title: Structural determinants for transport of lactase phlorizin-hydrolase in the early secretory pathway as a multi-domain membrane glycoprotein.
abstract: Lactase phlorizin-hydrolase (LPH) is a membrane anchored type I glycoprotein of the intestinal epithelium that is composed of four homologous structural domains. The role of each distinct domain in the intramolecular organization and function of LPH is not completely understood.
  
  
  
  Here, we analyzed the early events of LPH biosynthesis and trafficking by directed restructuring of the domain compositions.
  
  
  
  Removal of domain I (LPH∆1) results in a malfolded ER-localized protein. By contrast, LPH without domain II (LPH∆2) is normally transported along the secretory pathway, but does not dimerize nor is enzymatically active. Interestingly a polypeptide stretch in domain II between L735-R868exerts an intriguing role in modulating the trafficking behavior of LPH and its biological function. In fact, association of this stretch with transport-competent LPH chimeras results in their ER-arrest or aberrant trafficking. This stretch harbors a unique N-glycosylation site that is responsible for LPH retention in the ER via association with calnexin and facilitates proper folding of domains I and III before ER exit of LPH. Notably, a similar N-glycosylation site is also found in domain IV with comparable effects on the trafficking of LPH-derived molecules.
  
  
  
  Our study provides novel insights into the intramolecular interactions and the sequence of events involved in the folding, dimerization and transport of LPH.
  
  
  
  Elucidation of the structural-functional relevance of the domains in pro-LPH is crucial in unravelling and understanding the molecular basis of carbohydrate malabsorption disorders that are associated with lactase deficiency or lactase malfunction.
authors: Department of Physiological Chemistry, University of Veterinary Medicine Hannover, Hannover, Germany.
authors: Lena
authors: L
authors: Diekmann
authors: Institute of Physiology and Pathophysiology, Philipps-University Marburg, Marburg, Germany.
authors: Marc
authors: M
authors: Behrendt
authors: Department of Physiological Chemistry, University of Veterinary Medicine Hannover, Hannover, Germany.
authors: Mahdi
authors: M
authors: Amiri
authors: Department of Physiological Chemistry, University of Veterinary Medicine Hannover, Hannover, Germany. Electronic address: Hassan.Naim@tiho-hannover.de.
authors: Hassan Y
authors: HY
authors: Naim
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D037281:Calnexin / Q000378:metabolism; D002462:Cell Membrane / Q000201:enzymology; D002522:Chlorocebus aethiops; D004721:Endoplasmic Reticulum / Q000378:metabolism; D006031:Glycosylation; D010694:Lactase-Phlorizin Hydrolase / Q000737:chemistry* / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000737:chemistry* / Q000378:metabolism*; D018832:Molecular Chaperones / Q000378:metabolism; D011485:Protein Binding; D000072417:Protein Domains; D020836:Protein Structure, Quaternary; D017433:Protein Structure, Secondary; D021381:Protein Transport; D055571:Secretory Pathway; D017384:Sequence Deletion; D013329:Structure-Activity Relationship; D013347:Subcellular Fractions / Q000201:enzymology
chemical_list: D008562:Membrane Glycoproteins; D018832:Molecular Chaperones; D037281:Calnexin; D010694:Lactase-Phlorizin Hydrolase
keywords: Dimerization; Human intestinal lactase-phlorizin hydrolase; Intramolecular organization; Protein domain; Trafficking