title: Unique COPII component AtSar1a/AtSec23a pair is required for the distinct function of protein ER export in Arabidopsis thaliana.
abstract: Secretory proteins traffic from endoplasmic reticulum (ER) to Golgi via the coat protein complex II (COPII) vesicle, which consists of five cytosolic components (Sar1, Sec23-24, and Sec13-31). In eukaryotes, COPII transport has diversified due to gene duplication, creating multiple COPII paralogs. Evidence has accumulated, revealing the functional heterogeneity of COPII paralogs in protein ER export. Sar1B, the small GTPase of COPII machinery, seems to be specialized for large cargo secretion in mammals. Arabidopsis contains five Sar1 and seven Sec23 homologs, and AtSar1a was previously shown to exhibit different effects on α-amylase secretion. However, mechanisms underlying the functional diversity of Sar1 paralogs remain unclear in higher organisms. Here, we show that the Arabidopsis Sar1 homolog AtSar1a exhibits distinct localization in plant cells. Transgenic Arabidopsis plants expressing dominant-negative AtSar1a exhibit distinct effects on ER cargo export. Mutagenesis analysis identified a single amino acid, Cys84, as being responsible for the functional diversity of AtSar1a. Structure homology modeling and interaction studies revealed that Cys84 is crucial for the specific interaction of AtSar1a with AtSec23a, a distinct Arabidopsis Sec23 homolog. Structure modeling and coimmunoprecipitation further identified a corresponding amino acid, Cys484, on AtSec23a as being essential for the specific pair formation. At the cellular level, the Cys484 mutation affects the distinct function of AtSec23a on vacuolar cargo trafficking. Additionally, dominant-negative AtSar1a affects the ER export of the transcription factor bZIP28 under ER stress. We have demonstrated a unique plant pair of COPII machinery function in ER export and the mechanism underlying the functional diversity of COPII paralogs in eukaryotes.
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Yonglun
authors.initials: Y
authors.lastname: Zeng
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Kin Pan
authors.initials: KP
authors.lastname: Chung
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Baiying
authors.initials: B
authors.lastname: Li
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China; School of Life Sciences, Center for Protein Science and Crystallography, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Ching Man
authors.initials: CM
authors.lastname: Lai
authors.affiliation: Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720;
authors.forename: Sheung Kwan
authors.initials: SK
authors.lastname: Lam
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Xiangfeng
authors.initials: X
authors.lastname: Wang
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Yong
authors.initials: Y
authors.lastname: Cui
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Caiji
authors.initials: C
authors.lastname: Gao
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Ming
authors.initials: M
authors.lastname: Luo
authors.affiliation: School of Life Sciences, Center for Protein Science and Crystallography, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China;
authors.forename: Kam-Bo
authors.initials: KB
authors.lastname: Wong
authors.affiliation: Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California, Berkeley, CA 94720;
authors.forename: Randy
authors.initials: R
authors.lastname: Schekman
authors.affiliation: School of Life Sciences, Centre for Cell and Developmental Biology and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China; Shenzhen Research Institute, The Chinese University of Hong Kong, Shenzhen 518057, China ljiang@cuhk.edu.hk.
authors.forename: Liwen
authors.initials: L
authors.lastname: Jiang
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000235:genetics / Q000378:metabolism*; D059865:Endoplasmic Reticulum Stress / Q000502:physiology; D008958:Models, Molecular; D020125:Mutation, Missense; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism; D021381:Protein Transport / Q000502:physiology; D033921:Vesicular Transport Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D033921:Vesicular Transport Proteins
keywords: ER export; Sar1; Sec23; coat protein complex II; functional diversity