An eukaryotic RuvB-like protein (RUVBL1) essential for growth

The p300/CBP acetyltransferases function as transcriptional coactivators of beta-catenin in vertebrates

Wnt growth factors regulate a variety of developmental processes by altering specific gene expression patterns. In vertebrates beta-catenin acts as transcriptional activator, which is needed to overcome target gene repression by Groucho/TLE proteins, and to permit promoter activation as the final consequence of Wnt signaling. However, the molecular mechanisms of transcriptional activation by beta-catenin are only poorly understood. Here we demonstrate that the closely related acetyltransferases p300 and CBP potentiate beta-catenin-mediated activation of the siamois promoter, a known Wnt target. beta-catenin and p300 also synergize to stimulate a synthetic reporter gene construct, whereas activation of the cyclin D1 promoter by beta-catenin is refractory to p300 stimulation. Axis formation and activation of the beta-catenin target genes siamois and Xnr-3 in Xenopus embryos are sensitive to the E1A oncoprotein, a known inhibitor of p300/CBP. The C-terminus of beta-catenin interacts directly with a region overlapping the CH-3 domain of p300. p300 could participate in alleviating promoter repression imposed by chromatin structure and in recruiting the basal transcription machinery to promoters of particular Wnt target genes.

Human TIP49b/RUVBL2 gene: genomic structure, expression pattern, physical link to the human CGB/LHB gene cluster on chromosome 19q13.3

Bacterial DNA helicase RuvB protein is an essential component in homologous recombination and DNA double-strand break repair. Here, we report the gene structure of TIP49b/RUVBL2, a second putative human homologue of the bacterial RuvB gene. This gene contains 15 exons and 14 introns. The TIP49b/RUVBL2 open reading frame encodes a protein of 463 amino acids, showing 43% identity with the RUVBL1 protein. The TIP49b/RUVBL2 gene is physically linked to the human CGB/LHB gene cluster on chromosome 19q13.3. Genomic sequence analysis revealed that the TIP49b/RUVBL2 gene is very close (55 nucleotides in length) to the LHB gene, in the opposite orientation. The very close co-location of the mouse homologues of the human TIP49b/RUVBL2 and LHB genes was also conserved on mouse chromosome 7. Co-ordinated transcriptional regulation between the TIP49b/RUVBL2 and LHB genes was not observed. TIP49b/RUVBL2, like RUVBL1, was expressed ubiquitously in all human tissues examined and more strongly in testis. As TIP49b/RUVBL2 is expected to be involved in recombination repair and is located in a chromosome region frequently amplified in breast cancer, we quantified TIP49b/RUVBL2 gene expression by using real-time quantitative RT-PCR in a series of breast tumour samples. None of the tumour samples showed an altered TIP49b/RUVBL2 transcription level relative to normal breast tissue.

An ATPase/helicase complex is an essential cofactor for oncogenic transformation by c-Myc

The c-Myc transactivation domain was used to affinity purify tightly associated nuclear proteins. Two of these proteins were identified as TIP49 and a novel related protein called TIP48, both of which are highly conserved in evolution and contain ATPase/helicase motifs. TIP49 and TIP48 are complexed with c-Myc in vivo, and binding is dependent on a c-Myc domain essential for oncogenic activity. A missense mutation in the TIP49 ATPase motif acts as a dominant inhibitor of c-Myc oncogenic activity but does not inhibit normal cell growth, indicating that functional TIP49 protein is an essential mediator of c-Myc oncogenic transformation. The TIP49 and TIP48 ATPase/helicase proteins represent a novel class of cofactors recruited by transcriptional activation domains that function in diverse pathways.

Isolation, molecular characterization, and tissue-specific expression of ECP-51 and ECP-54 (TIP49), two homologous, interacting erythroid cytosolic proteins

We isolated two proteins, ECP-51 and ECP-54, from human erythrocyte cytosol by affinity chromatography using a peptide of the integral membrane protein stomatin as bait. Partial amino acid sequence information obtained by microsequencing allowed us to clone the respective cDNAs. Analysis of the nucleotide sequences revealed that ECP-51 and ECP-54 are homologous (44.2% amino acid identity) and contain ATP-binding sites. ECP-54 was identified as TIP49/RUVBL1/NMP238, which is a component of a large nuclear protein complex, possibly the RNA polymerase II holoenzyme; ECP-51 is a novel protein. Using the two-hybrid system, we showed that these proteins interact with each other. The interaction of ECP-51 and ECP-54 with the stomatin peptide and the localization to the nucleus and cytoplasm suggest an additional function for these proteins as chaperone components.

TIP49b, a new RuvB-like DNA helicase, is included in a complex together with another RuvB-like DNA helicase, TIP49a

We previously reported that TIP49a is a novel mammalian DNA helicase showing structural similarity with the bacterial recombination factor RuvB. In this study, we isolated a new TIP49a-related gene, termed TIP49b, from human and yeast cells. TIP49b also resembled RuvB, thus suggesting that TIP49a and TIP49b are included in a gene family. Like TIP49a, TIP49b was abundantly expressed in the testis and thymus. Enzyme assays revealed that TIP49b was an single-stranded DNA-stimulated ATPase and ATP-dependent DNA helicase. Most of the enzymatic properties of TIP49b were the same as those of TIP49a, whereas the polarity of TIP49b DNA helicase activity (5' to 3') was the opposite to that of TIP49a. TIP49b and TIP49a bound to each other and were included in the same complex of approximately 700 kDa in a cell. We found that TIP49b was an essential gene for the growth of Saccharomyces cerevisiae, as is the TIP49a gene, suggesting that TIP49b does not complement the TIP49a function and vice versa. From these observations, we suggest that TIP49b plays an essential role in the cellular processes involved in DNA metabolism.

A rat RuvB-like protein, TIP49a, is a germ cell-enriched novel DNA helicase

We have isolated a novel nuclear protein with a molecular mass of 49 kDa (TIP49a) from rat liver. The rat TIP49a showed structural resemblance to several bacterial RuvBs and also displayed Walker A and B motifs. We overproduced the recombinant TIP49a in Escherichia coli and purified it to near homogeneity. Biochemical investigations demonstrated that TIP49a possessed ATPase activity that was stimulated by single-stranded DNA but neither by double-stranded DNA nor by any forms of RNA polymers tested. Moreover, a UV cross-linking assay indicated TIP49a specifically interacted with ATP. Interestingly, we found that DNA duplex was unwound by the recombinant TIP49a in the presence of ATP or dATP. Optimal concentrations of ATP and Mg2+ for the helicase activity were 1-2 mM and 0.25-1 mM, respectively. Displacement of the DNA strand occurred in the 3' to 5' direction with respect to the single-stranded DNA flanking the duplex. Western blot analysis revealed that TIP49a was abundantly expressed in testes and moderately in spleen, thymus, and lung. In mouse seminiferous tubules, the protein was restrictively observed in germ lineages from late pachytene spermatocytes to round spermatids. From these observations, we propose that TIP49a is a novel DNA helicase and may play a role in nuclear processes such as recombination and transcription.