The ability of p53 to function as a transcription factor is instrumental in facilitating the response to cellular stress, and p300/CBP proteins, which act as coactivators for diverse transcription factors, participate in regulating p53 activity. We report a novel cofactor for p300 that facilitates the p53 response by augmenting p53-dependent transcription and apoptosis. JMY and p300 associate in physiological conditions, and, during the cellular stress response, the p300/JMY complex is recruited to activated p53. The bax gene is efficiently activated by JMY, and protein isoforms that arise through alternative splicing alter the functional outcome of the p53 response. The results provide compelling evidence that the p300/JMY coactivator complex plays a central role in facilitating the p53 response.

The p300/CREB-binding protein (CBP) family of proteins consists of coactivators that influence the activity of a wide variety of transcription factors. Although the mechanisms that allow p300/CBP proteins to achieve transcriptional control are not clear, it is believed that the regulation of chromatin is an important aspect of the process. Here, we describe a new level of p300-dependent control mediated through the functional interaction between p300/CBP and members of the family of nucleosome assembly proteins (NAP), which includes NAP1, NAP2, and TAF1. We find that NAP proteins, which have previously been implicated in the regulation of transcription factor binding to chromatin, augment the activity of different p300 targets, including p53 and E2F, through a process that is likely to involve the physical interaction between p300 and NAP. NAP proteins can form oligomers, and the results show that NAP proteins can bind to both core histones and p300 coactivator proteins, perhaps in a multicomponent ternary complex. We also provide data in support of the idea that histones can influence the interaction between p300 and NAP protein. These results argue that NAP is a functionally important component of the p300 coactivator complex and suggest that NAP may serve as a point of integration between transcriptional coactivators and chromatin.

Studies in tissue culture cells have implicated p300 and CBP acetyltransferases in myogenic regulatory factor (MRF) mediated transcription and terminal differentiation of skeletal muscle cells. However, in vivo data placing p300 and CBP on myogenic differentiation pathways are not yet available. In this report we provide genetic evidence that p300 but not CBP acetyltransferase (AT) activity is required for myogenesis in the mouse and in embryonic stem (ES) cells. A fraction of embryos carrying a single p300 AT- deficient allele exhibit impaired MRF expression, delayed terminal differentiation and a reduced muscle mass. In mouse embryos lacking p300 protein, Myf-5 induction is severely attenuated. Similarly, ES cells homozygous for a p300 AT or a p300 null mutation fail to activate Myf5 and MyoD transcription efficiently, while Pax3, acting genetically upstream of these MRFs, is expressed. In contrast, ES cells lacking CBP AT activity express MyoD and Myf5 and undergo myogenic differentiation. These data reveal a specific requirement for p300 and its AT activity in the induction of MRF gene expression and myogenic cell fate determination in vivo.

p300 and CBP are large nuclear acetyltransferases exhibiting a complex multi‐domain structure. Mouse embryos nullizygous for either p300 or Cbp die at midgestation, while heterozygotes are viable but in part display defects in neurulation or bone morphogenesis. To directly examine the contribution of the acetyltransferase (AT) activity to mouse development, we have abrogated this function by a knock‐in approach. Remarkably, a single AT‐deficient allele of p300 or Cbp leads to embryonic or neonatal lethality, indicating that the mutant alleles are dominant. Formation of the cardiovascular system, the lung and the small intestine are strongly impaired in p300 AT and to a much lesser extent in Cbp AT mutant embryos, a difference that is also reflected by the defects in gene expression. Embryonic stem cells homozygous for either the p300 AT or a p300 null mutation respond differently to BMP2 stimulation, indicating that the two alleles are not equivalent. Unexpectedly, the p300 AT‐mutant cells upregulate BMP‐inducible genes to levels similar or even higher than observed in wild‐type cells.

NALP5 appears to be a tissue-specific autoantigen involved in hypoparathyroidism in patients with APS-1. Autoantibodies against NALP5 appear to be highly specific and may be diagnostic for this prominent component of APS-1.

The pRb tumour suppressor protein is an essential component of the cell-cycle clock, integrating both positive and negative signals for cellular growth and proliferation with the transcription machinery. pRb exerts its tumour suppression function by both antagonizing and synergizing with downstream effectors, such as E2F. pRb has two modes of action, it can inactivate E2F transcription activity or it can assemble an active repression complex with E2F. Apart from E2F, pRb interacts with various factors to promote cellular differentiation. The differentiation properties of pRb are likely to contribute partly to its tumour suppressor function. It is also clear that pRb is a master regulator for transcription. It can both activate and repress transcription in a context-dependent manner. pRb interacts directly with histone acetyltransferase, histone deacetylases and SWI/SNF proteins, all of which are classes of proteins involved in chromatin remodelling. Last, but not least, pRb regulates transcription driven by all three polymerases, thereby integrating the cell-cycle clock with the biosynthetic capacity of the cell in controlling cellular proliferation and growth.

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It has been proposed that the decline in protein synthesis observed in aging organisms may result from a decrease in elongation factor EF-la. Transgenic Drosophila melanogaster flies carrying an additional copy of the EF-la gene under control of a heat-inducible promoter have an extended lifespan, further indicating that the EF-la gene may play an important role in determining longevity. To test this hypothesis, we have quantitated EF-1amRNA, EF-la protein, and the EF-la complex-formation activity in these transgenic flies. Furthermore, we have tested whether the transgene construct is functiona-i.e., whether transgenic mRNA is induced when flies are grown at higher temperature. The results show that although there is a clear difference in mean lifespan between the EF-la transgenic (E) flies and the control transgenic (C) flies, E flies do not express more EF-la protein or mRNA than C flies kept at the same experimental conditions. Although the transgene can be induced when E flies are heat-shocked at 3rC, transgenic mRNA is not detectable in E flies aged at 29°C. In both lines, the loss in catalytic activity with age is the same. We conclude that the E flies examined here do not live longer because of overexpressing the EF-1a gene.scription of the EF-la gene. The eukaryotic polypeptidechain-elongation factor EF-la is a GTP-binding protein that catalyzes the binding of aminoacyl-tRNA to the ribosome (13). Based on Webster's experiments, Shepherd et al. (14) addressed the question of whether expression of the EF-la gene may be directly involved in determining the lifespan of the fruit fly. They transformed Drosophila with a P-element vector containing a cDNA copy of the EF-la gene under control of the inducible hsp7O (70-kDa heat shock protein) gene promoter. At 250C the flies carrying the EF-la P element (E flies) lived significantly longer than the flies carrying a control P element (C flies). This difference in mean lifespan was increased when flies were kept at 29.50C. The authors concluded that the heat-induced overexpression of the EF-la gene led to increased lifespan of the flies and thus that there might be a positive correlation between the expression of the EF-la gene and longevity. Their results suggested that an important aging control might be exerted at the level of transcription of the EF-la gene. We have tested this hypothesis by measuring the expression of EF-la in these transgenic E and C flies both at the mRNA and at the protein level. The lifespan of each species, including man, is different, but genetically defined. According to the program theory of aging, the longevity of animal species, as well as the lifespan of their cells in culture, is determined by genetically controlled processes similar to those that control early development. Several attempts have been made to demonstrate the existence oflongevity-determining genes (1-5), but nothing is known about possible targets for such genes at the cellular or at the molecular level. Most higher organisms contain mitotic as well as p...