title: Plant Sar1 isoforms with near-identical protein sequences exhibit different localisations and effects on secretion.
abstract: In plants, differentiation of subdomains of the endoplasmic reticulum (ER) dedicated to protein export, the ER export sites (ERES), is influenced by the type of export-competent membrane cargo to be delivered to the Golgi. This raises a fundamental biological question: is the formation of transport intermediates at the ER for trafficking to the Golgi always regulated in the same manner? To test this, we followed the distribution and activity of two plant Sar1 isoforms. Sar1 is the small GTPase that regulates assembly of COPII (coat protein complex II) on carriers that transport secretory cargo from ER to Golgi. We show that, in contrast to a tobacco Sar1 isoform, the two Arabidopsis Sar1 GTPases were localised at ERES, independently of co-expression of Golgi-destined membrane cargo in tobacco cells. Although both isoforms labelled ERES, one was found to partition with the membrane fraction to a greater extent. The different distribution of fluorescent fusions of the two isoforms was influenced by the nature of an amino acid residue at the C-terminus of the protein, suggesting that the requirements for membrane association of the two GTPases are not equal. Furthermore, functional analyses based on the secretion of the bulk flow marker alpha-amylase indicated that over-expression of GTP-restricted mutants of the two isoforms caused different levels of ER export inhibition. These novel results indicate a functional heterogeneity among plant Sar1 isoforms.
authors.affiliation: Department of Biology, University of Saskatchewan, SK, Canada.
authors.forename: Sally L
authors.initials: SL
authors.lastname: Hanton
authors.forename: Laurent
authors.initials: L
authors.lastname: Chatre
authors.forename: Loren A
authors.initials: LA
authors.lastname: Matheson
authors.forename: Marika
authors.initials: M
authors.lastname: Rossi
authors.forename: Michael A
authors.initials: MA
authors.lastname: Held
authors.forename: Federica
authors.initials: F
authors.lastname: Brandizzi
mesh_terms: D000595:Amino Acid Sequence; D018274:Electroporation; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics*; D020033:Protein Isoforms / Q000737:chemistry / Q000235:genetics; D050683:R-SNARE Proteins / Q000737:chemistry / Q000235:genetics*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000235:genetics* / Q000378:metabolism; D000516:alpha-Amylases / Q000235:genetics / Q000378:metabolism
chemical_list: D010940:Plant Proteins; D020033:Protein Isoforms; D050683:R-SNARE Proteins; D000516:alpha-Amylases