Journal of cell science. Supplement最新文献

NF-kappa B is a dimeric protein that serves to initiate gene transcription in higher eukaryotic cells in response to mainly pathogenic stimuli. Its activity is controlled by a third inhibitory subunit, called I kappa B. When I kappa B is bound, NF-kappa B cannot bind to DNA or enter the nucleus but is stored in a latent cytoplasmic form. Upon stimulation of cells I kappa B is released, which allows the activation of NF-kappa B. We have analyzed the molecular mechanism underlying the removal of I kappa B-alpha. Distinct extracellular stimuli lead to a phosphorylation of I kappa B-alpha of serines 32 and 36 by a yet unidentified kinase. These modifications do not directly dissociate I kappa B from NF-kappa B but render the inhibitor highly susceptible for proteolytic degradation by, presumably, the proteasome. In this paper, we report for the first time that higher molecular mass forms of I kappa B-alpha occur under conditions that lead to a phosphorylation of I kappa B-alpha and activation of NF-kappa B. These I kappa B-alpha variants had discrete molecular masses and were most prominent in cells overexpressing I kappa B-alpha, suggesting the covalent modification of I kappa B-alpha by ubiquitin conjugation. The proteasome inhibitor Cbz-Ile-Glu(O-t-Bu)-Ala-leucinal (PSI), which stabilizes the phospho form of I kappa B-alpha, only slightly increased the amount of conjugates indicating that the conjugation of I kappa B-alpha with ubiquitin was the rate-limiting step in I kappa B-alpha degradation, and not its phosphorylation or proteolysis. Our data suggest that conjugation of I kappa B-alpha with ubiquitin is an intermediate reaction in the phosphorylation-controlled degradation of I kappa B-alpha and the subsequent activation of NF-kappa B.

Myc is a nuclear phosphoprotein which controls cellular proliferation, most likely by regulating gene activity. The finding that the neuronal model cell line PC12 lacks the Myc DNA binding partner, the Max protein, and the demonstration that Myc is a repressor of gene activity as well as a transactivator, lead to models for Myc action in regulating cell growth.

There is increasing interest in studying how specific metabolic activities within the nucleus are organised into functional domains. The best known example is the nucleolus where rRNA genes are transcribed and rRNA processed and assembled into ribosomal units. Other subnuclear domains have been known for many years through morphological studies but are only recently being analysed at the molecular level. Here we focus on an evolutionarily conserved nuclear domain, called the coiled body, which contains splicing snRNPs. We review recent literature on the coiled body and discuss a possible model for its biological function.

Bidirectional molecular trafficking between the nucleus and the cytoplasm of eukaryotic cells occurs through the nuclear pore complexes (NPCs), approximately 120 megadalton supramolecular assemblies embedded in the double-membraned nuclear envelope. Significant progress has been made in elucidating the three-dimensional (3-D) architecture of the NPC, and in identifying, characterizing, and cloning and sequencing NPC proteins. Several of these have now been localized within the 3-D structure of the NPC. Nevertheless, there still remain major questions relating to the conformation, molecular composition and functional roles of distinct NPC components. Here we review recent structural studies from our group and others which have contributed toward dissecting the molecular architecture of the NPC. We also present our results on the molecular characterization of some NPC components, and on the elucidation of their functional roles in mediated nucleocytoplasmic transport.

The Wilms' tumour suppressor gene (WT1) encodes a protein(s) with 4 zinc fingers that is essential for the development of the genitourinary system. A considerable body of evidence exists to support the idea that WT1 binds DNA and functions as a transcription factor. However, we have shown recently by confocal microscopy and immunoprecipitation studies that a significant proportion of WT1 is associated with splice factors in kidney cell lines, fetal tissues and transfected Cos cells. Different isoforms of WT1 are produced by an alternative splice that leads to the presence or absence of a 3 amino acid insertion (KTS) between zinc fingers 3 and 4. We have shown that these different forms localise differently in the nucleus. The +KTS form mainly localises with splice factors, the -KTS form mainly with transcription factors. Here we propose a model to account for these different localisations. Also, we discuss the possible significance of these findings.

