title: Integral membrane protein sorting to vacuoles in plant cells: evidence for two pathways.
abstract: Plant cells may contain two functionally distinct vacuolar compartments. Membranes of protein storage vacuoles (PSV) are marked by the presence of alpha-tonoplast intrinsic protein (TIP), whereas lytic vacuoles (LV) are marked by the presence of gamma-TIP. Mechanisms for sorting integral membrane proteins to the different vacuoles have not been elucidated. Here we study a chimeric integral membrane reporter protein expressed in tobacco suspension culture protoplasts whose traffic was assessed biochemically by following acquisition of complex Asn-linked glycan modifications and proteolytic processing, and whose intracellular localization was determined with confocal immunofluorescence. We show that the transmembrane domain of the plant vacuolar sorting receptor BP-80 directs the reporter protein via the Golgi to the LV prevacuolar compartment, and attaching the cytoplasmic tail (CT) of gamma-TIP did not alter this traffic. In contrast, the alpha-TIP CT prevented traffic of the reporter protein through the Golgi and caused it to be localized in organelles separate from ER and from Golgi and LV prevacuolar compartment markers. These organelles had a buoyant density consistent with vacuoles, and alpha-TIP protein colocalized in them with the alpha-TIP CT reporter protein when the two were expressed together in protoplasts. These results are consistent with two separate pathways to vacuoles for membrane proteins: a direct ER to PSV pathway, and a separate pathway via the Golgi to the LV.
authors.affiliation: Institute of Biological Chemistry, Washington State University, Pullman, Washington 99164-6340, USA.
authors.forename: L
authors.initials: L
authors.lastname: Jiang
authors.forename: J C
authors.initials: JC
authors.lastname: Rogers
mesh_terms: D000595:Amino Acid Sequence; D020346:Aquaporins; D001483:Base Sequence; D001693:Biological Transport, Active; D003546:Cysteine Endopeptidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017931:DNA Primers / Q000235:genetics; D004721:Endoplasmic Reticulum / Q000378:metabolism; D004792:Enzyme Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006056:Golgi Apparatus / Q000378:metabolism; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D015388:Organelles / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D010944:Plants / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D010947:Plants, Toxic; D011499:Protein Processing, Post-Translational; D011523:Protoplasts / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism
chemical_list: D020346:Aquaporins; D017931:DNA Primers; D004792:Enzyme Precursors; D008565:Membrane Proteins; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; C091845:gamma-TIP protein, plant; C078044:alpha-TIP protein, Phaseolus vulgaris; D003546:Cysteine Endopeptidases; C076060:proaleurain