title: Calnuc binds to LRP9 and affects its endosomal sorting.
abstract: Calnuc is an ubiquitous Ca(++)-binding protein found in the cytoplasm where it binds different Galpha subunits, in the Golgi lumen where it constitutes a major Ca(++) storage pool, and outside the cell. We identified LDLR-related protein 9 (LRP9) as the first transmembrane protein shown to interact directly with Calnuc. LRP9 is a member of a new subfamily of the LDLR superfamily that cycles between the trans-Golgi network (TGN) and endosomes through a mechanism dependent on clathrin adaptor GGA proteins. The aim of the present study was to characterize the interaction between Calnuc and LRP9. Various biochemical assays showed that the N-terminus of Calnuc interacts with an arginine-rich region in the cytosolic tail of LRP9. Confocal microscopy showed that Calnuc colocalizes with LRP9 at the surface of the TGN and early endosomes. Depletion of Calnuc by small interfering RNA (siRNA) missorted LRP9 in the late endosome/lysosome compartments and enhanced its lysosomal degradation. This phenotype was rescued by the expression of siRNA-resistant wild-type Calnuc as well as cytoplasmic Calnuc, indicating that the cytoplasmic pool of Calnuc is involved in LRP9 endosomal sorting to prevent the delivery of LRP9 to lysosomes. This is the first report showing that Calnuc plays a role in receptor trafficking.
authors.affiliation: Department of Pharmacology, Faculty of Medicine, Université de Sherbrooke, QC, Canada.
authors.forename: Julie
authors.initials: J
authors.lastname: Brodeur
authors.forename: Heidi
authors.initials: H
authors.lastname: Larkin
authors.forename: Rémi
authors.initials: R
authors.lastname: Boucher
authors.forename: Caroline
authors.initials: C
authors.lastname: Thériault
authors.forename: Samuel Chayer
authors.initials: SC
authors.lastname: St-Louis
authors.forename: Hugo
authors.initials: H
authors.lastname: Gagnon
authors.forename: Christine
authors.initials: C
authors.lastname: Lavoie
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D011992:Endosomes / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D026502:LDL-Receptor Related Proteins / Q000235:genetics / Q000378:metabolism*; D008247:Lysosomes / Q000378:metabolism; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D009419:Nerve Tissue Proteins; D000080866:Nucleobindins; D011485:Protein Binding; D021381:Protein Transport / Q000502:physiology*; D034741:RNA, Small Interfering / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D020798:Two-Hybrid System Techniques; D021601:trans-Golgi Network / Q000378:metabolism
chemical_list: D002135:Calcium-Binding Proteins; D004268:DNA-Binding Proteins; D026502:LDL-Receptor Related Proteins; C419827:LRP3 protein, human; C538941:Lrp10 protein, mouse; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D000080866:Nucleobindins; D034741:RNA, Small Interfering; D011993:Recombinant Fusion Proteins