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

The conversion of signal transducer and activator of transcription (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 functioning 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.