title: Characterization of AtSEC12 and AtSAR1. Proteins likely involved in endoplasmic reticulum and Golgi transport.
abstract: Transport of cargo proteins from the endoplasmic reticulum (ER) to the cis-Golgi network is mediated by protein-coated vesicles. The coat, called COPII coat, consists of proteins that are recruited from the cytosol and interact with integral membrane proteins of the ER. In yeast, both cytosolic proteins (Sec13/31, Sec23/24, and Sar1) and ER-associated proteins (Sec12 and others) have been purified and characterized and it has been possible to demonstrate transport vesicle formation in vitro. Arabidopsis thaliana homologs of Sar1 and Sec12 have recently been identified, but little is known about the properties of the proteins or their subcellular distribution. Here we demonstrate that AtSAR1, a 22-kD protein that binds GTP, and AtSEC12, a 43-kD GTP-exchange protein, are both associated with the ER. However, about one-half of the cellular AtSAR1 is present in the cytosol. When AtSAR1 is overexpressed in transgenic plants, the additional protein is also cytosolic. When tissue-culture cells are cold-shocked (12 h at 8 degrees C), AtSAR1 levels appeared to decline and a larger proportion of the total protein was found in the cytosol. Given the known function of AtSAR1 in yeast, we propose that the amount of ER-associated AtSAR1 is an indication of the intensity of the secretory process. Thus, we expect that such a cold shock will adversely affect ER-to-Golgi transport of proteins.
authors.affiliation: Michigan State University-Department of Energy Plant Research Laboratory, East Lansing 48824-1312, USA.
authors.forename: M
authors.initials: M
authors.lastname: Bar-Peled
authors.forename: N V
authors.initials: NV
authors.lastname: Raikhel
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D001693:Biological Transport, Active; D003600:Cytosol / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005656:Fungal Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D006056:Golgi Apparatus / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors; D008562:Membrane Glycoproteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008970:Molecular Weight; D020559:Monomeric GTP-Binding Proteins; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins
chemical_list: D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; D008562:Membrane Glycoproteins; D010940:Plant Proteins; C071239:SEC12 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins; D019204:GTP-Binding Proteins; D020559:Monomeric GTP-Binding Proteins; C077395:SAR1 protein, S cerevisiae