The DNA-Binding and tau 2 Transactivation Domains of the Rat Glucocorticoid Receptor Constitute a Nuclear Matrix-Targeting Signal

Using an ATP-depletion paradigm to augment glucocorticoid receptor (GR) binding to the nuclear matrix, we have identified a minimal segment of the receptor that constitutes a nuclear matrix targeting signal (NMTS). While previous studies implicated a role for the receptor's DNA-binding domain in nuclear matrix targeting, we show here that this domain of rat GR is necessary, but not sufficient, for matrix targeting. A minimal NMTS can be generated by linking the rat GR DNA-binding domain to either its tau 2 transactivation domain in its natural context, or a heterologous transactivation domain derived from the Herpes simplex virus VP16 protein. The transactivation and nuclear matrix-targeting activities of tau 2 are separable, as transactivation mutants were identified that either inhibited or had no apparent effect on matrix targeting of tau 2. A functional interaction between the NMTS of rat GR and the RNA-binding nuclear matrix protein hnRNP U was revealed in cotransfection experiments in which hnRNP U overexpression was found to interfere with the transactivation activity of GR derivatives that possess nuclear matrix-binding capacity. We have therefore ascribed a novel function to a steroid hormone transactivation domain that could be an important component of the mechanism used by steroid hormone receptors to regulate genes in their native configuration within the nucleus.