A novel action of insulin on phosphoprotein formation by mammary gland explants.

Studies on the induction of casein synthesis in competent daughter cells formed in mammary gland explants have revealed a new action of insulin. Treatment with prolactin, in the absence of insulin, is responsible for the formation of some polypeptides which are destined to become phosphoproteins, or for the formation of a unique protein-kinase, or both. In any case, insulin is required for participation of the prolactin-induced factor(s) in a protein-kinase reaction.