The Identities Of Membrane Steroid Receptors PDF Download

In addition to the well-characterized genomic mechanism of steroid action that uses intracellular receptors, steroid hormones also signal through nongenomic processes that use membrane receptors. A membrane receptor for corticosterone (CORT) has been described in brains of the roughskin newt (Taricha granulosa). This receptor is believed to be a G-protein coupled receptor because corticosterone binding is inhibited by guanyl nucleotides and enhanced by Mg2. The studies described in this thesis use biochemical, pharmacological and molecular techniques to characterize the newt neuronal membrane glucocorticoid receptor (mGR) in pursuit of its molecular identification. The mGR was successfully solubilized from newt neuronal membranes and conditions were defined that maintained corticosterone binding activity for further study. The solubilized receptor was partially purified using standard chromatographic techniques and an immobilized ligand affinity resin (CORT-Sepharose). These chromatographic studies were combined with the use of a novel photoaffinity ligand (azido-CORT) to biochemically characterize the mGR protein, finding that it is an acidic glycoprotein with an apparent molecular weight of 63 kDa and an isoelectric point of approximately 5.0. Pharmacological studies with mGR showed that a subset of kappa opioid ligands displaced corticosterone from the receptor binding site with K[subscript i] values in the nanomolar to low-micromolar range. The interaction of mGR with kappa opioid ligands was specific because no mu-, delta-, or orphanin-specific opioid ligands were effective at displacing corticosterone from the receptor. These data suggest that the newt neuronal mGR may be a kappa-opioid like receptor. Finally, molecular studies were used to clone a novel newt brain protein, neuronal axonal protein 22 (NAP-22), that was identified in a protein differential display strategy designed to identify mGR. Studies with the cloned and expressed NAP-22 protein suggest that it is not the mGR but, instead, may be a mGR-associated protein. These studies provided new information about the biochemical and pharmacological properties of mGR, and may have discovered a protein that is associated with the newt neuronal mGR.