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PDBsum entry 4h5i
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Protein transport
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PDB id
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4h5i
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References listed in PDB file
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Key reference
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Title
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The structure of sec12 implicates potassium ion coordination in sar1 activation.
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Authors
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C.Mcmahon,
S.M.Studer,
C.Clendinen,
G.P.Dann,
P.D.Jeffrey,
F.M.Hughson.
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Ref.
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J Biol Chem, 2012,
287,
43599-43606.
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PubMed id
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Abstract
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Coat protein II (COPII)-coated vesicles transport proteins and lipids from the
endoplasmic reticulum to the Golgi. Crucial for the initiation of COPII coat
assembly is Sec12, a guanine nucleotide exchange factor responsible for
activating the small G protein Sar1. Once activated, Sar1/GTP binds to
endoplasmic reticulum membranes and recruits COPII coat components (Sec23/24 and
Sec13/31). Here, we report the 1.36 Å resolution crystal structure of the
catalytically active, 38-kDa cytoplasmic portion of Saccharomyces cerevisiae
Sec12. Sec12 adopts a β propeller fold. Conserved residues cluster around a
loop we term the "K loop," which extends from the N-terminal propeller
blade. Structure-guided site-directed mutagenesis, in conjunction with in vitro
and in vivo functional studies, reveals that this region of Sec12 is
catalytically essential, presumably because it makes direct contact with Sar1.
Strikingly, the crystal structure also reveals that a single potassium ion
stabilizes the K loop; bound potassium is, moreover, essential for optimum
guanine nucleotide exchange activity in vitro. Thus, our results reveal a novel
role for a potassium-stabilized loop in catalyzing guanine nucleotide exchange.
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