAT) proteins in living cells. We identified five U-STAT homodimers-STAT1, STAT3, STAT4, STAT5A, and STAT5B-and two heterodimers-STAT1:-STAT2 and STAT5A:STAT5B-and performed semi -quantitative assessments of the forces and characterizations of binding interfaces that support them. One STAT protein- STAT6-was found to be monomeric. This comprehensive analysis of latent STAT self-assembly lays bare considerable structural and functional diversity in the ways that link STAT dimerization before and after activation.