Glucagon Precursors Identified by Immunoprecipitation of Products of Cell-free Translation of Messenger RNA

Polyadenylated RNA extracted from anglerfish islets was translated in a wheat germ cell-free system containing [³⁵S]methionine in the presence and absence of microsomal membranes prepared from a canine pancreas. Labeled translation products were analyzed by immunoprecipitation with an antiserum to porcine glucagon, followed by electrophoresis of the translation products and immunoprecipitated proteins on SDS polyacrylamide gels. In the absence of microsomal membranes two proteins of M_r = 14,500 and M_r = 12,500 were specifically immunoprecipitated with antiglucagon serum. Addition of microsomal membranes to the translation reactions resulted in a diminution of the labeled protein of M_r = 14,500 and a marked increase in the immunoreactive protein of M_r = 12,500. The protein of M_r = 12,500 was resistant to degradation by proteolytic enzymes added to translation reactions, indicating that it was segregated within microsomal vesicles. These results are consistent with synthesis of anglerfish islet glucagon in the form of a pre-prohormonal precursor (M_r = 14,500) containing a leader sequence that is cotranslationally cleaved from the protein by enzymes associated with microsomal membranes to produce a smaller intermediate prohormonal precursor (M_r = 12,500) of pancreatic glucagon (M_r = 3500).