title: Two dipolar α-helices within hormone-encoding regions of proglucagon are sorting signals to the regulated secretory pathway.
abstract: Proglucagon is expressed in pancreatic α cells, intestinal L cells, and some hypothalamic and brainstem neurons. Tissue-specific processing of proglucagon yields three major peptide hormones as follows: glucagon in the α cells and glucagon-like peptides (GLP)-1 and -2 in the L cells and neurons. Efficient sorting and packaging into the secretory granules of the regulated secretory pathway in each cell type are required for nutrient-regulated secretion of these proglucagon-derived peptides. Our previous work suggested that proglucagon is directed into granules by intrinsic sorting signals after initial processing to glicentin and major proglucagon fragment (McGirr, R., Guizzetti, L., and Dhanvantari, S. (2013) J. Endocrinol. 217, 229-240), leading to the hypothesis that sorting signals may be present in multiple domains. In the present study, we show that the α-helices within glucagon and GLP-1, but not GLP-2, act as sorting signals by efficiently directing a heterologous secretory protein to the regulated secretory pathway. Biophysical characterization of these peptides revealed that glucagon and GLP-1 each encode a nonamphipathic, dipolar α-helix, whereas the helix in GLP-2 is not dipolar. Surprisingly, glicentin and major proglucagon fragment were sorted with different efficiencies, thus providing evidence that proglucagon is first sorted to granules prior to processing. In contrast to many other prohormones in which sorting is directed by ordered prodomains, the sorting determinants of proglucagon lie within the ordered hormone domains of glucagon and GLP-1, illustrating that each prohormone has its own sorting "signature."
authors.affiliation: From the Departments of Medical Biophysics.
authors.forename: Leonardo
authors.initials: L
authors.lastname: Guizzetti
authors.affiliation: the Metabolism/Diabetes and Imaging Programs, Lawson Health Research Institute, London, Ontario N6A 4V2, Canada.
authors.forename: Rebecca
authors.initials: R
authors.lastname: McGirr
authors.affiliation: From the Departments of Medical Biophysics, the Metabolism/Diabetes and Imaging Programs, Lawson Health Research Institute, London, Ontario N6A 4V2, Canada Pathology, and Medicine, University of Western Ontario, London, Ontario N6A 3K7 and sdhanvan@uwo.ca.
authors.forename: Savita
authors.initials: S
authors.lastname: Dhanvantari
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D015153:Blotting, Western; D005934:Glucagon / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D052216:Glucagon-Like Peptide 1 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D053767:Glucagon-Like Peptide 2 / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D008647:Mesocricetus; D018613:Microscopy, Confocal; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016366:Open Reading Frames / Q000235:genetics; D016716:PC12 Cells; D010455:Peptides / Q000737:chemistry / Q000378:metabolism; D052336:Proglucagon / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011499:Protein Processing, Post-Translational; D017433:Protein Structure, Secondary; D021381:Protein Transport; D051381:Rats; D055571:Secretory Pathway; D022142:Secretory Vesicles / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D053767:Glucagon-Like Peptide 2; D010455:Peptides; D052336:Proglucagon; D052216:Glucagon-Like Peptide 1; D005934:Glucagon
keywords: Confocal Microscopy; Dipolar α-Helix; GLP-1; Glucagon; Intracellular Trafficking; Peptide Hormones; Proglucagon; Protein Secretion; Protein Sorting; Secretory Granules
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