Evidence for a Signal Sequence at the N Terminus of the Common Precursor to Adrenocorticothrophin and β‐Lipotropin in Mouse Pituitary Cells

The precursor to corticotropin and β‐endorphin was synthesized in a reticulocyte cell‐free system under the direction of mRNA from mouse AtT‐20 pituitary tumor cells in the presence of [³H]proline, [³H]phenylalanine, [³H]leucine, [³H]valine, [³H]isoleucine or [³⁵S]methionine. Automatic Edman degradation of the radioactive cell‐free product showed the following N‐terminal sequence: Pro‐1, Met‐2, Leu‐11, Leu‐12, Leu‐13, Leu‐15, Leu‐16, Leu‐17, Ile‐21 and Val‐23. The corticotropin‐endorphin precursor was also labeled in AtT‐20 cells with [³H]valine, [³H]leucine, [³H]tryptophan, [³H]serine, [³⁵S]methionine or [³⁵S]cysteine. Automatic Edman degradation of the radioactive intact cell form gave the following N‐terminal sequence: Trp‐1, Cys‐2, Leu‐3, Ser‐5, Ser‐6, Val‐7, Cys‐8, Leu‐11, Leu‐17, Leu‐180 and tentatively Met‐27. The sequence of the intact cell form from AtT‐20 cells matches the sequence of the cell‐free form of bovine pituitary precursor beginning at Trp‐27, as determined by recombinant DNA technology [Nakanishi, S., Inoue, A., Kita, T., Nakamura, M., Chang, A. C. Y., Cohen, S. N., and Numa, S. (1979) Nature (Lond) 278, 423–427]. The sequence of the mouse pituitary mRNA‐directed cell‐free translation product also matches the bovine precursor beginning at Pro‐2. The results suggest that both the mouse and bovine precursors possess a signal sequence of 26 amino acids which is cleaved in intact cells. CNBr cleavage of [³⁵S]cysteine‐labelled intact cell precursor gave rise to an N‐terminal fragment of a size compatible with the presence of a methionyl residue at or near position 27.