Characterization of a common precursor to corticotropin and beta-lipotropin: cell-free synthesis of the precursor and identification of corticotropin peptides in the molecule.

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Abstract
MRNA was isolated from cultures of AtT-20/D-16v [mouse pituitary] tumor cells and translated in a mRNA-dependent reticulocyte cell-free system. The corticotropin (ACTH) product was purified by a double-antibody immunoprecipitation procedure using antisera specific for the .alpha.(1-24) sequence of ACTH. The product is shown by sodium dodecyl sulfate/gel electrophoresis and gel filtration on guanidine.cntdot.HCl columns to be homogeneous with an apparent molecular weight (Mr) of 28,500. A product with the same Mr is synthesized when membrane-bound polysomes from D-16v cells are allowed to complete their nascent chains in a reticulocyte cell-free system. Mr 31,000 ACTH isolated from tumor cells was separated into 3 proteins of different apparent Mr: 29,000, 32,000 and 34,000. The cell-free product contains the same lysine-, methionine-, and phenylalanine-labeled tryptic peptides as the Mr 29,000 ACTH synthesized in the tumor cells. Tryptic peptide analysis also reveals the presence of the .alpha.(1-39) sequence in the Mr 28,500 cell-free product and suggests that there is only 1 copy of this sequence in the Mr 28,500 molecule.

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