Energy minimized models of the interaction of the N-terminal region of Tat with the activation loop of CDK9.

(A) Phosphorylation of Ser175 at the activation loop of CDK9 favors a hydrogen bonding interaction with Lys12 of wild type Tat. (B) pSer175 interactions with Lys12 are unaffected by the H13L mutation. (C) Impact of Ser175 phosphorylation on preexisting intermolecular interactions between CDK9 and Tat. (D) Modeling of K12N Tat/CDK9 interactions with or without the phosphate moiety at Ser175. (E) Tat interactions with CDK9 S175A in the wildtype and H13L backgrounds. (F) Tat interactions with the CDK9 phosphomimetic mutants S175D and S175E.