Binding of activated progesterone receptor to microsomes

Area of Science:

• Biochemistry
• Molecular Biology
• Endocrinology

Background:

• Specific binding of steroid hormones to cellular microsomes is a known phenomenon in various tissues.
• The hen oviduct is a key tissue for studying steroid hormone action and receptor dynamics.

Purpose of the Study:

• To characterize the steroid hormone receptor found in hen oviduct microsomes.
• To compare the properties of the microsomal receptor with the activated cytosolic receptor.
• To investigate the mechanism and nature of microsomal binding of the cytosolic receptor.

Main Methods:

• Solubilization of microsomal receptors.
• Physicochemical characterization including sedimentation analysis and Stokes radius determination.
• Assessment of dissociation rates and binding to polyanions.
• Immunoprecipitation using antibodies against the cytosolic receptor.
• Saturation binding assays to determine Bmax.
• RNase treatment to assess the role of RNA.

Main Results:

• The hen oviduct microsomal receptor exhibits properties similar to the activated cytosolic receptor (e.g., 4 S sedimentation, 5.5 nm Stokes radius, slow dissociation, polyanion adsorption).
• The microsomal receptor is immunoprecipitated by an antibody against the cytosolic receptor.
• Microsomes demonstrate saturable binding of the cytosolic receptor (Bmax ~300 fmol/mg protein), even in non-target tissues.
• RNase treatment significantly decreases this microsomal binding.

Conclusions:

• The activated cytosolic steroid hormone receptor binds to microsomes with high affinity.
• Microsomal binding is likely mediated by an interaction with RNA, suggesting RNA's role in receptor localization or function.