RESUMO
Canonical histones (H2A, H2B, H3, and H4) are present in all eukaryotes where they package genomic DNA and participate in numerous cellular processes, such as transcription regulation and DNA repair. In addition to the canonical histones, there are many histone variants, which have different amino acid sequences, possess tissue-specific expression profiles, and function distinctly from the canonical counterparts. A number of histone variants, including both core histones (H2A/H2B/H3/H4) and linker histones (H1/H5), have been identified to date. Htz1 (H2A.Z) and CENP-A (CenH3) are present from yeasts to mammals, and H3.3 is present from Tetrahymena to humans. In addition to the prevalent variants, others like H3.4 (H3t), H2A.Bbd, and TH2B, as well as several H1 variants, are found to be specific to mammals. Among them, H2BFWT, H3.5, H3.X, H3.Y, and H4G are unique to primates (or Hominidae). In this review, we focus on localization and function of primate- or hominidae-specific histone variants.
Assuntos
Histonas/classificação , Primatas/genética , Primatas/fisiologia , Sequência de Aminoácidos , Animais , Encéfalo , Neoplasias da Mama , Nucléolo Celular , DNA , Regulação da Expressão Gênica , Histonas/genética , Humanos , Mamíferos , FilogeniaRESUMO
RecQL5, a mammalian RecQ family protein, is involved in the regulation of transcription elongation, DNA damage response, and DNA replication. Here, we identified and characterized an alternative splicing isoform of RECQL5 (RECQL5ß1), which contains 17 additional amino acid residues within the RECQL5 KIX domain when compared with the canonical isoform (RECQL5ß). RECQL5ß1 had a markedly decreased binding affinity to RNA polymerase II (Pol II) and poorly competed with the transcription elongation factor TFIIS for binding to Pol II. As a result, this isoform has a weaker activity for repression of transcription elongation. In contrast, we discovered that RECQL5ß1 could bind stronger to MRE11, which is a primary sensor of DNA double-strand breaks (DSBs). Furthermore, we found that RECQL5ß1 promoted DNA repair in the RECQL5ß1 rescue cells. These results suggest that RECQL5ß mainly functions as a transcription repressor, while the newly discovered RECQL5ß1 has a specialized role in DNA damage response. Taken together, our data suggest a cellular-functional specialization for each KIX splicing isoform in the cell.
Assuntos
Reparo do DNA , Proteína Homóloga a MRE11/metabolismo , RNA Polimerase II/metabolismo , RecQ Helicases/metabolismo , Transcrição Gênica , Linhagem Celular , DNA/metabolismo , Quebras de DNA de Cadeia Dupla , Células HEK293 , Células HeLa , Humanos , Células MCF-7 , Ligação Proteica , Isoformas de Proteínas , Fatores de Elongação da Transcrição/metabolismoRESUMO
The hominidae-specific histone variant H4G is expressed in breast cancer patients in a stage-dependent manner. H4G localizes primarily in the nucleoli via its interaction with nucleophosmin (NPM1). H4G is involved in rDNA transcription and ribosome biogenesis, which facilitates breast cancer cell proliferation. However, the molecular mechanism underlying this process remains unknown. Here, we show that H4G is not stably incorporated into nucleolar chromatin, even with the chaperoning assistance of NPM1. H4G likely form transient nucleosome-like-structure that undergoes rapid dissociation. In addition, the nucleolar chromatin in H4GKO cells is more compact than WT cells. Altogether, our results suggest that H4G relaxes the nucleolar chromatin and enhances rRNA transcription by forming destabilized nucleosome in breast cancer cells.
