Schultz et al. (1998) Proc. Natl. Acad. Sci. USA 95, 5857-5864
Letunic et al. (2012) Nucleic Acids Res , doi:10.1093/nar/gkr931

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SAR Sar1p-like members of the Ras-family of small GTPases

SMART accession number:	SM00178
Description:	Yeast SAR1 is an essential gene required for transport of secretory proteins from the endoplasmic reticulum to the Golgi apparatus.
Interpro abstract (IPR006687):	Small GTPases form an independent superfamily within the larger class of regulatory GTP hydrolases. This superfamily contains proteins that control a vast number of important processes and possess a common, structurally preserved GTP-binding domain [(PUBMED:2122258), (PUBMED:1898771)]. Sequence comparisons of small G proteins from various species have revealed that they are conserved in primary structures at the level of 30-55% similarity [(PUBMED:2029511)]. Crystallographic analysis of various small G proteins revealed the presence of a 20 kDa catalytic domain that is unique for the whole superfamily [(PUBMED:1898771), (PUBMED:2196171)]. The domain is built of five alpha helices (A1-A5), six beta-strands (B1-B6) and five polypeptide loops (G1-G5). A structural comparison of the GTP- and GDP-bound form, allows one to distinguish two functional loop regions: switch I and switch II that surround the gamma-phosphate group of the nucleotide. The G1 loop (also called the P-loop) that connects the B1 strand and the A1 helix is responsible for the binding of the phosphate groups. The G3 loop provides residues for Mg(2+) and phosphate binding and is located at the N terminus of the A2 helix. The G1 and G3 loops are sequentially similar to Walker A and Walker B boxes that are found in other nucleotide binding motifs. The G2 loop connects the A1 helix and the B2 strand and contains a conserved Thr residue responsible for Mg(2+) binding. The guanine base is recognised by the G4 and G5 loops. The consensus sequence NKXD of the G4 loop contains Lys and Asp residues directly interacting with the nucleotide. Part of the G5 loop located between B6 and A5 acts as a recognition site for the guanine base [(PUBMED:11995995)]. The small GTPase superfamily can be divided into at least 8 different families, including: Arf small GTPases. GTP-binding proteins involved in protein trafficking by modulating vesicle budding and uncoating within the Golgi apparatus. Ran small GTPases. GTP-binding proteins involved in nucleocytoplasmic transport. Required for the import of proteins into the nucleus and also for RNA export. Rab small GTPases. GTP-binding proteins involved in vesicular traffic. Rho small GTPases. GTP-binding proteins that control cytoskeleton reorganisation. Ras small GTPases. GTP-binding proteins involved in signalling pathways. Sar1 small GTPases. Small GTPase component of the coat protein complex II (COPII) which promotes the formation of transport vesicles from the endoplasmic reticulum (ER). Mitochondrial Rho (Miro). Small GTPase domain found in mitochondrial proteins involved in mitochondrial trafficking. Roc small GTPases domain. Small GTPase domain always found associated with the COR domain. The SAR1 [(PUBMED:1396601), (PUBMED:8138575)] protein, first identified in budding yeast, is a 21 kDa GTP- binding protein involved in vesicular transport between the endoplasmic reticulum and the Golgi [(PUBMED:8262187)]. It is a GTP-binding protein that takes part in the formation of secretory vesicles by binding to an ER type II membrane protein, Sec12p [(PUBMED:1396601)]. It is evolutionary conserved and seems to be present in all eukaryotes. SAR1 is generally included in the RAS 'superfamily' of small GTP-binding proteins, but it is only slightly related to other RAS proteins. It also differs from RAS proteins in that it lacks cysteine residues at the C terminus and is therefore not subject to prenylation. SAR1 is slightly related to ARFs.
GO process:	intracellular protein transport (GO:0006886)
GO component:	intracellular (GO:0005622)
GO function:	GTP binding (GO:0005525)
Family alignment:	View or CHROMA formatCLUSTALW formatMSF formatFASTA formatPIR format

There are 272 SAR domains in 272 proteins in SMART's nrdb database.

Click on the following links for more information.

• Evolution (species in which this domain is found)

• Click on to expand nodes. To display all proteins with a SAR domain in a specific node, click on it.

  This tree shows only several representative species. The complete taxonomic breakdown of all proteins with SAR domain is also avaliable.

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  Go to specific node: Anopheles gambiae, Arabidopsis thaliana, Caenorhabditis elegans, Drosophila melanogaster, Homo sapiens, Mus musculus, Rattus norvegicus, Saccharomyces cerevisiae, Takifugu rubripes

• Cellular role (predicted cellular role)

• Binding / catalysis: GTP-hydrolysis

• Literature (relevant references for this domain)

• Primary literature is listed below; Automatically-derived, secondary literature is also avaliable.

  • Kuge O et al.
  • Sar1 promotes vesicle budding from the endoplasmic reticulum but not Golgi compartments.
  • J Cell Biol. 1994; 125: 51-65
  • Display abstract

  • Two new members (Sar1a and Sar1b) of the SAR1 gene family have been identified in mammalian cells. Using immunoelectron microscopy, Sar1 was found to be restricted to the transitional region where the protein was enriched 20-40-fold in vesicular carriers mediating ER to Golgi traffic. Biochemical analysis revealed that Sar1 was essential for an early step in vesicle budding. A Sar1-specific antibody potently inhibited export of vesicular stomatitis virus glycoprotein (VSV-G) from the ER in vitro. Consistent with the role of guanine nucleotide exchange in Sar1 function, a trans-dominant mutant (Sar1a[T39N]) with a preferential affinity for GDP also strongly inhibited vesicle budding from the ER. In contrast, Sar1 was not found to be required for the transport of VSV-G between sequential Golgi compartments, suggesting that components active in formation of vesicular carriers mediating ER to Golgi traffic may differ, at least in part, from those involved in intra-Golgi transport. The requirement for novel components at different stages of the secretory pathway may reflect the recently recognized differences in protein transport between the Golgi stacks as opposed to the selective sorting and concentration of protein during export from the ER.

