title: Carboxypeptidase E and Secretogranin III Coordinately Facilitate Efficient Sorting of Proopiomelanocortin to the Regulated Secretory Pathway in AtT20 Cells.
abstract: Proopiomelanocortin (POMC) is a multivalent prohormone that can be processed into at least 7 biologically active peptide hormones. Processing can begin in the trans-Golgi network (TGN) and continues in the secretory granules of the regulated secretory pathway (RSP). Sorting of POMC into these granules is a complex process. Previously, a membrane-associated form of carboxypeptidase E (CPE) was shown to bind to POMC and facilitate its trafficking into these granules. More recently, secretogranin III (SgIII) was also found to affect POMC trafficking. Here, we show by RNA silencing that CPE and SgIII play a synergistic role in the trafficking of POMC to granules of the RSP in AtT20 cells. Reduction of either protein resulted in increased constitutive secretion of POMC and chromogranin A, which was increased even further when both proteins were reduced together, indicative of missorting at the TGN. In SgIII-reduced cells, POMC accumulated in a compartment that cofractionated and colocalized with syntaxin 6, a marker of the TGN, on sucrose density gradients and in immunocytochemistry, respectively, indicating an accumulation of this protein in the presumed sorting compartment. Regulated secretion of ACTH, as a measure of sorting and processing of POMC in mature granules, was reduced in the SgIII down-regulated cells but was increased in the CPE down-regulated cells. These results suggest that multiple sorting systems exist, providing redundancy to ensure the important task of continuous and accurate trafficking of prohormones to the granules of the RSP for the production of peptide hormones.
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Niamh X
authors.initials: NX
authors.lastname: Cawley
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Trushar
authors.initials: T
authors.lastname: Rathod
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Sigrid
authors.initials: S
authors.lastname: Young
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Hong
authors.initials: H
authors.lastname: Lou
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Nigel
authors.initials: N
authors.lastname: Birch
authors.affiliation: Section on Cellular Neurobiology (N.X.C., T.R., S.Y., H.L., Y.P.L.), Program in Developmental Neuroscience, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892-4480; and School of Biological Sciences (N.B.), Centre for Brain Research and Brain Research New Zealand, Rangahau Roro Aotearoa, University of Auckland, New Zealand.
authors.forename: Y Peng
authors.initials: YP
authors.lastname: Loh
mesh_terms: D000818:Animals; D043423:Carboxypeptidase H / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D002864:Chromogranins / Q000235:genetics / Q000378:metabolism*; D052680:Corticotrophs / Q000378:metabolism*; D051379:Mice; D011333:Pro-Opiomelanocortin / Q000378:metabolism*; D021381:Protein Transport / Q000502:physiology; D034622:RNA Interference; D055571:Secretory Pathway / Q000502:physiology*; D022142:Secretory Vesicles / Q000378:metabolism*
chemical_list: D002864:Chromogranins; C506167:secretogranin III; D011333:Pro-Opiomelanocortin; D043423:Carboxypeptidase H