Identification and characterization of a biosynthetic precursor (preprolactin) of ovine pituitary prolactin

Poly(A)-containing mRNA prepared from ovine anterior pituitary glands stimulated the incorporation of radioactivity into protein in a wheat germ cell-free system from 7- to 25-fold over background. Approximately 30% of the total protein synthesized could be precipitated by antiserum to prolactin, indicating that a molecule which shares antigenic determinants with prolactin was being synthesized. Analysis of the immunoprecipitable cell-free translation product, by electrophoresis on polyacrylamide gels in the presence of sodium dodecyl sulphate demonstrated the presence of a prolactin-like molecule with a molecular weight greater than that of prolactin itself. Partial N-terminal sequence analysis of the immunoprecipitable translation product showed that it contained one methionine residue at position 1 and several leucine residues within the first 30 amino acids. Experiments with labelled initiator and internal met tRNAMet indicated that the product being synthesized contained the initiating methionine and was probably the initial translation product of prolactin mRNA.