Immature granules are not major sites for segregation of constitutively secreted granule content proteins in NIT-1 insulinoma cells.
Immature secretory granules (ISG's) are sites of segregation of proteins destined for secretion by unregulated pathways from those stored in mature secretory granules in endocrine cells. To determine whether significant soluble protein sorting occurs in ISG's, the secretion of soluble versions of the pancreatic protein GP2 (GP2-GPI(-)) and placental alkaline phosphatase (SEAP) was analyzed in NIT-1 cells. By immunofluorescence microscopy, neither protein localized to SG's in transfected cells. Their secretion was secretagogue-independent in pulse-chase radiolabeling experiments even at early times of chase, while a small increase in the secretion of amylase, which is known to enter ISG's, could be detected. Finally, in sucrose gradient fractionation experiments, SEAP was present in light density fractions. We conclude that while some proteins, such as amylase, have a limited intrinsic capacity to enter ISG's, the segregation of proteins secreted via the constitutive pathway from SG content proteins occurs primarily in the trans Golgi network.
Rindler
M J
MJ
Department of Cell Biology and Kaplan Cancer Center, New York University School of Medicine, 550 First Avenue, New York, NY 10016, USA. rindlm01@med.nyu.edu
Colomer
V
V
Jin
Y
Y
D000469:Alkaline Phosphatase / Q000378:metabolism; D000681:Amylases / Q000378:metabolism; D000818:Animals; D015415:Biomarkers / Q000032:analysis; D043423:Carboxypeptidase H; D002268:Carboxypeptidases / Q000378:metabolism; D002458:Cell Fractionation; D058851:GPI-Linked Proteins; D013006:Growth Hormone / Q000378:metabolism; D007340:Insulinoma; D007515:Islets of Langerhans / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000378:metabolism; D008856:Microscopy, Fluorescence; D021381:Protein Transport; D022142:Secretory Vesicles / Q000737:chemistry* / Q000502:physiology; D014407:Tumor Cells, Cultured
D015415:Biomarkers; C489339:GP2 protein, human; D058851:GPI-Linked Proteins; D007527:Isoenzymes; D008562:Membrane Glycoproteins; C055791:germ-cell AP isoenzyme; D013006:Growth Hormone; D000469:Alkaline Phosphatase; D000681:Amylases; D002268:Carboxypeptidases; D043423:Carboxypeptidase H