Conversion of Pre-Proparathyroid Hormone to Proparathyroid Hormone by Dog Pancreatic Microsomes†

In the wheat germ extract and reticulocyte lysate cell-free protein synthesizing systems, the major translation product of bovine parathyroid hormone mRNA was a precursor of parathyroid hormone, pre-proparathyroid hormone and little or no proparathyroid hormone or parathyroid hormone was produced. If dog pancreatic rough microsomes were added to the wheat germ system, a new radioactive protein was synthesized which coelectrophoresed with proparathyroid hormone on acrylamide gels containing sodium dodecyl sulfate and urea-acid acrylamide gels at pH 4. Analysis by Edman degradation of this protein labeled with [3H]lysine revealed lysines at positions 1, 4, and 5 as expected for authentic proparathyroid hormone. Conversion of pre-proparathyroid hormone to proparathyroid hormone occurred only if the membranes were present while pre-proparathyroid hormone was being synthesized. Inhibition of the conversion activity by a nonionic detergent, Nonidet P40, suggested that the proteolytic activity was associated with the membranes. Radioactive proparathyroid hormone but not pre-proparathyroid hormone was protected from digestion by proteolytic enzymes added posttranslationally to the reactions that contained membranes suggesting that proparathyroid hormone had been transported across the microsomal membrane into the lumen of the vesicles. Although analyzed less extensively, the addition of microsomal membranes to the reticulocyte lysate cell-free system is also required for the synthesis of proparathyroid hormone. The fraction of pre-proparathyroid hormone that is converted to proparathyroid hormone in the reticulocyte lysate is about two times that observed in the wheat germ cell-free system. These data demonstrate that, in completely heterologous cell-free systems, pre-proparathyroid hormone can be accurately processed to form proparathyroid hormone by a proteolytic activity associated with microsomal membranes.