Mannose 6-phosphate receptors are sorted from immature secretory granules via adaptor protein AP-1, clathrin, and syntaxin 6-positive vesicles.
The occurrence of clathrin-coated buds on immature granules (IGs) of the regulated secretory pathway suggests that specific transmembrane proteins are sorted into these buds through interaction with cytosolic adaptor proteins. By quantitative immunoelectron microscopy of rat endocrine pancreatic beta cells and exocrine parotid and pancreatic cells, we show for the first time that the mannose 6-phosphate receptors (MPRs) for lysosomal enzyme sorting colocalize with the AP-1 adaptor in clathrin-coated buds on IGs. Furthermore, the concentrations of both MPR and AP-1 decline by approximately 90% as the granules mature. Concomitantly, in exocrine secretory cells lysosomal proenzymes enter and then are sorted out of IGs, just as was previously observed in beta cells (Kuliawat, R., J. Klumperman, T. Ludwig, and P. Arvan. 1997. J. Cell Biol. 137:595-608). The exit of MPRs in AP-1/clathrin-coated buds is selective, indicated by the fact that the membrane protein phogrin is not removed from maturing granules. We have also made the first observation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor, syntaxin 6, which has been implicated in clathrin-coated vesicle trafficking from the TGN to endosomes (Bock, J.B., J. Klumperman, S. Davanger, and R.H. Scheller. 1997. Mol. Biol. Cell. 8:1261-1271) that enters and then exits the regulated secretory pathway during granule maturation. Thus, we hypothesize that during secretory granule maturation, MPR-ligand complexes and syntaxin 6 are removed from IGs by AP-1/clathrin-coated vesicles, and then delivered to endosomes.
Klumperman
J
J
Department of Cell Biology and Center for Electron Microscopy, University of Utrecht, School of Medicine, 3584CX Utrecht, The Netherlands. j.klumperman@lab.azu.nl
Kuliawat
R
R
Griffith
J M
JM
Geuze
H J
HJ
Arvan
P
P
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D000818:Animals; D002401:Cathepsin B / Q000032:analysis / Q000378:metabolism; D002966:Clathrin / Q000032:analysis*; D003594:Cytoplasmic Granules / Q000737:chemistry* / Q000378:metabolism; D004792:Enzyme Precursors / Q000032:analysis / Q000378:metabolism; D006056:Golgi Apparatus / Q000737:chemistry / Q000648:ultrastructure; D007515:Islets of Langerhans / Q000737:chemistry; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D008562:Membrane Glycoproteins / Q000032:analysis; D008565:Membrane Proteins / Q000032:analysis*; D009363:Neoplasm Proteins / Q000032:analysis; D010179:Pancreas / Q000737:chemistry; D010306:Parotid Gland / Q000737:chemistry; D011384:Proinsulin / Q000032:analysis; D017027:Protein Tyrosine Phosphatases; D050765:Qa-SNARE Proteins; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D017527:Receptor, IGF Type 2 / Q000032:analysis* / Q000378:metabolism; D054635:Receptor-Like Protein Tyrosine Phosphatases, Class 8
D033965:Adaptor Protein Complex alpha Subunits; D033942:Adaptor Proteins, Vesicular Transport; D002966:Clathrin; D004792:Enzyme Precursors; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009363:Neoplasm Proteins; D050765:Qa-SNARE Proteins; D017527:Receptor, IGF Type 2; D011384:Proinsulin; C102889:PTPRN2 protein, human; D017027:Protein Tyrosine Phosphatases; D054635:Receptor-Like Protein Tyrosine Phosphatases, Class 8; C057832:procathepsin B; D002401:Cathepsin B; D007545:Isoproterenol