Distribution of casein mRNA between free and membrane-bound polysomes during induction of lactogenesis in the rabbit

Abstract

The presence of casein mRNA in free and membrane-bound polysomes was estimated during lactogenesis induced by prolactin injected into pseudopregnant rabbits and during the course of pregnancy. Casein mRNA was visualized by polyacrylamide gel electrophoresis after isolation by poly(U)-Sepharose chromatography and quantified by hybridization with its complementary DNA. Synthesis of casein by both kinds of polysomes was evaluated by translation in a cell-free system followed by immunoprecipitation of the casein synthesized. The presence of nascent casein chains in polysomes was determined by their capacity to bind I 125I] anti-casein. The ability of tissue to secrete casein was estimated by immunoprecipitation of the labelled casein in the medium during incubation of mammary explants with [ 14C] aminoacids. The concentration of casein mRNA was always higher in membrane-bound than in free polysomes. Prolactin increased simultaneously the concentration in both types of polysomes. In the lactating rabbit, about 95% and 5% of total polysomal casein mRNA was found in membrane-bound and in free polysomes respectively. In the pseudopregnant rabbit, before prolactin treatment, these values were 65% and 35%. Under the action of prolactin, this proportion was progressively shifted; after 4 days treatment, the balance observed at lactation was reached. The capacity of the tissue to secrete casein was induced more slowly than casein synthesis itself.