title: Arabidopsis ROCK1 transports UDP-GlcNAc/UDP-GalNAc and regulates ER protein quality control and cytokinin activity.
abstract: The formation of glycoconjugates depends on nucleotide sugars, which serve as donor substrates for glycosyltransferases in the lumen of Golgi vesicles and the endoplasmic reticulum (ER). Import of nucleotide sugars from the cytosol is an important prerequisite for these reactions and is mediated by nucleotide sugar transporters. Here, we report the identification of REPRESSOR OF CYTOKININ DEFICIENCY 1 (ROCK1, At5g65000) as an ER-localized facilitator of UDP-N-acetylglucosamine (UDP-GlcNAc) and UDP-N-acetylgalactosamine (UDP-GalNAc) transport in Arabidopsis thaliana. Mutant alleles of ROCK1 suppress phenotypes inferred by a reduced concentration of the plant hormone cytokinin. This suppression is caused by the loss of activity of cytokinin-degrading enzymes, cytokinin oxidases/dehydrogenases (CKXs). Cytokinin plays an essential role in regulating shoot apical meristem (SAM) activity and shoot architecture. We show that rock1 enhances SAM activity and organ formation rate, demonstrating an important role of ROCK1 in regulating the cytokinin signal in the meristematic cells through modulating activity of CKX proteins. Intriguingly, genetic and molecular analysis indicated that N-glycosylation of CKX1 was not affected by the lack of ROCK1-mediated supply of UDP-GlcNAc. In contrast, we show that CKX1 stability is regulated in a proteasome-dependent manner and that ROCK1 regulates the CKX1 level. The increased unfolded protein response in rock1 plants and suppression of phenotypes caused by the defective brassinosteroid receptor bri1-9 strongly suggest that the ROCK1 activity is an important part of the ER quality control system, which determines the fate of aberrant proteins in the secretory pathway.
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany;
authors.forename: Michael C E
authors.initials: MC
authors.lastname: Niemann
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany;
authors.forename: Isabel
authors.initials: I
authors.lastname: Bartrina
authors.affiliation: Department of Cellular Chemistry, Hannover Medical School, D-30625 Hannover, Germany;
authors.forename: Angel
authors.initials: A
authors.lastname: Ashikov
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany;
authors.forename: Henriette
authors.initials: H
authors.lastname: Weber
authors.affiliation: Laboratory of Growth Regulators and Department of Chemical Biology and Genetics, Centre of the Region Haná for Biotechnological and Agricultural Research, Palacký University and Institute of Experimental Botany, Academy of Sciences of the Czech Republic, CZ-78371, Olomouc, Czech Republic; and.
authors.forename: Ondřej
authors.identifier: http://orcid.org/0000-0003-3452-0154
authors.initials: O
authors.lastname: Novák
authors.affiliation: Laboratory of Growth Regulators and Department of Chemical Biology and Genetics, Centre of the Region Haná for Biotechnological and Agricultural Research, Palacký University and Institute of Experimental Botany, Academy of Sciences of the Czech Republic, CZ-78371, Olomouc, Czech Republic; and.
authors.forename: Lukáš
authors.initials: L
authors.lastname: Spíchal
authors.affiliation: Laboratory of Growth Regulators and Department of Chemical Biology and Genetics, Centre of the Region Haná for Biotechnological and Agricultural Research, Palacký University and Institute of Experimental Botany, Academy of Sciences of the Czech Republic, CZ-78371, Olomouc, Czech Republic; and.
authors.forename: Miroslav
authors.initials: M
authors.lastname: Strnad
authors.affiliation: Department of Applied Genetics and Cell Biology, University of Natural Resources and Life Sciences, 1190 Vienna, Austria.
authors.forename: Richard
authors.initials: R
authors.lastname: Strasser
authors.affiliation: Department of Cellular Chemistry, Hannover Medical School, D-30625 Hannover, Germany;
authors.forename: Hans
authors.initials: H
authors.lastname: Bakker
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany;
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmülling
authors.affiliation: Institute of Biology/Applied Genetics, Dahlem Centre of Plant Sciences, Freie Universität Berlin, D-14195 Berlin, Germany; tower@zedat.fu-berlin.de.
authors.forename: Tomáš
authors.initials: T
authors.lastname: Werner
mesh_terms: D017360:Arabidopsis / Q000378:metabolism* / Q000648:ultrastructure; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D003583:Cytokinins / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D018519:Meristem / Q000378:metabolism / Q000648:ultrastructure; D010641:Phenotype; D014536:Uridine Diphosphate N-Acetylgalactosamine / Q000378:metabolism*; D014537:Uridine Diphosphate N-Acetylglucosamine / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D003583:Cytokinins; C000597092:ROCK1 protein, Arabidopsis; D014537:Uridine Diphosphate N-Acetylglucosamine; D014536:Uridine Diphosphate N-Acetylgalactosamine
keywords: CKX; ROCK1; cytokinin; nucleotide sugars; shoot meristem