The removal of introns from precursor messenger RNAs occurs in a large complex, the spliceosome, that contains many proteins and five small nuclear RNAs (snRNAs). The snRNAs interact with the intron-containing substrate RNA and with each other to form a dynamic network of RNA interactions that define the intron and promote splicing. There is evidence that protein splicing factors play important roles in regulating RNA interactions in the spliceosome. PRP8 is a highly conserved protein that is associated in particles with the U5 snRNA and directly binds the substrate RNA in spliceosomes. UV crosslinking has been used to map the binding sites, and shows extensive interaction between PRP8 protein and the 5' exon prior to the first step of splicing and with the 3' splice site region subsequently. It is proposed that PRP8 protein may stabilize fragile interactions between the U5 snRNA and exon sequences at the splice sites, to anchor and align them in the catalytic centre of the spliceosome.

The 5' cap structure of RNA polymerase II transcripts and the poly(A) tail found at the 3' end of most mRNAs have been demonstrated to play multiple roles in gene expression and its regulation. In the first part of this review we will concentrate on the role played by the cap in pre-mRNA splicing and how it may contribute to efficient and specific substrate recognition. In the second half, we will discuss the roles that polyadenylation has been demonstrated to play in RNA metabolism and will concentrate in particular on an elegant mechanism where regulation of polyadenylation is used to control gene expression.

The E2F1 transcription factor, in co-operation with DP1, controls the expression of several S-phase specific genes. This activity is most likely responsible for the oncogenic and S-phase inducing properties of E2F1, suggesting that this transcription factor plays a key role in regulating the cell cycle. The transcriptional activation functions of E2F1 are resident in a small C-terminal domain which can independently activate transcription. Here we review the protein-protein interactions which impinge upon and regulate this activation domain and put forward some models on their mechanism of action.

The multiple origins of eukaryotic chromosomes vary in the time of their initiation during S phase. In the chromosomes of Saccharomyces cerevisiae the presence of a functional telomere causes nearby origins to delay initiation until the second half of S phase. The key feature of telomeres that causes the replication delay is the telomeric sequence (C(1-3)A/G(1-3)T) itself and not the proximity of the origin to a DNA end. A second group of late replicating origins has been found at an internal position on chromosome XIV. Four origins, spanning approximately 140 kb, initiate replication in the second half of S phase. At least two of these internal origins maintain their late replication time on circular plasmids. Each of these origins can be separated into two functional elements: those sequences that provide origin function and those that impose late activation. Because the assay for determining replication time is costly and laborious, it has not been possible to analyze in detail these 'late' elements. We report here the development of two new assays for determining replication time. The first exploits the expression of the Escherichia coli dam methylase in yeast and the characteristic period of hemimethylation that transiently follows the passage of a replication fork. The second uses quantitative hybridization to detect two-fold differences in the amount of specific restriction fragments as a function of progress through S phase. The novel aspect of this assay is the creation in vivo of a non-replicating DNA sequence by site-specific pop-out recombination. This non-replicating fragment acts as an internal control for copy number within and between samples. Both of these techniques are rapid and much less costly than the more conventional density transfer experiments that require CsCl gradients to detect replicated DNA. With these techniques it should be possible to identify the sequences responsible for late initiation, to search for other late replicating regions in the genome, and to begin to analyze the effect that altering the temporal program has on chromosome function.

DNA replication is a pivotal event in the cell cycle and, as a consequence, is tightly controlled in eukaryotic cells. The initiation of DNA replication is dependent upon the completion of mitosis and upon the commitment to complete the cell cycle made during G(1). Characterisation of the protein factors required for initiating DNA replication is essential to understand how the cell cycle is regulated. Recent results indicate that initiation complexes assemble in multiple stages during the cell cycle. First, origins are bound by the multisubunit origin recognition complex (ORC) which is essential for DNA replication in vivo. ORC, present at little more than one complete complex per replication origin, binds to origins immediately after initiation in the previous cell cycle. ORC binding occurs by the recognition of a bipartite sequence that includes the essential ARS consensus sequence (ACS) and the functionally important B1 element adjacent to the ACS. A novel pre-replicative complex (pre-RC) assembles at origins at the end of mitosis in actively cycling cells and remains at origins until DNA replication initiates. Finally, Dbf4, which is periodically synthesised at the end of G(1), interacts with replication origins. Dbf4-origin interaction requires an intact ACS strongly suggesting that interaction occurs through ORC. Dbf4 interacts with and is required for the activation of the Cdc7 protein kinase and together, Dbf4 and Cdc7 are required for the G(1)-S transition. Separate regions of Dbf4 are required for Cdc7- and origin-interaction suggesting that Dbf4 may act to recruit Cdc7 to replication origins where phosphorylation of some key component may cause origin firing.