title: Identification of SEC12, SED4, truncated SEC16, and EKS1/HRD3 as multicopy suppressors of ts mutants of Sar1 GTPase.
abstract: The yeast SAR1 gene encodes a low-molecular-weight GTPase which is essential for the formation of transport vesicles from the endoplasmic reticulum (ER). To understand how the Sar1p function is regulated in its GTPase cycle, we searched for multicopy suppressors of sar1 temperature-sensitive mutants and identified SEC12, SED4, truncated SEC16, and EKS1. EKS1 turns out to be identical to HRD3, which was independently isolated as a gene implicated in the degradation of an HMG-CoA reductase isozyme, Hmg2p. In this paper, we show that the product of EKS1/HRD3 is a type-I transmembrane glycoprotein and resides in the ER. The eks1/hrd3 disrupted cells are normal in growth and transport of cargo proteins, but missecrete BiP (Kar2p). The overexpression of EKS1/HRD3, which stabilizes Hmg2p, did not affect the stability of wild-type or mutant Sar1p or any early Sec proteins we examined. These results suggest that the role of Eks1p/Hrd3p is not involved in the ER protein degradation in general but rather required for the maintenance of the ER membrane functions. The novel genetic interactions unveiled between SAR1, SEC12, SEC16, and SED4 will provide useful information as to how the complex machinery of vesicle budding operates.
authors.affiliation: Molecular Membrane Biology Laboratory, RIKEN, Wako, Saitama, 351-0198, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Saito
authors.forename: T
authors.initials: T
authors.lastname: Yamanushi
authors.forename: T
authors.initials: T
authors.lastname: Oka
authors.forename: A
authors.initials: A
authors.lastname: Nakano
mesh_terms: D004721:Endoplasmic Reticulum / Q000378:metabolism; D005656:Fungal Proteins / Q000235:genetics* / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000235:genetics* / Q000378:metabolism; D018628:Gene Dosage; D016153:Genes, Suppressor; D020662:Guanine Nucleotide Exchange Factors; D008562:Membrane Glycoproteins / Q000235:genetics* / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism; D020559:Monomeric GTP-Binding Proteins; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D033921:Vesicular Transport Proteins
chemical_list: D005656:Fungal Proteins; D020662:Guanine Nucleotide Exchange Factors; C104622:HRD3 protein, S cerevisiae; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; C071239:SEC12 protein, S cerevisiae; C096367:SEC16 protein, S cerevisiae; C096368:SED4 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