Binding of the 7SK snRNA turns the HEXIM1 protein into a P-TEFb (CDK9/cyclin T) inhibitor

• Published in: The EMBO journal Vol. 23; no. 13; pp. 2608 - 2619
• Main Authors: Michels, Annemieke A, Fraldi, Alessandro, Li, Qintong, Adamson, Todd E, Bonnet, François, Nguyen, Van Trung, Sedore, Stanley C, Price, Jason P, Price, David H, Lania, Luigi, Bensaude, Olivier
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 07-07-2004; Blackwell Publishing Ltd; Nature Publishing Group
• Subjects: Amino Acid Motifs; Amino Acid Sequence; Amino acids; Cyclin T; Cyclin-Dependent Kinase 9 - metabolism; Cyclins - metabolism; Electrophoretic Mobility Shift Assay; Escherichia coli - genetics; Glutathione Transferase - metabolism; HeLa Cells; HEXIM1; Humans; Mammals; MAQ1; Models, Biological; Molecular Sequence Data; Mutation; P-TEFb; Positive Transcriptional Elongation Factor B - antagonists & inhibitors; Positive Transcriptional Elongation Factor B - genetics; Positive Transcriptional Elongation Factor B - metabolism; Precipitin Tests; Protein Structure, Tertiary; Recombinant Fusion Proteins - chemistry; Recombinant Fusion Proteins - isolation & purification; Recombinant Fusion Proteins - metabolism; RNA polymerase II; RNA, Small Nuclear - metabolism; RNA-Binding Proteins - chemistry; RNA-Binding Proteins - genetics; RNA-Binding Proteins - isolation & purification; RNA-Binding Proteins - metabolism; transcription; Two-Hybrid System Techniques; Yeasts
• ISSN: 0261-4189; 1460-2075
• DOI: 10.1038/sj.emboj.7600275

The positive transcription elongation factor b (P‐TEFb) plays a pivotal role in productive elongation of nascent RNA molecules by RNA polymerase II. Core active P‐TEFb is composed of CDK9 and cyclin T. In addition, mammalian cell extracts contain an inactive P‐TEFb complex composed of four components, CDK9, cyclin T, the 7SK snRNA and the MAQ1/HEXIM1 protein. We now report an in vitro reconstitution of 7SK‐dependent HEXIM1 association to purified P‐TEFb and subsequent CDK9 inhibition. Yeast three‐hybrid tests and gel‐shift assays indicated that HEXIM1 binds 7SK snRNA directly and a 7SK snRNA‐recognition motif was identified in the central part of HEXIM1 (amino acids (aa) 152–155). Data from yeast two‐hybrid and pull‐down assay on GST fusion proteins converge to a direct binding of P‐TEFb to the HEXIM1 C‐terminal domain (aa 181–359). Consistently, point mutations in an evolutionarily conserved motif (aa 202–205) were found to suppress P‐TEFb binding and inhibition without affecting 7SK recognition. We propose that the RNA‐binding domain of HEXIM1 mediates its association with 7SK and that P‐TEFb then enters the complex through association with HEXIM1.