Anti-apoptotic Bcl-2 fails to form efficient complexes with pro-apoptotic Bak to protect from Celecoxib-induced apoptosis.

The non-steroidal anti-inflammatory drug Celecoxib is a specific inhibitor of cyclooxygenase-2. Apart from its inhibitor function, Celecoxib induces apoptosis through the intrinsic pathway which is controlled by the Bcl-2 family members. In Jurkat T lymphoma cells, treatment with Celecoxib results in a rapid decline of the anti-apoptotic Bcl-2-related protein Mcl-1. The depletion of Mcl-1 is sufficient for apoptosis induction and can be blocked by overexpression of Bcl-xL but not by the close homologue Bcl-2. The present investigation analyzed the mechanism by which Bcl-xL prevents apoptosis induction whereas Bcl-2 failed to. Our data show that the involvement of the orphan nuclear receptor Nur77/TR3 specifically targeting Bcl-2 but not Bcl-xL was not involved in Celecoxib-induced apoptosis. Surprisingly, BH3-only proteins Bid, Bim, and Puma of the Bcl-2 family were not needed either. However, unlike Bcl-2, Mcl-1, and Bcl-xL sequestered Bak preventing it from activation through a direct interaction. Thus, when abundantly expressed, Bcl-xL can substitute for the loss of Mcl-1 whereas Bcl-2, incapable of forming a high affinity complex with Bak, could not.

Daxx is an H3.3-specific histone chaperone and cooperates with ATRX in replication-independent chromatin assembly at telomeres.

The histone variant H3.3 is implicated in the formation and maintenance of specialized chromatin structure in metazoan cells. H3.3-containing nucleosomes are assembled in a replication-independent manner by means of dedicated chaperone proteins. We previously identified the death domain associated protein (Daxx) and the alpha-thalassemia X-linked mental retardation protein (ATRX) as H3.3-associated proteins. Here, we report that the highly conserved N terminus of Daxx interacts directly with variant-specific residues in the H3.3 core. Recombinant Daxx assembles H3.3/H4 tetramers on DNA templates, and the ATRX-Daxx complex catalyzes the deposition and remodeling of H3.3-containing nucleosomes. We find that the ATRX-Daxx complex is bound to telomeric chromatin, and that both components of this complex are required for H3.3 deposition at telomeres in murine embryonic stem cells (ESCs). These data demonstrate that Daxx functions as an H3.3-specific chaperone and facilitates the deposition of H3.3 at heterochromatin loci in the context of the ATRX-Daxx complex.

Distinct factors control histone variant H3.3 localization at specific genomic regions.

The incorporation of histone H3 variants has been implicated in the epigenetic memory of cellular state. Using genome editing with zinc-finger nucleases to tag endogenous H3.3, we report genome-wide profiles of H3 variants in mammalian embryonic stem cells and neuronal precursor cells. Genome-wide patterns of H3.3 are dependent on amino acid sequence and change with cellular differentiation at developmentally regulated loci. The H3.3 chaperone Hira is required for H3.3 enrichment at active and repressed genes. Strikingly, Hira is not essential for localization of H3.3 at telomeres and many transcription factor binding sites. Immunoaffinity purification and mass spectrometry reveal that the proteins Atrx and Daxx associate with H3.3 in a Hira-independent manner. Atrx is required for Hira-independent localization of H3.3 at telomeres and for the repression of telomeric RNA. Our data demonstrate that multiple and distinct factors are responsible for H3.3 localization at specific genomic locations in mammalian cells.

New functions for an old variant: no substitute for histone H3.3.

Histone proteins often come in different variants serving specialized functions in addition to their fundamental role in packaging DNA. The metazoan histone H3.3 has been most closely associated with active transcription. Its role in histone replacement at active genes and promoters is conserved to the single histone H3 in yeast. However, recent genetic studies in flies have challenged its importance as a mark of active chromatin, and revealed unexpected insights into essential functions of H3.3 in the germline. With strikingly little amino acid sequence difference to the canonical H3, H3.3 therefore accomplishes a surprising variety of cellular and developmental processes.

In situ observation of protein phosphorylation by high-resolution NMR spectroscopy.

Although the biological significance of protein phosphorylation in cellular signaling is widely appreciated, methods to directly detect these post-translational modifications in situ are lacking. Here we introduce the application of high-resolution NMR spectroscopy for observing de novo protein phosphorylation in vitro and in Xenopus laevis egg extracts and whole live oocyte cells. We found that the stepwise modification of adjacent casein kinase 2 (CK2) substrate sites within the viral SV40 large T antigen regulatory region proceeded in a defined order and through intermediate substrate release. This kinase mechanism contrasts with a more intuitive mode of CK2 action in which the kinase would remain substrate bound to perform both modification reactions without intermediate substrate release. For cellular signaling pathways, the transient availability of partially modified CK2 substrates could exert important switch-like regulatory functions.