Isolation of human, swine, and rat prepepsinogens and calf preprochymosin, and determination of the primary structures of their NH₂-terminal signal sequences

The total RNAs were extracted from human, swine, rat, and calf gastric mucosae, and translated in vitro in the presence of radiolabeled amino acids using a wheat germ cell-free system. Upon sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of the translation products, a protein band with a molecular weight of about 43,000 was obtained in each case as one of the major products. These products could be specifically immunoprecipitated with a corresponding anti-pepsinogen or anti-chymosin antiserum. Radiosequence analysis of these translation products purified by SDS-polyacrylamide gel electrophoresis showed that each of them is a precursor form, i.e., prepepsinogen or preprochymosin, having an amino-terminal extension peptide (signal sequence) comprising 15 (human and swine) or 16 (rat and calf) amino acid residues. The primary structures of these signal sequences were determined to be as follows: -15 -10 -5 -1Human prepepsinogen: Met-Lys-Trp-Leu-Leu-Leu-Leu-Gly-Leu-Val-Ala-Leu-Ser-Glu-Cys-Swine prepepsinogen: Met-Lys-Trp-Leu-Leu-Leu-Leu-Ser-Leu-Val-Val-Leu-Ser-Glu-Cys-Rat prepepsinogen:Met-Lya-Trp-Met-Val-Val-Ala-Leu-Leu-Cys-Leu-Pro-Leu-Leu-Glu-Ala-Calf prepepsinogen:Met-Arg-Cys-Leu-Val-Val-Leu-Leu-Ala-Val-Phe-Ala-Leu-Ser-Gln-Gly-These signal sequences share common characteristics with those of other pre-secretory proteins, i.e., the presence of positive charges in the NH₂-terminal region, hydrophobic amino acid clusters in the interior part, and amino acids with short side chains at the site of cleavage by the signal peptidase.