Biomedical subjects

A Moustakas

Publications and source records attributed to A Moustakas.

25 records · Page 2Linked to original sources

PubMed1994

The types II and III transforming growth factor-beta receptors form homo-oligomers.

Affinity-labeling experiments have detected hetero-oligomers of the types I, II, and III transforming growth factor beta (TGF-beta) receptors which mediate intracellular signaling by TGF-beta, but the oligomeric state of the individual receptor types remains unknown. Here we use two types of experiments to show that a major portion of the receptor types II and III forms homo-oligomers both in the absence and presence of TGF-beta. Both experiments used COS-7 cells co-transfected with combinations of these receptors carrying different epitope tags at their extracellular termini. In immunoprecipitation experiments, radiolabeled TGF-beta was bound and cross-linked to cells co-expressing two differently tagged type II receptors. Sequential immunoprecipitations using anti-epitope monoclonal antibodies showed that type II TGF-beta receptors form homo-oligomers. In cells co-expressing epitope-tagged types II and III receptors, a low level of co-precipitation of the ligand-labeled receptors was observed, indicating that some hetero-oligomers of the types II and III receptors exist in the presence of ligand. Antibody-mediated cross-linking studies based on double-labeling immunofluorescence explored co-patching of the receptors at the cell surface on live cells. In cells co-expressing two differently tagged type II receptors or two differently tagged type III receptors, forcing one receptor into micropatches by IgG induced co-patching of the receptor carrying the other tag, labeled by noncross-linking monovalent Fab'. These studies showed that homo-oligomers of the types II and III receptors exist on the cell surface in the absence or presence of TGF-beta 1 or -beta 2. In cells co-expressing types II and III receptors, the amount of heterocomplexes at the cell surface was too low to be detected in the immunofluorescence co-patching experiments, confirming that hetero-oligomers of the types II and III receptors are minor and probably transient species.

Affinity Labels↗

PubMed1994

TGF-beta receptors: structure and function.

The transforming growth factor beta (TGF-beta) superfamily of dimeric polypeptide growth factors regulate cell growth and differentiation and play important roles in embryonic development and the immune response. The molecular cloning of three types of high affinity cell-surface receptors (type I, type II and type III) for these growth factors provide insights into their poorly understood mechanisms of action. Here we focus on the structure of these receptors and on the complex interactions which occur between different receptors that are necessary for ligand-induced signal transduction. The type I and type II receptors are both related transmembrane serine/threonine kinase receptors which may hetero-oligomerize to transduce proper signals. The type III receptor modulates the binding of ligands to the signaling complex of type I and type II receptors. These findings suggest that serine/threonine phosphorylation and heteromeric receptor interactions are important elements of TGF-beta induced signaling.

Animals↗

PubMed1993

The transforming growth factor beta receptors types I, II, and III form hetero-oligomeric complexes in the presence of ligand.

Transforming growth factors beta (TGF-beta s) are disulfide-linked dimers. In Rat-1 cells both radioiodinated TGF-beta 1 and -beta 2 bind to and can be chemically cross-linked to type I and II receptors (which are thought to mediate effects of cell growth suppression and gene activation), to type III proteoglycan receptors, and to a novel approximately 50-kDa protein. After detergent solubilization of cells that were cross-linked with radioiodinated TGF-beta, antibodies specific for the type II receptor precipitated labeled receptor types I and III as well as type II. In these cells, the type III receptor is the predominant TGF-beta-binding protein, and antibodies specific for it precipitate mainly this cross-linked receptor. Thus, in the presence of TGF-beta ligand, receptor types II and III and types II and I form heteromeric complexes. The majority of the type III receptor does not associate with receptor types I and II, probably reflecting the relative amounts of the three receptors on the surface of Rat-1 cells. Since TGF-beta 1 but not TGF-beta 2 binds to the exoplasmic domain of the type II receptor in the absence of the type III receptor, and since both TGF-beta 1 and -beta 2 bind with high affinity to the type III receptor, we suggest that TGF-beta 2, and possibly TGF-beta 1, bind initially to the type III receptor. The TGF-beta 2-type III receptor complex would then interact with a type II receptor, thus modulating the affinity of the type II receptor for TGF-beta 2.

