SARSar1p-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:
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)|
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- Evolution (species in which this domain is found)
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- Structure (3D structures containing this domain)
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