Solubilization and Molecular Weight Estimation of Prolactin Receptors from Rana catesbeiana Tadpole Liver and Tail Fin*

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Abstract
Membrane receptors from Rana catesbeiana tadpole tissues and female Sprague-Dawley rat livers were solubilized, and their molecular weights were determined by disc gel electrophoresis in 0.1% Triton χ-100 polyacrylamide gels of various acrylamide concentrations (4–8%). Electrophoresis of membrane proteins solubilized in 1.0% Triton χ100 resulted in a single major band at all gel concentrations that demonstrated specific PRL-binding activity. A second peak of radioactivity, corresonding to unbound labeled PRL, was distinct from the PRL-binding activity at all gel concentrations. Solubilization with the nonionic detergent did not alter the PRL-binding molecule, as indicated by identical mobilities of the PRL-bound receptor complex from membranes incubated with PRL before solubilization and of receptors incubated with PRL after solubilization at all gel concentrations studied. The ability of the PRL receptor to specifically bind to PRL was also preserved in Triton χ100 extracts of membranes. Radius and molecular weight determinations from the calculated slopes of the log of the relative mobilities of the solubilized binding components at various gel concentrations (Ferguson plots) indicated that the PRL-binding component of the rat liver (170,000 daltons) is almost twice the size of the PRL-receptor complex from either of the tadpole tissues, liver (114,000 daltons) and tail fin (103,000 daltons). However, the charges of the PRL receptors differ for each tissue. These data indicate that the physical characteristics of the solubilized PRL receptors differ for the amphibian and mammalian tissues. Furthermore, although no apparent difference in molecular weight is noted for the PRL-receptor complexes isolated from the two amphibian tissues, a significant difference in charge is evident. This difference in charge, along with previous reports of other differences, may indicate that the tadpole tail fin and liver membrane PRL receptors are different.