Post-translational modification and mitochondrial relocalization of histone H3 during apoptosis induced by staurosporine.

Post-translational modifications (PTMs) of histones such as phosphorylation, acetylation, and ubiquitination, collectively referred to as the "histone-code", have been known to regulate gene expression and chromatin condensation for over a decade. They are also implicated in processes such as DNA repair and apoptosis. However, the study of the phosphorylation of histones has been mainly focused on chromosome condensation and mitosis. Therefore, the phosphorylation of histones in apoptosis is not fully understood. It was recently demonstrated by Tang et al. that histones are released from nucleosome during apoptosis, an observation that is in agreement with our findings. In addition to the release of histones, the dephosphorylation of histone H3 at Thr-3 and Ser-10 was observed during apoptosis in some cancer cells. Our data suggest that the modification and release of histones could serve markers of apoptosis in human cancer cells. We also suggest that the released histones, especially H3, could be translocated to mitochondria during apoptosis.

RNA-binding proteins of mammalian mitochondria.

A UV-cross-linking assay was used to identify RNA-binding proteins in mammalian mitochondria. A number of these proteins were detected ranging in molecular mass from 15 to 120 kDa. All of the mRNA-binding activities were localized to the matrix except for two proteins which are primarily associated with the inner membrane. None of the polypeptides is specific for binding mitochondrial mRNAs since all bound mRNAs from other sources with comparable efficiency. Some preference for binding mRNA over tRNA or homoribopolymers was observed with several of the proteins. A protein with characteristic pentatricopeptide repeat motifs found in many RNA binding proteins was identified associated with the small subunit of the mitochondrial ribosome.

Identification of mammalian mitochondrial translational initiation factor 3 and examination of its role in initiation complex formation with natural mRNAs.

Human mitochondrial translational initiation factor 3 (IF3(mt)) has been identified from the human expressed sequence tag data base. Using consensus sequences derived from conserved regions of the bacterial IF3, several partially sequenced cDNA clones were identified, and the complete sequence was assembled in silico from overlapping clones. IF3(mt) is 278 amino acid residues in length. MitoProt II predicts a 97% probability that this protein will be localized in mitochondria and further predicts that the mature protein will be 247 residues in length. The cDNA for the predicted mature form of IF3(mt) was cloned, and the protein was expressed in Escherichia coli in a His-tagged form. The mature form of IF3(mt) has short extensions on the N and C termini surrounding a region homologous to bacterial IF3. The region of IF3(mt) homologous to prokaryotic factors ranges between 21-26% identical to the bacterial proteins. Purified IF3(mt) promotes initiation complex formation on mitochondrial 55 S ribosomes in the presence of mitochondrial initiation factor 2 (IF2(mt)), [(35)S]fMet-tRNA, and either poly(A,U,G) or an in vitro transcript of the cytochrome oxidase subunit II gene as mRNA. IF3(mt) shifts the equilibrium between the 55 S mitochondrial ribosome and its subunits toward subunit dissociation. In addition, the ability of E. coli initiation factor 1 to stimulate initiation complex formation on E. coli 70 S and mitochondrial 55 S ribosomes was investigated in the presence of IF2(mt) and IF3(mt).