Characterization of a common precursor to corticotropin and beta-lipotropin: identification of beta-lipotropin peptides and their arrangement relative to corticotropin in the precursor synthesized in a cell-free system.

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Abstract
Radioactive proteins synthesized in an mRNA-dependent reticulocyte cell-free system under the direction of mRNA from AtT-20/D-16v mouse [pituitary tumor] cells were isolated by specific immunoprecipitation using antiserum to either .alpha.(1-24)corticotropin or .beta.-endorphin [.beta.(61-91) lipotropin]. Each immunoprecipitate was fractionated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and contained only 1 labeled protein with an apparent MW of 28,500. Tryptic peptide analysis of the Mr [relative MW] 28,500 corticotropin and .beta.-lipotropin molecules isolated from the gels demonstrated that the 2 proteins had the same lysine, methionine and tryptophan peptides. Four tryptic peptides from the cell-free product exhibited the same electrophoretic and chromatographic mobilities as marker tryptic peptides from bovine .beta.-melanotropin and porcine .beta.-endorphin. The identification of these peptides was confirmed by amino acid composition studies with a variety of labeled amino acids. The .beta.-lipotropin tryptic peptides were also located carboxy terminal to the corticotropin tryptic peptides.

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