Ecdysteroid receptors of the blowfly Calliphora vicina

Abstract

Ecdysteroids, the molting hormones of arthropods, act like vertebrate steroid hormones by binding to an intracellular receptor protein. We have recently isolated a protein from nuclei of blowfly larvae which has satisfied the requirements of an ecdysteroid receptor. The receptor was partially purified and its ecdysteroid-binding properties were characterized. The availability of receptor preparations which have been stabilized by partial purification now enables us to study the general DNA-binding properties of ecdysteroid receptors.DNA-binding characteristics of ecdysteroid receptors were studied with calf thymus DNA. Affinity for DNA was observed both in the presence and in the absence of steroid ligand but the ligand clearly enhanced binding of receptors to DNA. Receptor preparations contained a heterogeneous mixture of receptors; up to 25% of DNA-binding receptors, and nonbinding forms of ecdysteroid receptors. The ability to bind to DNA was subject to inactivation which was not affected by partial purification, but which could be decelerated by dilution of the receptor preparation. Thus, dilution resulted in a spurious activation of DNA binding. A genuine activation, which would have led to an increase in the percentage of the DNA-binding form of the ecdysteroid receptor complex, was not observed.