title: The subcellular distribution of calnexin is mediated by PACS-2.
abstract: Calnexin is an endoplasmic reticulum (ER) lectin that mediates protein folding on the rough ER. Calnexin also interacts with ER calcium pumps that localize to the mitochondria-associated membrane (MAM). Depending on ER homeostasis, varying amounts of calnexin target to the plasma membrane. However, no regulated sorting mechanism is so far known for calnexin. Our results now describe how the interaction of calnexin with the cytosolic sorting protein PACS-2 distributes calnexin between the rough ER, the MAM, and the plasma membrane. Under control conditions, more than 80% of calnexin localizes to the ER, with the majority on the MAM. PACS-2 knockdown disrupts the calnexin distribution within the ER and increases its levels on the cell surface. Phosphorylation by protein kinase CK2 of two calnexin cytosolic serines (Ser554/564) reduces calnexin binding to PACS-2. Consistent with this, a Ser554/564 Asp phosphomimic mutation partially reproduces PACS-2 knockdown by increasing the calnexin signal on the cell surface and reducing it on the MAM. PACS-2 knockdown does not reduce retention of other ER markers. Therefore, our results suggest that the phosphorylation state of the calnexin cytosolic domain and its interaction with PACS-2 sort this chaperone between domains of the ER and the plasma membrane.
authors.affiliation: Department of Cell Biology, University of Alberta, Edmonton, Alberta, T6G2H7, Canada.
authors.forename: Nathan
authors.initials: N
authors.lastname: Myhill
authors.forename: Emily M
authors.initials: EM
authors.lastname: Lynes
authors.forename: Jalal A
authors.initials: JA
authors.lastname: Nanji
authors.forename: Anastassia D
authors.initials: AD
authors.lastname: Blagoveshchenskaya
authors.forename: Hao
authors.initials: H
authors.lastname: Fei
authors.forename: Katia
authors.initials: K
authors.lastname: Carmine Simmen
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Cooper
authors.forename: Gary
authors.initials: G
authors.lastname: Thomas
authors.forename: Thomas
authors.initials: T
authors.lastname: Simmen
mesh_terms: D048868:Adaptor Proteins, Signal Transducing / Q000502:physiology*; D000595:Amino Acid Sequence; D037281:Calnexin / Q000096:biosynthesis* / Q000737:chemistry / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism*; D003600:Cytosol / Q000737:chemistry; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005786:Gene Expression Regulation; D006367:HeLa Cells; D006801:Humans; D008954:Models, Biological; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D033921:Vesicular Transport Proteins
chemical_list: D048868:Adaptor Proteins, Signal Transducing; C499638:PACS2 protein, human; D033921:Vesicular Transport Proteins; D037281:Calnexin