  • Nakano A et al.
  • Mutational analysis of the Sar1 protein, a small GTPase which is essential for vesicular transport from the endoplasmic reticulum.
  • J Biochem (Tokyo). 1994; 116: 243-7
  • Display abstract

  • SAR1 encodes a 21-kDa GTPase, which is required for vesicle formation from the endoplasmic reticulum in yeast. Although it belongs to the expanding small GTPase superfamily, there are interesting structural features that are unique to the Sar1 protein. We performed a site-directed mutational study to identify the amino acid residues that are essential for the Sar1p function. Among seven mutants we constructed, four are functionless by themselves, while two confer temperature sensitivity to cells. When the mutant proteins are overproduced in wild-type cells, all of these six show a dominant negative effect on cell growth. The replacement by serine of the only cysteine residue present in Sar1p caused no significant change in the growth phenotype. These findings are not only important for analyzing the mechanism of the Sar1p action in yeast, but will also be very useful for studying the function of Sar1p counterparts in higher eukaryotes.

  • Oka T, Nakano A
  • Inhibition of GTP hydrolysis by Sar1p causes accumulation of vesicles that are a functional intermediate of the ER-to-Golgi transport in yeast.
  • J Cell Biol. 1994; 124: 425-34
  • Display abstract

  • The SAR1 gene product (Sar1p), a 21-kD GTPase, is a key component of the ER-to-Golgi transport in the budding yeast. We previously reported that the in vitro reconstitution of protein transport from the ER to the Golgi was dependent on Sar1p and Sec12p (Oka, T., S. Nishikawa, and A. Nakano. 1991. J. Cell Biol. 114:671-679). Sec12p is an integral membrane protein in the ER and is essential for the Sar1 function. In this paper, we show that Sar1p can remedy the temperature-sensitive defect of the sec12 mutant membranes, which is in the formation of ER-to-Golgi transport vesicles. The addition of Sar1p promotes vesicle formation from the ER irrespective of the GTP- or GTP gamma S-bound form, indicating that the active form of Sar1p but not the hydrolysis of GTP is required for this process. The inhibition of GTP hydrolysis blocks transport of vesicles to the Golgi and thus causes their accumulation. The accumulating vesicles, which carry Sar1p on them, can be separated from other membranes, and, after an appropriate wash that removes Sar1p, are capable of delivering the content to the Golgi when added back to fresh membranes. Thus we have established a new method for isolation of functional intermediate vesicles in the ER-to-Golgi transport. The sec23 mutant is defective in activation of Sar1 GTPase (Yoshihisa, T., C. Barlowe, and R. Schekman. 1993. Science (Wash. DC). 259:1466-1468). The membranes and cytosol from the sec23 mutant show only a partial defect in vesicle formation and this defect is also suppressed by the increase of Sar1p. Again GTP hydrolysis is not needed for the suppression of the defect in vesicle formation. Based on these results, we propose a model in which Sar1p in the GTP-bound form is required for the formation of transport vesicles from the ER and the GTP hydrolysis by Sar1p is essential for entering the next step of vesicular transport to the Golgi apparatus.

  • Nakano A, Muramatsu M
  • A novel GTP-binding protein, Sar1p, is involved in transport from the endoplasmic reticulum to the Golgi apparatus.
  • J Cell Biol. 1989; 109: 2677-91
  • Display abstract

  • SAR1, a gene that has been isolated as a multicopy suppressor of the yeast ER-Golgi transport mutant sec12, encodes a novel GTP-binding protein. Its nucleotide sequence predicts a 21-kD polypeptide that contains amino acid sequences highly homologous to GTP-binding domains of many ras-related proteins. Gene disruption experiments show that SAR1 is essential for cell growth. To test its function further, SAR1 has been placed under control of the GAL1 promoter and introduced into a haploid cell that had its chromosomal SAR1 copy disrupted. This mutant grows normally in galactose medium but arrests growth 12-15 h after transfer to glucose medium. At the same time, mutant cells accumulate ER precursor forms of a secretory pheromone, alpha-mating factor, and a vacuolar enzyme, carboxypeptidase Y. We propose that Sec12p and Sarlp collaborate in directing ER-Golgi protein transport.

• Structure (3D structures containing this domain)

• 3D Structures of SAR domains in PDB

  PDB code	Main view	Title
  1f6b		Crystal structure of sar1-gdp complex
  1m2o		Crystal structure of the sec23-sar1 complex
  2fa9		The crystal structure of sar1[h79g]-gdp provides insight into the coat-controlled gtp hydrolysis in the disassembly of cop ii
  2fmx		An open conformation of switch i revealed by sar1-gdp crystal structure at low mg(2+)
  2gao		Crystal structure of human sar1a in complex with gdp
  2qtv		Structure of sec23-sar1 complexed with the active fragment of sec31

• Links (links to other resources describing this domain)

• INTERPRO	IPR006687

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