Achstetter, T., Franzusoff, A., Field, C. & Schekman, R. SEC7 encodes an unusual, high molecular weight protein required for membrane traffic from the yeast Golgi apparatus. J. Biol. Chem. 263, 11711-11717

Department of Biochemistry, University of California, Berkeley 94720.
Journal of Biological Chemistry (Impact Factor: 4.57). 09/1988; 263(24):11711-7.
Source: PubMed


Saccharomyces cerevisiae with mutations at the sec7 locus are pleiotropically deficient in protein transport within the Golgi apparatus and proliferate a large array of Golgi cisternae at a restrictive growth temperature (37 degrees C). The SEC7 gene and its product (Sec7p) have been evaluated by molecular cloning and sequence analysis. Two genes that allow sec7 mutant cells to grow at 37 degrees C are represented in wild-type yeast DNA libraries. A single copy of the authentic SEC7 gene permits growth of mutant cells, whereas the other gene suppresses growth deficiency only when expressed from a multicopy plasmid. The SEC7 gene is contained on a 8.4-kilobase pair SphI restriction fragment, portions of which hybridize to a single 6-kilobase pair mRNA. The gene is essential for yeast vegetative growth. DNA sequence analysis of this region detects a single open reading frame with the potential to encode a 2008-amino acid-long hydrophilic protein of 230 kDa. Putative Sec7p contains an unusual, highly charged acidic domain of 125 amino acids with 29% glutamate, 18% aspartate, and 21% serine. Within this region, stretches of 14 consecutive glutamate residues and 13 consecutive glutamates/aspartates are predicted. This domain in Sec7p may serve a structural role to interact with lipids or proteins on the cytoplasmic surface of the Golgi apparatus.

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Available from: Tilman Achstetter
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    • "To investigate whether membrane transport from the late Golgi is involved in the formation of CUPS, we used strains bearing thermo-sensitive mutations in the ARF-GEF Sec7 and in the phosphatidylinositol 4-kinase Pik1, two essential proteins localized primarily at the late Golgi. In these mutant strains (sec7-1 and pik1-104), the export of cargo from the late Golgi is blocked or delayed (Achstetter et al., 1988; Franzusoff et al., 1991; Walch-Solimena and Novick, 1999). The analysis by fluorescence microscopy of growing sec7-1 and pik1-104 cells incubated at 37°C revealed that the distribution of Grh1-2×GFP in these conditions was similar to that observed in the control wild-type strain (Fig. 3, A and B). "
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    ABSTRACT: Upon starvation, Grh1, a peripheral membrane protein located at endoplasmic reticulum (ER) exit sites and early Golgi in Saccharomyces cerevisiae under growth conditions, relocates to a compartment called compartment for unconventional protein secretion (CUPS). Here we report that CUPS lack Golgi enzymes, but contain the coat protein complex II (COPII) vesicle tethering protein Uso1 and the Golgi t-SNARE Sed5. Interestingly, CUPS biogenesis is independent of COPII- and COPI-mediated membrane transport. Pik1- and Sec7-mediated membrane export from the late Golgi is required for complete assembly of CUPS, and Vps34 is needed for their maintenance. CUPS formation is triggered by glucose, but not nitrogen starvation. Moreover, upon return to growth conditions, CUPS are absorbed into the ER, and not the vacuole. Altogether our findings indicate that CUPS are not specialized autophagosomes as suggested previously. We suggest that starvation triggers relocation of secretory and endosomal membranes, but not their enzymes, to generate CUPS to sort and secrete proteins that do not enter, or are not processed by enzymes of the ER-Golgi pathway of secretion. © 2014 Cruz-Garcia et al.
    Full-text · Article · Dec 2014 · The Journal of Cell Biology
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    • "These two subfamilies comprise related proteins sharing the highly conserved catalytic Sec7 domain (Sec7d) and five conserved regions denoted DCB, HUS, HDS1, -2 and -3 domains [3] [4] [5]. GBF/Gea and BIG/Sec7 are both considered essential for Golgi function in Saccharomyces cerevisiae, Drosophila melanogaster and mammalian cells [6] [7] [8] [9] [10] [11] [12] [13] [14]. Moreover, they are the only Arf-GEF subfamilies common to all eukaryotes [3]. "
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    ABSTRACT: Golgi Arf1-guanine nucleotide exchange factors (GEFs) belong to two subfamilies: GBF/Gea and BIG/Sec7. Both are conserved across eukaryotes, but the physiological role of each is not well understood. Aspergillus nidulans has a single member of the early Golgi GBF/Gea-subfamily, geaA, and the late Golgi BIG/Sec7-subfamily, hypB. Both geaA and hypB are essential. hypB5 conditionally blocks secretion. We sought extragenic hypB5 suppressors and obtained geaA1. geaA1 results in Tyr1022Cys within a conserved GBF/Gea-specific S(Y/W/F)(L/I) motif in GeaA. This mutation alters GeaA localization. Remarkably, geaA1 suppresses hypBDelta, indicating that a single mutant Golgi Arf1-GEF suffices for growth.
    Full-text · Article · Nov 2014 · FEBS Letters
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    • "A third possibility is that AGEF-1 and GBF-1 are present on the same compartment yet performing different functions. Again in yeast, Gea2 and Sec7 are associated with the TGN performing distinct functions [52], [53]. At present we cannot distinguish between these possibilities, and more studies are required to shed light on the specific functions of ArfGEFs. "
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    ABSTRACT: Small GTPases of the Sar/Arf family are essential to generate transport containers that mediate communication between organelles of the secretory pathway. Guanine nucleotide exchange factor (GEFs) activate the small GTPases and help their anchorage in the membrane. Thus, GEFs in a way temporally and spatially control Sar1/Arf1 GTPase activation. We investigated the role of the ArfGEF GBF-1 in C. elegans oocytes and intestinal epithelial cells. GBF-1 localizes to the cis-Golgi and is part of the t-ER-Golgi elements. GBF-1 is required for secretion and Golgi integrity. In addition, gbf-1(RNAi) causes the ER reticular structure to become dispersed, without destroying ER exit sites (ERES) because the ERES protein SEC-16 was still localized in distinct punctae at t-ER-Golgi units. Moreover, GBF-1 plays a role in receptor-mediated endocytosis in oocytes, without affecting recycling pathways. We find that both the yolk receptor RME-2 and the recycling endosome-associated RAB-11 localize similarly in control and gbf-1(RNAi) oocytes. While RAB5-positive early endosomes appear to be less prominent and the RAB-5 levels are reduced by gbf-1(RNAi) in the intestine, RAB-7-positive late endosomes were more abundant and formed aggregates and tubular structures. Our data suggest a role for GBF-1 in ER structure and endosomal traffic.
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