title: Sucrase is an intramolecular chaperone located at the C-terminal end of the sucrase-isomaltase enzyme complex.
abstract: The sucrase-isomaltase enzyme complex (pro-SI) is a type II integral membrane glycoprotein of the intestinal brush border membrane. Its synthesis commences with the isomaltase (IM) subunit and ends with sucrase (SUC). Both domains reveal striking structural similarities, suggesting a pseudo-dimeric assembly of a correctly folded and an enzymatically active pro-SI. The impact of each domain on the folding and function of pro-SI has been analyzed by individual expression and coexpression of the individual subunits. SUC acquires correct folding, enzymatic activity and transport competence and is secreted into the external milieu independent of the presence of IM. By contrast, IM persists as a mannose-rich polypeptide that interacts with the endoplasmic reticulum resident molecular chaperone calnexin. This interaction is disrupted when SUC is coexpressed with IM, indicating that SUC competes with calnexin for binding of IM. The interaction between SUC and the membrane-anchored IM leads to maturation of IM and blocks the secretion of SUC into the external milieu. We conclude that SUC plays a role as an intramolecular chaperone in the context of the pro-SI protein. To our knowledge all intramolecular chaperones so far identified are located at the N-terminal end. SUC is therefore the first C-terminally located intramolecular chaperone in mammalian cells.
authors.affiliation: Department of Physiological Chemistry, School of Veterinary Medicine Hannover, Bünteweg 17, 30559 Hannover, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Jacob
authors.forename: Berit
authors.initials: B
authors.lastname: Pürschel
authors.forename: Hassan Y
authors.initials: HY
authors.lastname: Naim
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D019556:COS Cells; D002135:Calcium-Binding Proteins / Q000378:metabolism; D037281:Calnexin; D002522:Chlorocebus aethiops; D003001:Cloning, Molecular; D017931:DNA Primers; D019281:Dimerization; D004721:Endoplasmic Reticulum / Q000378:metabolism; D007700:Kinetics; D008562:Membrane Glycoproteins / Q000378:metabolism; D018832:Molecular Chaperones / Q000737:chemistry / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D017510:Protein Folding; D021122:Protein Subunits; D011994:Recombinant Proteins / Q000378:metabolism; D013393:Sucrase / Q000737:chemistry / Q000378:metabolism*; D013394:Sucrase-Isomaltase Complex / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D014162:Transfection
chemical_list: D002135:Calcium-Binding Proteins; D017931:DNA Primers; D008562:Membrane Glycoproteins; D018832:Molecular Chaperones; D021122:Protein Subunits; D011994:Recombinant Proteins; D037281:Calnexin; D013394:Sucrase-Isomaltase Complex; D013393:Sucrase