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Title
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Primary structure and function analysis of the Abrus precatorius agglutinin A chain by site-directed mutagenesis. Pro(199) Of amphiphilic alpha-helix H impairs protein synthesis inhibitory activity.
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Authors
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C.L.Liu,
C.C.Tsai,
S.C.Lin,
L.I.Wang,
C.I.Hsu,
M.J.Hwang,
J.Y.Lin.
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Ref.
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J Biol Chem, 2000,
275,
1897-1901.
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PubMed id
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Abstract
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Abrus agglutinin (AAG), a low-toxicity protein from the plant Abrus precatorius,
is less lethal than abrina (ABRa) in mice (LD(50) = 5 mg/kg versus 20 microg/kg
of body weight). Nucleotide sequence analysis of a cDNA clone encoding
full-length AAG showed an open reading frame with 1641 base pairs, corresponding
to a 547-amino acid residue preproprotein containing a signal peptide and a
linker region (two amino acid residues) between the AAG-A and AAG-B subunits.
AAG had high homology to ABRa (77.8%). The 13 amino acid residues involved in
catalytic function, which are highly conserved among abrins and ricins, were
also conserved within AAG-A. The protein synthesis inhibitory activity of AAG-A
(IC(50) = 3.5 nM) was weaker than that of ABRa-A (0.05 nM). Molecular modeling
followed by site-directed mutagenesis showed that Pro(199) of AAG-A, located in
amphiphilic helix H and corresponding to Asn(200) of ABRa-A, can induce bending
of helix H. This bending would presumably affect the binding of AAG-A to its
target sequence, GpApGpAp, in the tetraloop structure of the 28 S rRNA subunit
and could be one of the major factors contributing to the relatively weak
protein synthesis inhibitory activity and toxicity of AAG.
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