MANNOSE 6-PHOSPHATE RECEPTORS ARE SORTED FROM IMMATURE SECRETORY GRANULES VIA ADAPTER PROTEIN AP-1, CLATHRIN, AND SYNTAXIN 6-POSITIVE VESICLES

The occurrence of clathrin-coated buds on immature granules (IGs) of t he regulated secretory pathway suggests that specific transmembrane pr oteins are sorted into these buds through interaction with cytosolic a daptor proteins. By quantitative immunoelectron tron microscopy of rat endocrine pancreatic beta cells and exocrine parotid and pancreatic c ells, we show for the first time that the mannose 6-phosphate receptor s (MPRs) for lysosomal enzyme sorting colocalize with the AP-1 adaptor in clathrin-coated buds on IGs. Furthermore, the concentrations of bo th MPR and AP-1 decline by similar to 90% as the granules mature. Conc omitantly, in exocrine secretory cells lysosomal proenzymes enter and then are sorted out of IGs, just as was previously observed in beta ce lls (Kuliawat, R., J. Klumperman, T. Ludwig, and P. Arvan. 1997. J. Ce ll Biol. 137:595-608). The exit of MPRs in AP-1/clathrin-coated buds i s selective, indicated by the fact that the membrane protein phogrin i s not removed from maturing granules. We have also made the first obse rvation of a soluble N-ethylmaleimide-sensitive factor attachment prot ein receptor, syntaxin 6, which has been implicated in clathrin-coated vesicle trafficking from the TGN to endosomes (Bock, J.B., J. Klumper man, S. Davanger, and R.H. Scheller. 1997, Mel. Biol. Cell. 5:1261-:12 71) 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.