Preprolactin represents the initial product of prolactin mRNA translation.

mRNA isolated from a line of functional rat pituitary tumor cells (GH3) added to a cell-free protein synthesizing system derived from wheat embryo directed the biosynthesis of only one protein which is immunoprecipitated by prolactin antiserum. This protein, 2000--3,000 daltons heavier than prolactin and referred to as preprolactin, appeared to represent the initial, authentic product of translation since [35S]methionine donated by initiator Met-tRNAi was documented to be incorporated into the N-terminal portion; the molar ratio of leucine: initiator methionine was estimated to be 25--28:1, while prolactin contains 22 leucine residues. The mRNA coding for preprolactin was demonstrated to migrate at approximately 12--13S using rate zonal centrifugation. These data are compatible with the postulate that preprolactin represents the intact, initial product of prolactin mRNA translation.