title: Compensatory contributions of HEXIM1 and HEXIM2 in maintaining the balance of active and inactive positive transcription elongation factor b complexes for control of transcription.
abstract: Human positive transcriptional elongation factor b (P-TEFb), consisting of a cyclin-dependent kinase 9-cyclin T heterodimer, stimulates general and disease-specific transcriptional elongation by phosphorylating RNA polymerase II. The HEXIM1 protein, aided by the 7SK snRNA, sequesters P-TEFb into an inactive 7SK.HEXIM1.P-TEFb small nuclear ribonucleic acid particle for inhibition of transcription and, consequently, cell proliferation. Here we show that, like HEXIM1, a highly homologous protein named HEXIM2 also possesses the ability to inactivate P-TEFb to suppress transcription through a 7SK-mediated interaction with P-TEFb. Furthermore, HEXIM1 and HEXIM2 can form stable homo- and hetero-oligomers (most likely dimers), which may nucleate the formation of the 7SK small nuclear ribonucleic acid particle. Despite their similar functions, HEXIM1 and HEXIM2 exhibit distinct expression patterns in various human tissues and established cell lines. In HEXIM1-knocked down cells, HEXIM2 can functionally and quantitatively compensate for the loss of HEXIM1 to maintain a constant level of the 7SK/HEXIM-bound P-TEFb. Our results demonstrate that there is a tightly regulated cellular process to maintain the balance between active and inactive P-TEFb complexes, which controls global transcription as well as cell growth and differentiation.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA.
authors.forename: Jasper H N
authors.initials: JH
authors.lastname: Yik
authors.forename: Ruichuan
authors.initials: R
authors.lastname: Chen
authors.forename: Andrea C
authors.initials: AC
authors.lastname: Pezda
authors.forename: Qiang
authors.initials: Q
authors.lastname: Zhou
mesh_terms: D000595:Amino Acid Sequence; D005786:Gene Expression Regulation / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D042925:Positive Transcriptional Elongation Factor B / Q000235:genetics / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D017411:Ribonucleoproteins, Small Nuclear / Q000235:genetics / Q000378:metabolism*; D014157:Transcription Factors
chemical_list: C476123:HEXIM1 protein, human; C501428:HEXIM2 protein, human; D016601:RNA-Binding Proteins; D017411:Ribonucleoproteins, Small Nuclear; D014157:Transcription Factors; D042925:Positive Transcriptional Elongation Factor B