The nuclear-associated endoplasmic reticulum

Efficient transcription through an intron requires the binding of an Sm-type U1 snRNP with intact stem loop II to the splice donor

The mechanism behind the positive action of introns upon transcription and the biological significance of this positive feedback remains unclear. Functional ablation of splice sites within an HIV-derived env cDNA significantly reduced transcription that was rescued by a U1 snRNA modified to bind to the mutated splice donor (SD). Using this model we further characterized both the U1 and pre-mRNA structural requirements for transcriptional enhancement. U1 snRNA rescued as a mature Sm-type snRNP with an intact stem loop II. Position and sequence context for U1-binding is crucial because a promoter proximal intron placed upstream of the mutated SD failed to rescue transcription. Furthermore, U1-rescue was independent of promoter and exon sequence and is partially replaced by the transcription elongation activator Tat, pointing to an intron-localized block in transcriptional elongation. Thus, transcriptional coupling of U1 snRNA binding to the SD may licence the polymerase for transcription through the intron.

Localization of interferon-induced lupus inclusions demonstrated by computer image reconstruction of monensin-treated Daudi cells

A structural analysis of cells that contained the interferon-alpha-induced lupus inclusions (LI) was performed using a high-voltage electron microscope to determine the exact cellular location of LI and their association with normal cell organelles. LI were induced in the human B lymphoblastoid cell line, Daudi, by culturing with the pure recombinant human leukocyte interferon, IFLrA. Just prior to harvesting, a portion of the cells was treated with monensin to selectively swell the Golgi apparatus, and thereby simplify their identification using the electron microscope. Organellar associations between LI and the outer nuclear envelope and Golgi apparatus were identified in stereopairs of 1-micron sections prepared from both cells that were not treated with monensin and those that were treated with monensin. Serial 0.25-micron sections of the monensin-treated cells were prepared, and seven arbitrarily chosen cells were examined. Each of these cells contained a single LI, and it formed throughout an endoplasmic-reticulum region that made contact with both the outer nuclear envelope and the Golgi vesicles. Reconstruction of a cell by computer from the digitized negatives of serial sections clearly illustrated these relationships. This study reports the first determination of the association between LI and the Golgi apparatus. It also identifies the presence of only one LI in every cell, and the routine association of the LI with both the outer nuclear envelope and the Golgi apparatus. The unique cell location of LI formation suggests their functioning in membrane biogenesis, the trafficking of proteins to the plasma membrane or to cytoplasmic vesicles, or the processing of proteins for secretion.

Interaction between mRNA, ribosomes and the cytoskeleton

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The antiproliferative potency of histamine antagonists correlates with inhibition of binding of [3H]-histamine to novel intracellular receptors (HIC) in microsomal and nuclear fractions of rat liver

Previously, we identified in rat liver microsomes, low (microM) affinity histamine receptors (HIC), associated with antiestrogen binding sites (AEBS). N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine HCl (DPPE), a potent AEBS ligand, is a specific HIC antagonist. Through binding HIC, newly-formed intracellular histamine mediates, and DPPE inhibits, human platelet aggregation. We now provide evidence that histamine, mobilized from cytoplasmic stores, is a mediator of the mitogenic response to concanavalin A in mouse spleen cells. DNA synthesis and intracellular histamine levels are decreased over time by the histidine decarboxylase inhibitor, alpha-fluoromethylhistidine. For DPPE, H1 and H2 antagonists, rank order of potency to inhibit [3H]-histamine binding to HIC in rat liver microsomes correlates with antiproliferative potency. DPPE also competes for [3H]-histamine binding at low and high affinity sites in rat liver nuclei (IC50 approximately 2 microM). Thus, histamine may mediate growth through two intracellular subtypes of HIC.