title: pH-induced conversion of the transport lectin ERGIC-53 triggers glycoprotein release.
abstract: The recycling mannose lectin ERGIC-53 operates as a transport receptor by mediating efficient endoplasmic reticulum (ER) export of some secretory glycoproteins. Binding of cargo to ERGIC-53 in the ER requires Ca2+. Cargo release occurs in the ERGIC, but the molecular mechanism is unknown. Here we report efficient binding of purified ERGIC-53 to immobilized mannose at pH 7.4, the pH of the ER, but not at slightly lower pH. pH sensitivity of the lectin was more prominent when Ca2+ concentrations were low. A conserved histidine in the center of the carbohydrate recognition domain was required for lectin activity suggesting it may serve as a molecular pH/Ca2+ sensor. Acidification of cells inhibited the association of ERGIC-53 with the known cargo cathepsin Z-related protein and dissociation of this glycoprotein in the ERGIC was impaired by organelle neutralization that did not impair the transport of a control protein. The results elucidate the molecular mechanism underlying reversible lectin/cargo interaction and establish the ERGIC as the earliest low pH site of the secretory pathway.
authors.affiliation: Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Appenzeller-Herzog
authors.forename: Annie-Claude
authors.initials: AC
authors.lastname: Roche
authors.forename: Oliver
authors.initials: O
authors.lastname: Nufer
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Hauri
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001692:Biological Transport; D019556:COS Cells; D002118:Calcium / Q000737:chemistry / Q000494:pharmacology; D002241:Carbohydrates / Q000737:chemistry; D002738:Chloroquine / Q000494:pharmacology; D003432:Cross-Linking Reagents / Q000494:pharmacology; D004247:DNA / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006023:Glycoproteins / Q000737:chemistry*; D006026:Glycoside Hydrolases / Q000378:metabolism; D006639:Histidine / Q000737:chemistry; D006801:Humans; D006863:Hydrogen-Ion Concentration; D037102:Lectins / Q000737:chemistry; D008358:Mannose / Q000737:chemistry; D037241:Mannose-Binding Lectins / Q000737:chemistry*; D008565:Membrane Proteins / Q000737:chemistry*; D008856:Microscopy, Fluorescence; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011233:Precipitin Tests; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry; D013997:Time Factors; D014162:Transfection
chemical_list: D002241:Carbohydrates; D003432:Cross-Linking Reagents; D006023:Glycoproteins; C084004:LMAN1 protein, human; D037102:Lectins; D037241:Mannose-Binding Lectins; D008565:Membrane Proteins; D011994:Recombinant Proteins; D006639:Histidine; D002738:Chloroquine; D004247:DNA; D006026:Glycoside Hydrolases; D008358:Mannose; D002118:Calcium