TAFII70 Isoform-Specific Growth Suppression Correlates With Its Ability to Complex With the GADD45a Protein

Identification of NRAS Downstream Genes with CRISPR Activation Screening

Mutations in NRAS constitutively activate cell proliferation signaling in malignant neoplasms, such as leukemia and melanoma, and the clarification of comprehensive downstream genes of NRAS might lead to the control of cell-proliferative signals of NRAS-driven cancers. We previously established that NRAS expression and proliferative activity can be controlled with doxycycline and named as THP-1 B11. Using a CRISPR activation library on THP-1 B11 cells with the NRAS-off state, survival clones were harvested, and 21 candidate genes were identified. By inducting each candidate guide RNA with the CRISPR activation system, DOHH, HIST1H2AC, KRT32, and TAF6 showed higher cell-proliferative activity. The expression of DOHH, HIST1H2AC, and TAF6 was definitely upregulated with NRAS expression. Furthermore, MEK inhibitors resulted in the decreased expression of DOHH, HIST1H2AC, and TAF6 proteins in parental THP-1 cells. The knockdown of DOHH, HIST1H2AC, and TAF6 was found to reduce proliferation in THP-1 cells, indicating that they are involved in the downstream proliferation of NRAS. These molecules are expected to be new therapeutic targets for NRAS-mutant leukemia cells.

Mutational analysis of TAF6 revealed the essential requirement of the histone-fold domain and the HEAT repeat domain for transcriptional activation

TAF6, bearing the histone H4-like histone-fold domain (HFD), is a subunit of the core TAF module in TFIID and SAGA transcriptional regulatory complexes. We isolated and characterized several yeast TAF6 mutants bearing amino acid substitutions in the HFD, the middle region or the HEAT repeat domain. The TAF6 mutants were highly defective for transcriptional activation by the Gcn4 and Gal4 activators. CHIP assays showed that the TAF6-HFD and the TAF6-HEAT domain mutations independently abrogated the promoter occupancy of TFIID and SAGA complex in vivo. We employed genetic and biochemical assays to identify the relative contributions of the TAF6 HFD and HEAT domains. First, the temperature-sensitive phenotype of the HEAT domain mutant was suppressed by overexpression of the core TAF subunits TAF9 and TAF12, as well as TBP. The HFD mutant defect, however, was suppressed by TAF5 but not by TAF9, TAF12 or TBP. Second, the HEAT mutant but not the HFD mutant was defective for growth in the presence of transcription elongation inhibitors. Third, coimmunoprecipitation assays using yeast cell extracts indicated that the specific TAF6 HEAT domain residues are critical for the interaction of core TAF subunits with the SAGA complex but not with TFIID. The specific HFD residues in TAF6, although required for heterodimerization between TAF6 and TAF9 recombinant proteins, were dispensable for association of the core TAF subunits with TFIID and SAGA in yeast cell extracts. Taken together, the results of our studies have uncovered the non-overlapping requirement of the evolutionarily conserved HEAT domain and the HFD in TAF6 for transcriptional activation.

Incorporation of an internal ribosome entry site-dependent mechanism in arsenic-induced GADD45 alpha expression

We have previously shown that trivalent arsenic (arsenite, As(3+)) is able to induce GADD45 alpha expression in human bronchial epithelial cells through activation of c-Jun NH(2)-terminal kinase and nucleolin-dependent mRNA stabilization. In the present report, we show that As(3+) is capable of inducing translation of the GADD45 alpha protein through a cap-independent, or rather, an internal ribosome entry site (IRES)-dependent mechanism. In growth-arrested cells, As(3+) elevated the GADD45 alpha protein level in a dose- and time-dependent manner which did not correlate with the GADD45 alpha mRNA expression. Pretreatment of the cells with rapamycin, an inhibitor for the cap-dependent translation machinery through the suppression of mTOR and p70S6 kinase, failed to affect the induction of the GADD45 alpha protein induced by As(3+). Sequence analysis revealed a potential IRES element in the 5'-untranslated region of the GADD45 alpha mRNA. This IRES element in the 5'-untranslated region of the GADD45 alpha mRNA is functional in mediating As(3+)-induced translation of the GADD45 alpha protein in a dicistronic reporter gene activity assay. Immunoprecipitation and proteomic studies suggest that As(3+) impairs the assembly of the cap-dependent initiating complex for general protein translation but increases the association of human elongation factor 2 and human heterogeneous nuclear ribonucleoprotin with this complex. Thus, these results suggest that in growth-arrested cells, As(3+) is still capable of inducing GADD45 alpha expression through an IRES-dependent translational regulation.