The AFF4 scaffold binds human P-TEFb adjacent to HIV Tat
Ursula Schulze-Gahmen,
Heather Upton,
Andrew Birnberg,
Katherine Bao,
Seemay Chou,
Nevan J Krogan,
Qiang Zhou,
Tom Alber
Affiliations
Ursula Schulze-Gahmen
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Heather Upton
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Andrew Birnberg
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Katherine Bao
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Seemay Chou
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Nevan J Krogan
Department of Cellular and Molecular Pharmacology, University of California, San Francisco, San Francisco, United States; California Institute for Quantitative Biosciences, QB3, Berkeley, United States; J David Gladstone Institutes, San Francisco, United States
Qiang Zhou
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States
Tom Alber
Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, United States; California Institute for Quantitative Biosciences, QB3, Berkeley, United States
Human positive transcription elongation factor b (P-TEFb) phosphorylates RNA polymerase II and regulatory proteins to trigger elongation of many gene transcripts. The HIV-1 Tat protein selectively recruits P-TEFb as part of a super elongation complex (SEC) organized on a flexible AFF1 or AFF4 scaffold. To understand this specificity and determine if scaffold binding alters P-TEFb conformation, we determined the structure of a tripartite complex containing the recognition regions of P-TEFb and AFF4. AFF4 meanders over the surface of the P-TEFb cyclin T1 (CycT1) subunit but makes no stable contacts with the CDK9 kinase subunit. Interface mutations reduced CycT1 binding and AFF4-dependent transcription. AFF4 is positioned to make unexpected direct contacts with HIV Tat, and Tat enhances P-TEFb affinity for AFF4. These studies define the mechanism of scaffold recognition by P-TEFb and reveal an unanticipated intersubunit pocket on the AFF4 SEC that potentially represents a target for therapeutic intervention against HIV/AIDS.
