Two alternative processing pathways for a preprohormone: a bioactive form of secretin.

Abstract

An N-terminally 9-residue elongated form of secretin, secretin-(-9 to 27) amide, was isolated from porcine intestinal tissue and characterized. Current knowledge about peptide processing sites does not allow unambiguous prediction of the signal peptide cleavage site in preprosecretin but suggests cleavage in the region of residues -10 to -14 counted upstream from the N terminus of the hormone. However, the structure of the isolated peptide suggests that the cleavage between the signal peptide and the N-terminal propeptide occurs at the C-terminal side of residue -10. Moreover, the isolated peptide demonstrates that secretin can be fully processed C-terminally prior to the final N-terminal cleavage. The results from this report, and those from earlier studies, where C-terminally elongated variants were isolated, show that the processing of the secretin precursor may proceed by one of two alternative pathways, in which either of the two ends is processed first. The bioactivity of the N-terminally extended peptide on exocrine pancreatic secretion was lower than that of secretin, indicating the importance of the finally processed free N terminus of the hormone for interaction with secretin receptors.Secretin, a gastrointestinal hormone, was discovered by Bayliss and Starling in 1902 (1), isolated (2), and characterized as a 27-residue C-terminally amidated peptide (3). It is known to be produced in endocrine S cells of the small intestine (4, 5), pancreatic 13 cells (6), and possibly also in brain (7,8). Its best known function is stimulation of the secretion of bicarbonaterich pancreatic fluid (9).The structures of the rat and mouse secretin precursors as deduced from the corresponding cDNA sequences contain a signal sequence, an N-terminal flanking peptide, secretin, and a C-terminal extension peptide in that order (10, 11). The amino acid sequence of pig preprosecretin has also been so deduced, but it is probably lacking a few residues at the N terminus of the signal peptide (10). The amino acid sequence of the precursor is such that current knowledge about signal peptide cleavage sites (12) does not allow unambiguous prediction of this site. On the basis of the amino acid sequence, the cleavage of the signal peptide has been proposed to occur at the C-terminal side of any of the residues between -10 and -14 (10), most likely after residue -12, -13, or -14. Hence, for determination of the actual cleavage site, isolation of the corresponding N-terminally elongated form of secretin is necessary. In this report, we show that cleavage of the signal peptide probably occurs at the C-terminal side of residue -10, thus suggesting the position of cleavage and the length of the N-terminal flanking peptide.It has been shown earlier that other secretin proforms also exist in intestinal tissue. A proform, consisting of the secretin part together with its C-terminal flanking peptide, has been isolated, although the length of the latter was found to be 31 residues shorter than the amino acid sequence deduced fromThe pub...

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