Biomedical subjects

R B Lanz

Publications and source records attributed to R B Lanz.

6 recordsLinked to original sources

PubMed1999

A steroid receptor coactivator, SRA, functions as an RNA and is present in an SRC-1 complex.

Nuclear receptors play critical roles in the regulation of eukaryotic gene expression. We report the isolation and functional characterization of a novel transcriptional coactivator, termed steroid receptor RNA activator (SRA). SRA is selective for steroid hormone receptors and mediates transactivation via their amino-terminal activation function. We provide functional and mechanistic evidence that SRA acts as an RNA transcript; transfected SRA, unlike other steroid receptor coregulators, functions in the presence of cycloheximide, and SRA mutants containing multiple translational stop signals retain their ability to activate steroid receptor-dependent gene expression. Biochemical fractionation shows that SRA exists in distinct ribonucleoprotein complexes, one of which contains the nuclear receptor coactivator steroid receptor coactivator 1. We suggest that SRA may act to confer functional specificity upon multiprotein complexes recruited by liganded receptors during transcriptional activation.

Animals↗

PubMed1999

Nuclear receptor coregulators: cellular and molecular biology.

Nuclear receptor coregulators are coactivators or corepressors that are required by nuclear receptors for efficient transcripitonal regulation. In this context, we define coactivators, broadly, as molecules that interact with nuclear receptors and enhance their transactivation. Analogously, we refer to nuclear receptor corepressors as factors that interact with nuclear receptors and lower the transcription rate at their target genes. Most coregulators are, by definition, rate limiting for nuclear receptor activation and repression, but do not significantly alter basal transcription. Recent data have indicated multiple modes of action of coregulators, including direct interactions with basal transcription factors and covalent modification of histones and other proteins. Reflecting this functional diversity, many coregulators exist in distinct steady state precomplexes, which are thought to associate in promoter-specific configurations. In addition, these factors may function as molecular gates to enable integration of diverse signal transduction pathways at nuclear receptor-regulated promoters. This review will summarize selected aspects of our current knowledge of the cellular and molecular biology of nuclear receptor coregulators.

Animals↗

PubMed1995

A transcriptional repressor obtained by alternative translation of a trinucleotide repeat.

Triplet nucleotide repeats are ubiquitous and rapidly evolving sequences in eucaryotic genomes. They are sporadically found in coding regions of transcription regulators where they become translated in different homopolymeric aminoacid (HPAA) stretches, depending on the local frame. Poly(CAG) yields three different HPAAs (poly Gln, Ser or Ala). Current sequence databases indicate a clear bias in the size and frequency of these HPAAs according to the rule: (Gln)n > (Ser)n >> (Ala)n. Aiming to understand the reasons of this bias, we changed the translational reading frame of the highly polymorphic CAG-repeat that normally encodes poly-Gln in the N-terminal portion of the rat glucocorticoid receptor (GR). The GR mutant in which the CAG repeat is translated to poly-Ala (called GR[Ala]) is incapable of transactivation, but maintains competence for hormone binding, nuclear translocation and specific DNA binding. We show that GR desactivation is obtained only when a very precise threshold length of the repeat is reached. GR[Ala] displays a strong negative dominance when tested for transcriptional activation in vivo and may become useful for selective competition of receptor dependent activities in tissue culture cells and transgenic animals. We discuss the implications of our findings for the understanding of the evolutionary behaviour of trinucleotide repeats in coding sequences.

Amino Acid Sequence↗

PubMed1994

Active, interactive, and inactive steroid receptor mutants.

In transient co-transfection assays, there is extensive cross-interaction between glucocorticoid receptor (GR) domains. For example, mutation of the conserved Ile residue at position 484 (rat GR map) to cysteine allows a net separation of transactivation and DNA binding. We also observed that the ligand binding domain plays a key role in cooperative transactivation. Furthermore, some carboxy-located mutations markedly alter the response of GR to agonists and antagonists. Finally, different reading frames of the CAG repeat that normally produces an amino-located poly-Gln repeat profoundly affect GR transactivation without altering DNA or ligand binding. This trans-dominant negative phenotype, seen when the CAG repeat yields a poly-Ala stretch, may turn out to be an excellent tool for functional analysis of GR in transgenic organisms.

Amino Acid Sequence↗

PubMed1994

A conserved carboxy-terminal subdomain is important for ligand interpretation and transactivation by nuclear receptors.

Nuclear receptors share a highly conserved region located at the very carboxy-terminal part of the ligand-binding domain. Site-directed mutagenesis of conserved hydrophobic residues in this region was reported to create mouse and human glucocorticoid receptors (GRs) and estrogen receptors that cannot transactivate but apparently maintain all the other functions. We constructed analogous mutations in the rat GR to compare the mechanism of deficiency to our recently generated trans-dominant-negative mutant. We found that in the rat GR these carboxy-terminal mutations do not generate trans-dominant-negative receptors. We show that these GR mutants fail to bind dexamethasone properly, and hence receptor transformation and subsequent functions are abolished. Furthermore, we report the identification of a GR mutant that is strongly responsive to the antagonist RU486 but is silent with the agonist dexamethasone. We demonstrate that the reversal of the responsiveness is restricted to GR, since analogous mutations in related receptors do not generate similar phenotypes. Contrary to the case of the progesterone receptor, we show that carboxyl-end truncated GR mutants are not activated by RU486. We conclude that sequence conservation of this subdomain does not necessarily imply functional conservation. Chimeric constructs with GAL4 revealed the importance of protein-protein interactions to exert ligand discrimination, which is mediated by the carboxy-terminal subdomain.

Alanine↗

PubMed1989

A homologue of the axonally secreted protein axonin-1 is an integral membrane protein of nerve fiber tracts involved in neurite fasciculation.

Axonin-1 is a glycoprotein that is released from axons of cultured neurons (Stoeckli, E. T., P. F. Lemkin, T. B. Kuhn, M. A. Ruegg, M. Heller, and P. Sonderegger. 1989. Eur. J. Biochem. 180:249-258). It has recently been purified from the ocular vitreous fluid of the chicken embryo (Ruegg, M. A., E. T. Stoeckli, T. B. Kuhn, M. Heller, R. Zuellig, and P. Sonderegger. 1989. EMBO (Eur. Mol. Biol. Organ.) J. 8:55-63). Immunohistochemistry localized axonin-1 prevalently in developing nerve fiber tracts. The presence of anti-axonin-1 Fab fragments during axon growth in vitro resulted in antibody binding to the axonal surfaces and in a marked perturbation of the fasciculation pattern. Hence, a fraction of axonin-1 is associated with axonal membranes and, by operational criteria, qualifies as a cell adhesion molecule. The major proportion of membrane-associated axonin-1 co-solubilized with the integral membrane proteins. By physico-chemical, immunological, and protein-chemical criteria, the integral membrane form was found to be highly similar to soluble axonin-1. In common with a number of other cell adhesion molecules, both soluble and membrane-bound axonin-1 express the L2/HNK-1 and the L3 epitopes. Radioactive pulse-chase and double-labeling experiments revealed that the released form was not derived from the membrane-bound form by shedding from the membrane surface, but directly secreted from an intracellular pool. Due to its high degree of similarity to the membrane-associated form and the presence of the L2/HNK-1 and L3 epitopes, reported to be ligands in adhesive cell interactions, adhesive properties are postulated for secreted axonin-1. As a soluble adhesive protein, it may function as a regulator of cell adhesion around its most likely site of secretion, the growth cone.

Animals↗