Solution Structure of Tandem SH2 Domains from Spt6 Protein and Their Binding to the Phosphorylated RNA Polymerase II C-terminal Domain

Spt6 is a highly conserved transcription elongation factor and histone chaperone. It binds directly to the RNA polymerase II C-terminal domain (RNAPII CTD) through its C-terminal region that recognizes RNAPII CTD phosphorylation. In this study, we determined the solution structure of the C-terminal region of Saccharomyces cerevisiae Spt6, and we discovered that Spt6 has two SH2 domains in tandem. Structural and phylogenetic analysis revealed that the second SH2 domain was evolutionarily distant from canonical SH2 domains and represented a novel SH2 subfamily with a novel binding site for phosphoserine. In addition, NMR chemical shift perturbation experiments demonstrated that the tandem SH2 domains recognized Tyr1, Ser2, Ser5, and Ser7 phosphorylation of RNAPII CTD with millimolar binding affinities. The structural basis for the binding of the tandem SH2 domains to different forms of phosphorylated RNAPII CTD and its physiological relevance are discussed. Our results also suggest that Spt6 may use the tandem SH2 domain module to sense the phosphorylation level of RNAPII CTD.