title: Calcium and pH-dependent packing and release of the gel-forming MUC2 mucin.
abstract: MUC2, the major colonic mucin, forms large polymers by N-terminal trimerization and C-terminal dimerization. Although the assembly process for MUC2 is established, it is not known how MUC2 is packed in the regulated secretory granulae of the goblet cell. When the N-terminal VWD1-D2-D'D3 domains (MUC2-N) were expressed in a goblet-like cell line, the protein was stored together with full-length MUC2. By mimicking the pH and calcium conditions of the secretory pathway we analyzed purified MUC2-N by gel filtration, density gradient centrifugation, and transmission electron microscopy. At pH 7.4 the MUC2-N trimer eluted as a single peak by gel filtration. At pH 6.2 with Ca(2+) it formed large aggregates that did not enter the gel filtration column but were made visible after density gradient centrifugation. Electron microscopy studies revealed that the aggregates were composed of rings also observed in secretory granulae of colon tissue sections. The MUC2-N aggregates were dissolved by removing Ca(2+) and raising pH. After release from goblet cells, the unfolded full-length MUC2 formed stratified layers. These findings suggest a model for mucin packing in the granulae and the mechanism for mucin release, unfolding, and expansion.
authors.affiliation: Department of Medical Biochemistry and Cell Biology,  University of Gothenburg, SE-405 30 Gothenburg,  Sweden.
authors.forename: Daniel
authors.initials: D
authors.lastname: Ambort
authors.forename: Malin E V
authors.initials: ME
authors.lastname: Johansson
authors.forename: Jenny K
authors.initials: JK
authors.lastname: Gustafsson
authors.forename: Harriet E
authors.initials: HE
authors.lastname: Nilsson
authors.forename: Anna
authors.initials: A
authors.lastname: Ermund
authors.forename: Bengt R
authors.initials: BR
authors.lastname: Johansson
authors.forename: Philip J B
authors.initials: PJ
authors.lastname: Koeck
authors.forename: Hans
authors.initials: H
authors.lastname: Hebert
authors.forename: Gunnar C
authors.initials: GC
authors.lastname: Hansson
mesh_terms: D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D005782:Gels / Q000378:metabolism*; D020397:Goblet Cells / Q000378:metabolism / Q000648:ultrastructure; D006863:Hydrogen-Ion Concentration; D051379:Mice; D008810:Mice, Inbred C57BL; D008958:Models, Molecular; D055262:Mucin-2 / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D017434:Protein Structure, Tertiary
chemical_list: D005782:Gels; D055262:Mucin-2; D002118:Calcium