title: Endoplasmic reticulum-resident proteins are constitutively transported to vacuoles for degradation.
abstract: Soluble endoplasmic reticulum (ER)-resident proteins have very long lives because of their ER residency. This residency depends largely on ER-retrieval signals at their C-terminus. We examined the long-term destiny of endogenous ER-resident proteins, a lumenal binding protein (BiP) and a protein disulfide isomerase (PDI), with cultured cells of Arabidopsis. ER residents, in contrast to vacuolar proteinases, were considerably degraded in cells at the stationary phase. A subcellular fractionation analysis suggested that ER residents were transported into the vacuoles, which accumulated the residents lacking the ER-retrieval signals. We showed that the PDI located in the vacuoles had high mannose glycans, but not complex glycans, which suggested that the ER resident was transported to the vacuoles independent of the medial/trans-Golgi complex. To visualize the pathway of transport of ER-resident proteins, tobacco BY-2 cells were transformed with a chimeric gene encoding an ER-targeted green fluorescent protein (30 kDa GFP-HDEL). In the transformed cells at the stationary phase, GFP fluorescence was observed in the vacuoles. A subcellular fractionation revealed that a trimmed form of 27 kDa GFP was localized in the vacuoles. Treatment with E-64d, an inhibitor of papain-type cysteine proteinases that inhibits the degradation of GFP in the vacuoles, resulted in a stable accumulation of 27 kDa GFP in the vacuoles, even in the logarithmic phase. Our results suggest that endogenous ER residents are transported constitutively to the vacuoles by bypassing the Golgi complex and are then degraded.
authors.affiliation: Department of Botany, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Kentaro
authors.initials: K
authors.lastname: Tamura
authors.forename: Kenji
authors.initials: K
authors.lastname: Yamada
authors.forename: Tomoo
authors.initials: T
authors.lastname: Shimada
authors.forename: Ikuko
authors.initials: I
authors.lastname: Hara-Nishimura
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism*; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000378:metabolism; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum / Q000378:metabolism; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D010940:Plant Proteins / Q000378:metabolism; D030821:Plants, Genetically Modified; D019704:Protein Disulfide-Isomerases / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D012995:Solubility; D014026:Nicotiana; D014617:Vacuoles / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D002352:Carrier Proteins; D008164:Luminescent Proteins; D010940:Plant Proteins; D011993:Recombinant Fusion Proteins; C077221:BIP protein, Arabidopsis; D049452:Green Fluorescent Proteins; D019704:Protein Disulfide-Isomerases