hide
Free keywords:
AF4/FMR2 family member 4 (AFF4); C-terminal homology domain (CHD); RNA polymerase II; X-ray crystallography; dimerization; gene regulation; phosphorylation; super elongation complex; transcription elongation factor
Abstract:
AF4/FMR2 family member 4 (AFF4) is the scaffold protein of the multi-subunit super-elongation complex (SEC), which plays key roles in the release of RNA polymerase (Pol) II from promoter-proximal pausing and in the transactivation of HIV-1 transcription. AFF4 consists of an intrinsically disordered N-terminal region that interacts with other SEC subunits and a C-terminal homology domain (CHD) that is conserved among AF4/FMR2 family proteins, including AFF1, AFF2, AFF3, and AFF4. Here, we solved the X-ray crystal structure of the CHD in human AFF4 (AFF4-CHD) to 2.2 Å resolution and characterized its biochemical properties. The structure disclosed that AFF4-CHD folds into a novel domain that consists of eight helices and is distantly related to tetratrico peptide repeat (TPR) motifs. Our analyses further revealed that AFF4-CHD mediates the formation of an AFF4 homodimer or an AFF1-AFF4 heterodimer. Results from fluorescence anisotropy experiments suggested that AFF4-CHD interacts with both RNA and DNA in vitro. Furthermore, we identified a surface loop region in AFF4-CHD as a substrate for the P-TEFb kinase cyclin-dependent kinase 9 (CDK9), which triggers release of Pol II from promoter-proximal pausing sites. In conclusion, the AFF-CHD structure and biochemical analyses reported here reveal the molecular basis for the homo- and heterodimerization of AFF proteins and implicate the AFF4-CHD in nucleic acid interactions. The high conservation of the CHD among several other proteins suggests that our results are relevant also for understanding other CHD-containing proteins and their dimerization behavior.
