Hexamethylene Bis Acetamide Inducible Protein 1 (HEXIM1)

HEXIM1 inhibited the ability of C2TA (MHC2TA) to activate transcription of MHC class II genes in HeLa cells. This inhibition depended on the cyclin T1 (CCNT1)-binding domain of HEXIM1. In vitro binding experiments and chromatin immunoprecipitation analyses showed that HEXIM1 competed with C2TA for binding to CCNT1, which together with CDK9 forms the positive transcription elongation factor B (PTEFB), and sequestered PTEFB from C2TA. HeLa cells depleted of HEXIM1 by small interfering RNA displayed increased C2TA activity and induction of C2TA-dependent genes.The deduced 41-kD protein is 85.3% homologous to human HIS1. Northern and Western blot analyses revealed wide expression during postnatal development, with highest levels in heart, skeletal muscle, and brain.