The sorting of proglucagon to secretory granules is mediated by carboxypeptidase E and intrinsic sorting signals

J Endocrinol. 2013 Apr 15;217(2):229-40. doi: 10.1530/JOE-12-0468. Print 2013 May.


Proglucagon is expressed in pancreatic alpha cells, intestinal L cells and brainstem neurons. Tissue-specific processing of proglucagon yields the peptide hormones glucagon in the alpha cell and glucagon-like peptide (GLP)-1 and GLP-2 in L cells. Both glucagon and GLP-1 are secreted in response to nutritional status and are critical for regulating glycaemia. The sorting of proglucagon to the dense-core secretory granules of the regulated secretory pathway is essential for the appropriate secretion of glucagon and GLP-1. We examined the roles of carboxypeptidase E (CPE), a prohormone sorting receptor, the processing enzymes PC1/3 and PC2 and putative intrinsic sorting signals in proglucagon sorting. In Neuro 2a cells that lacked CPE, PC1/3 and PC2, proglucagon co-localised with the Golgi marker p115 as determined by quantitative immunofluorescence microscopy. Expression of CPE, but not of PC1/3 or PC2, enhanced proglucagon sorting to granules. siRNA-mediated knockdown of CPE disrupted regulated secretion of glucagon from pancreatic-derived alphaTC1-6 cells, but not of GLP-1 from intestinal cell-derived GLUTag cells. Mutation of the PC cleavage site K70R71, the dibasic R17R18 site within glucagon or the alpha-helix of glucagon, all significantly affected the sub-cellular localisation of proglucagon. Protein modelling revealed that alpha helices corresponding to glucagon, GLP-1 and GLP-2, are arranged within a disordered structure, suggesting some flexibility in the sorting mechanism. We conclude that there are multiple mechanisms for sorting proglucagon to the regulated secretory pathway, including a role for CPE in pancreatic alpha cells, initial cleavage at K70R71 and multiple sorting signals.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Carboxypeptidase H / antagonists & inhibitors
  • Carboxypeptidase H / drug effects
  • Carboxypeptidase H / metabolism*
  • Cell Line
  • Cells, Cultured
  • Enteroendocrine Cells / metabolism
  • Enteroendocrine Cells / pathology
  • Glucagon-Like Peptide 1 / metabolism
  • Glucagon-Like Peptide 2 / metabolism
  • Mice
  • Neuroblastoma / metabolism
  • Neuroblastoma / pathology
  • Pancreas / metabolism
  • Pancreas / pathology
  • Proglucagon / metabolism*
  • RNA, Small Interfering / pharmacology
  • Secretory Vesicles / metabolism*
  • Signal Transduction / physiology*


  • Glucagon-Like Peptide 2
  • RNA, Small Interfering
  • Proglucagon
  • Glucagon-Like Peptide 1
  • Carboxypeptidase H