Biosynthesis of the secreted 24 K isoforms of prolactin

PRL exists within the mammotroph population in a number of different molecular forms. Several of these forms are best described as isoforms, as they have the same mol wt (24 K), but differ in their net charges. In this study we have used in vitro translation assays to ascertain the number of 24 K translation products of normal pituitary messenger RNA (mRNA), and, finding only one, have used both in vitro translation assays and subcellular fractionation to determine the intracellular site of the posttranslational modification of this single translation product. Translation of mRNA from normal pituitary tissue or GH3 cells resulted in the apparent production of a number of pre-PRLs, but in only a single rough microsome-processed form of PRL, 24 K isoform 2. Longer term translation assays utilizing a variety of isotopes failed to show any evidence for rough microsomal posttranslational modification of isoform 2. Subcellular fractionation, using a discontinuous sucrose gradient, however, produced a membrane-bound large secretory granule fraction which, when isolated, contained essentially only isoform 2, and which had the capacity to convert isoform 2 to isoforms 3 and 3' by posttranslational phosphorylation.