Abstract
The intrinsically disordered scaffold proteins AFF1/4 and the transcription elongation factors ELL1/2 are core components of the super elongation complex required for HIV-1 proviral transcription. Here we report the 2.0-Å resolution crystal structure of the human ELL2 C-terminal domain bound to its 50-residue binding site on AFF4, the ELLBow. The ELL2 domain has the same arch-shaped fold as the tight junction protein occludin. The ELLBow consists of an N-terminal helix followed by an extended hairpin that we refer to as the elbow joint, and occupies most of the concave surface of ELL2. This surface is important for the ability of ELL2 to promote HIV-1 Tat-mediated proviral transcription. The AFF4–ELL2 interface is imperfectly packed, leaving a cavity suggestive of a potential binding site for transcription-promoting small molecules.
Highlights
The intrinsically disordered scaffold proteins AFF1/4 and the transcription elongation factors ELL1/2 are core components of the super elongation complex required for human immunodeficiency virus (HIV)-1 proviral transcription
The super elongation complex (SEC) consists of the Cyclin-dependent kinase CDK9 and Cyclin T (CycT1 or T2), together known as P-TEFb8; one of either of the intrinsically disordered (ID) scaffold proteins AFF1 or AFF4; one of either ENL or AF9; and one of either of the RNA polymerase elongation factors ELL1 or ELL2
Following the initial mapping of the AFF4 and ELL2 interaction sites to approximately residues 318–337 of the former and 519–640 of the latter[20] (Fig. 1a), we sought to isolate a stable form of this monomeric complex for crystallization (Supplementary Fig. 1a)
Summary
The intrinsically disordered scaffold proteins AFF1/4 and the transcription elongation factors ELL1/2 are core components of the super elongation complex required for HIV-1 proviral transcription. The ELLBow consists of an N-terminal helix followed by an extended hairpin that we refer to as the elbow joint, and occupies most of the concave surface of ELL2 This surface is important for the ability of ELL2 to promote HIV-1 Tat-mediated proviral transcription. The SEC consists of the Cyclin-dependent kinase CDK9 and Cyclin T (CycT1 or T2), together known as P-TEFb8; one of either of the intrinsically disordered (ID) scaffold proteins AFF1 or AFF4 (refs 5,6); one of either ENL or AF9; and one of either of the RNA polymerase elongation factors ELL1 or ELL2 (refs 5,9,10). The structure of P-TEFb lacking the C-terminal IDR of CycT1 has been determined in complex with HIV-1 Tat[15] and the N-terminal 60 residues of AFF4 (refs 16,17) This structure shows that AFF4 residues 32–67 bind as an extended strand a. We set out to visualize the last of the three known interfaces critical for AFF4 function, its binding site for ELL1/2
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