Minimal activators that bind to the KIX domain of p300/CBP identified by phage display screening

Human gene therapy approaches involving transcription factors often rely on artificial activation domains for transcriptional activation. These domains are often large (e.g., 80 amino acids for VP16), recruit multiple co-activation complexes at once, and offer no fine control over the level of transcription. In an attempt to understand the sequence and structural requirements of a minimal mammalian activator, we employed a molecular diversity approach with a peptide phage display library composed of random eight-amino acid peptides. Using the KIX domain of the mammalian co-activators p300 and CBP as target, we discovered a family of synthetic binding peptides. These peptides share significant homology with natural KIX domain ligands, and are shown to bind an overlapping, yet distinct, surface of p300/CREB-binding protein (CBP). When fused to a heterologous DNA binding domain, these synthetic peptides function as titratable, modular, and potent transcriptional activators in living cells through specific recruitment of p300/CBP, with the level of transcriptional activation proportional to the affinity of the synthetic peptide for the KIX domain. Taken together, our data demonstrate that a molecular diversity approach can be used to discover minimal, co-activator domain-specific synthetic activators, and that transcriptional activation can be modulated as desired at the level of co-activator recruitment.

Progesterone, but not progesterone-independent activation of progestin receptors by a mating stimulus, rapidly decreases progestin receptor immunoreactivity in female rat brain

Recent studies suggest that progestin receptors may be activated in vivo by neurotransmitters in the absence of ligand. More specifically, vaginal-cervical stimulation (VCS) can influence sexual behavior by activating progestin receptors in the absence of progesterone. Another way to test if progestin receptors are influenced by particular stimuli is to examine progestin receptor immunostaining. We report that progestin receptor immunoreactivity is decreased in the forebrain of estradiol-primed ovariectomized (OVX) rats within 1 h after a subcutaneous injection of progesterone, a time by which rapid down-regulation of progestin receptors does not seem to have occurred. In estradiol-primed OVX rats, VCS also decreased progestin receptor immunoreactivity within 1 h in the medial preoptic area, but not in any other area examined. To determine if the decrease in immunoreactivity by VCS was due to adrenal secretions or by ligand-independent activation of progestin receptors, we repeated the experiment in estradiol-primed OVX/adrenalectomized rats. Prior removal of the adrenal glands blocked the rapid decrease in progestin receptor immunoreactivity, even though data from other experiments suggest that progestin receptors are activated by VCS at this time. These studies suggest the possibility that progestin receptors may be affected differentially by progesterone-dependent or by progesterone-independent pathways. This raises the possibility that activation of progestin receptors by these two distinct pathways may lead to different neuronal consequences.