Some transcription factors contain stretches of polyglutamine encoded by repeats of the trinucleotide CAG. Expansion of the CAG repeat in the androgen receptor (AR) has been correlated with the incidence and severity of X-linked spinal and bulbar muscular atrophy (Kennedy's disease). In order to understand the relationship of this mutation to AR function, we constructed ARs that varied in the position and size of the polyglutamine tract, and assayed for the abilities of these mutant receptors to bind androgen and to activate transcription of several different AR-responsive reporter genes. Elimination of the tract in both human and rat AR resulted in elevated transcriptional activation activity, strongly suggesting that the presence of the polyglutamine tract is inhibitory to transactivation. Progressive expansion of the CAG repeat in human AR caused a linear decrease of transactivation function. Importantly, expansion of the tract did not completely eliminate AR activity. We postulate that this residual AR activity may be sufficient for development of male primary and secondary sex characteristics, but may fall below a threshold level of activity necessary for normal maintenance of motor neuron function. This functional abnormality may be representative of other genetic diseases that are associated with CAG expansion mutations in open reading frames, such as spinocerebellar ataxia type I and Huntington's disease.
BCL-2 is a 26-kDa integral membrane protein that represses apoptosis by an unknown menism. Recent findings indicate that Ca+ release from the endoplasmic reticulum (ER) mediates apoptosis in mouse lymphoma cells. In view of growing evidence that BCL-2 localizes to the ER, as well as mitochondria and the perinuclear membrane, we investigated the possibility that BCL-2 represses apoptosis by regulating CaW+ fluxes through the ER membrane. A cDNA encoding BCL-2 was introduced into WEHI7.2 cells and two subclones, W.Hb12 and W.Hbl3, which express high and low levels ofBCL-2 mRNA and protein, respectively, were isolated.
Glucocorticoids, a class of steroid hormones, associate specifically with intracellular receptors, facilitating a conformational change that converts the receptor in vitro to a DNA-binding protein and in vivo to a nuclear species that activates a class of transcriptional enhancers termed glucocorticoid response elements (GREs). The DNA sequences recognized specifically by the hormone-receptor complex correspond directly to those required for GRE enhancement. The structural transition that accompanies steroid binding, 'receptor transformation', has been monitored by changes in receptor chromatographic properties, accessibility to monoclonal antibodies, association with other receptor subunits or with heterologous proteins, and aqueous two-phase partition coefficient. However, the significance of the structural change for the biological activity of the receptor is not understood. We have used cloned rat glucocorticoid-receptor coding sequences to produce and characterize a novel class of receptor mutants that elicit GRE enhancer function in transfected cells even in the absence of hormone. The constitutive activity of those receptor derivatives, together with mapping studies that distinguish between the DNA- and hormone-binding domains of the receptor, imply that the conformational change corresponding to receptor transformation may simply unmask pre-existing functional domains for DNA binding, enhancer activation, or both.
The glucocorticoid receptor protein, in association with cognate hormonal ligands, binds with high affinity to specific DNA sequences termed glucocorticoid response elements (GREs) which can function as hormone-dependent transcriptional enhancers; thus, the receptor is a regulable enhancer-activating protein. We have constructed cell lines expressing different levels of glucocorticoid receptor, and demonstrate that the extent of a structural alteration in the chromatin at a characterized GRE, as well as the magnitude of several transcriptional responses elicited by the receptor, are roughly proportional to the number of receptor molecules per cell. Thus, for three independent glucocorticoid-responsive transcription units examined in our HTC-derived cell lines, the receptor appears to be a primary regulatory factor. Moreover, the results suggest that other cellular factors required for the assembly and function of GREs and transcription initiation complexes must be produced in excess relative to their levels of utilization at normal receptor concentrations.
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