Amino Acid Sequence↗

PubMed1993

Growth inhibition by transforming growth factor beta (TGF-beta) type I is restored in TGF-beta-resistant hepatoma cells after expression of TGF-beta receptor type II cDNA.

The growth of human hepatoma Hep 3B cells is potently inhibited by TGF-beta 1 (ID50 = 0.2 ng/ml, 8 pM). A mutant cell line was derived that was not inhibited in growth by TGF-beta 1 at 5 ng/ml (200 pM) and that lacked TGF-beta receptor type II (TGF-beta RII) gene. Transfection of the cloned cDNA for human TGF-beta RII to this mutant cell line restored receptor expression as well as the inhibition in growth by TGF-beta 1. In both wild-type and mutant cells stably transfected with TGF-beta RII cDNA, TGF-beta RII coimmunoprecipitated with TGF-beta receptor type I in the presence of ligand. These experiments provide direct evidence for the role of TGF-beta RII in the inhibitory effect of TGF-beta on growth and suggest that TGF-beta RII acts by means of a heteromeric surface complex with TGF-beta receptor type I.

Affinity Labels↗

PubMed1993

Alterations of the three short open reading frames in the Rous sarcoma virus leader RNA modulate viral replication and gene expression.

The Rous sarcoma virus (RSV) leader RNA has three short open reading frames (ORF1 to ORF3) which are conserved in all avian sarcoma-leukosis retroviruses. Effects on virus propagation were determined following three types of alterations in the ORFs: (i) replacement of AUG initiation codons in order to prohibit ORF translation, (ii) alterations of the codon context around the AUG initiation codon to enhance translation of the normally silent ORF3, and (iii) elongation of the ORF coding sequences. Mutagenesis of the AUG codons for ORF1 and ORF2 (AUG1 and AUG2) singly or together delayed the onset of viral replication and cell transformation. In contrast, mutagenesis of AUG3 almost completely suppressed these viral activities. Mutagenesis of ORF3 to enhance its translation inhibited viral propagation. When the mutant ORF3 included an additional frameshift mutation which extended the ORF beyond the initiation site for the gag, gag-pol, and env proteins, host cells were initially transformed but died soon thereafter. Elongation of ORF1 from 7 to 62 codons led to the accumulation of transformation-defective virus with a delayed onset of replication. In contrast, viruses with elongation of ORF1 from 7 to 30 codons, ORF2 from 16 to 48 codons, or ORF3 from 9 to 64 codons, without any alterations in the AUG context, exhibited wild-type phenotypes. These results are consistent with a model that translation of the ORFs is necessary to facilitate virus production.

Animals↗

PubMed1993

Effects of the open reading frames in the Rous sarcoma virus leader RNA on translation.

Three short open reading frames (ORFs) reside in the 5' leader of Rous sarcoma virus (RSV) and are conserved in all avian sarcoma-leukosis retroviruses. Both extensions of the lengths of the ORFs and alterations in their initiation codons affect viral replication and gene expression. To determine whether the effects on viral replication were due to translational regulation mediated by the ORFs, we examined translation following mutation of the initiation and termination codons of each of the three ORFs. We found that the ORFs marginally enhanced downstream gene expression. Moreover, repression of downstream gene translation was proportional to the lengths of the elongated ORFs and depended on the initiation contexts of the AUG codons. Although the ORFs play a major role in viral activities, their effects on translation were relatively minor. Rather, the ORFs may affect the fate of unspliced avian retroviral RNA in chronically infected cells by participating in the sorting of viral RNA for either translation or encapsidation into virions.

Animals↗

PubMed1989

A mutation in the short 5'-proximal open reading frame on Rous sarcoma virus RNA alters virus production.

The 5'-proximal open reading frame on Rous sarcoma virus RNA encodes a seven-amino-acid peptide and is conserved in all avian sarcoma-leukosis retroviruses. Ribosome-binding site analysis in intact chick cells showed that the 5'-proximal AUG codon is a strong site for initiation of translation in vivo. Removal of the 5'-proximal AUG codon by site-specific mutagenesis resulted in a virus with a reduced ability either to replicate or to transform a population of chicken embryo fibroblasts. These results establish a procedure for determining sites of ribosome binding and initiation of translation on mRNAs in intact eucaryotic cells and strongly suggest that the 5'-proximal open reading frame (or its AUG codon) on Rous sarcoma virus RNA has an important role in regulating viral activity.

Animals↗