The positive transcription elongation factor (P-TEFb) comprises a kinase, CDK9, and a Cyclin T1 or T2. Its activity is inhibited by association with the HEXIM1 or HEXIM2 protein bound to 7SK small nuclear RNA. HEXIM1 and HEXIM2 were found to form stable homoand hetero-oligomers. Using yeast two-hybrid and transfection assays, we have now shown that the C-terminal domains of HEXIM proteins directly interact with each other. Hydrodynamic parameters measured by glycerol gradient ultracentrifugation and gel-permeation chromatography demonstrate that both purified recombinant and cellular HEXIM1 proteins form highly anisotropic particles. Chemical cross-links suggest that HEXIM1 proteins form dimers. The multimeric nature of HEXIM1 is maintained in P-TEFb⅐HEXIM1⅐7SK RNA complexes. Multiple P-TEFb modules are found in the inactive P-TEFb⅐HEXIM1⅐7SK complexes. It is proposed that 7SK RNA binding to a HEXIM1 multimer promotes the simultaneous recruitment and hence inactivation of multiple P-TEFb units.The positive transcription elongation factor (P-TEFb) 1 comprises a protein kinase, CDK9, and a Cyclin T1, T2, or K (1, 2). It is required for transcription elongation of most class II genes. P-TEFb phosphorylates numerous substrates, including the C-terminal domain (CTD) of RNA polymerase II and the Spt5 subunit of the DRB sensitivity-inducing factor (DSIF). The DSIF prevents transcription from proceeding efficiently after initiation. The kinase activity of P-TEFb antagonizes this inhibitory effect. Several class II genes such as heat-shock or U2 small nuclear RNA genes do not have a strong requirement for P-TEFb activity to elongate, but rather to terminate, transcription properly (3, 4).Recent studies have indicated that two major forms of PTEFb are present in equivalent amounts in HeLa cell lysates (5). The active form consists of "core" P-TEFb, CDK9 and Cyclin T1 or T2. The inactive form consists of CDK9, Cyclin T1 or T2, MAQ1/HEXIM1, and 7SK RNA (5-8). The 7SK RNA is an abundant class III noncoding RNA (9) detected in vertebrates, cephalochordates, and mollusks.2 Binding of 7SK RNA to HEXIM1 turns this protein into a P-TEFb inhibitor (10). In response to treatments that arrest transcription (5-8) or following cardiac hypertrophic stimuli (11, 12), HEXIM1 and 7SK RNA dissociate from P-TEFb. As a consequence, the P-TEFb activity is up-regulated.HEXIM1 expression is up-regulated in smooth muscle cells treated with hexamethylene-bisacetamide (13) or down-regulated by estrogen in breast cancer cells and therefore named EDG-1 (14). HEXIM1 has also been reported as a protein accumulating in heart tissues during early embryogenesis and therefore named CLP-1 (cardiac lineage protein) (15). Disruption of the HEXIM1/CLP1 gene results in heart defects leading to death at birth in homozygote CLP-1(Ϫ/Ϫ) mice (16). Because P-TEFb plays a general role in class II gene expression, such a late developmental defect suggests the existence of a compensatory mechanism at the cellular level. Indeed, a BLAST search within genome and Express...

The HEXIM1 protein, in association with 7SK snRNA, binds and inhibits the kinase activity of P-TEFb (CDK9/cyclin T). P-TEFb activity is crucial for efficient transcription elongation of viral and cellular genes. HEXIM1 was originally isolated as a protein up-regulated by hexamethylene bisacetamide (HMBA), a prototypical inducer of differentiation. To determine the causative role of HEXIM1 during cell differentiation we analyzed the biochemical and functional consequences of HEXIM1 protein levels in several in vitro differentiation systems. We found that HEXIM1 mRNA and protein levels are up-regulated during differentiation of murine erythroleukemia cells upon treatment with HMBA or DMSO. Stimulation of HEXIM1 is not restricted to hematopoietic cells, as induction of phenotypic differentiation of neuroblastoma cells by retinoic acid results in up-regulation of HEXIM1. Moreover, ectopic expression of HEXIM1 causes growth inhibition and promotes neuronal differentiation. These findings highlight a crucial role of HEXIM1 protein during cell differentiation.

Some parkin polymorphisms appear to be risk factors for sporadic or familial PD. The functional effects of these coding polymorphisms need to be established, and further studies on parkin polymorphisms in PD should be undertaken.