Production of EGF-containing polypeptides in Xenopus oocytes microinjected with submaxillary gland mRNA

The biosynthesis of epidermal growth factor (EGF), a 6045 dalton mitogen produced in the mouse submaxillary gland under androgen regulation, was studied using Xenopus oocytes. Microinjection of total, unfractionated gland mRNA together with [35S]cysteine resulted in the production of a secretory polypeptide of approximately 9000 daltons, specifically immunoprecipitable with anti-EGF antibodies. A minor amount of a similarly immunoreactive 9000 dalton secretory polypeptide was produced from the sucrose gradient 9S fraction of gland mRNA. Other, more intensely labeled polypeptides, a cytoplasmic 125 000 dalton and a secretory 110 000 dalton protein were immunoprecipitated from oocytes injected with the greater than 25S mRNA fraction. The biosynthesis of both can hardly be detected in oocytes injected with unfractionated mRNA. All three polypeptides are produced under androgen regulation and share common immunoreactive properties. Northern blot analysis using a 76 nucleotide synthetic EGF cDNA probe revealed hybridization with a single 28S mRNA species. This, and the apparent interrelation between the three polypeptides, suggest that a gland-specific processing protein, encoded by a 9S mRNA, is required to produce the 9000 dalton pro-EGF from the nascent translation product of EGF mRNA.