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PDBsum entry 2ksz
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Metal binding protein
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PDB id
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2ksz
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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 solution structure of the mg2+ form of soybean calmodulin isoform 4 reveals unique features of plant calmodulins in resting cells.
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Authors
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H.Huang,
H.Ishida,
H.J.Vogel.
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Ref.
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Protein Sci, 2010,
19,
475-485.
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PubMed id
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Abstract
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Soybean calmodulin isoform 4 (sCaM4) is a plant calcium-binding protein,
regulating cellular responses to the second messenger Ca(2+). We have found that
the metal ion free (apo-) form of sCaM4 possesses a half unfolded structure,
with the N-terminal domain unfolded and the C-terminal domain folded. This
result was unexpected as the apo-forms of both soybean calmodulin isoform 1
(sCaM1) and mammalian CaM (mCaM) are fully folded. Because of the fact that free
Mg(2+) ions are always present at high concentrations in cells (0.5-2 mM), we
suggest that Mg(2+) should be bound to sCaM4 in nonactivated cells. CD studies
revealed that in the presence of Mg(2+) the initially unfolded N-terminal domain
of sCaM4 folds into an alpha-helix-rich structure, similar to the Ca(2+) form.
We have used the NMR backbone residual dipolar coupling restraints (1)D(NH),
(1)D(C alpha H alpha), and (1)D(C'C alpha) to determine the solution structure
of the N-terminal domain of Mg(2+)-sCaM4 (Mg(2+)-sCaM4-NT). Compared with the
known structure of Ca(2+)-sCaM4, the structure of the Mg(2+)-sCaM4-NT does not
fully open the hydrophobic pocket, which was further confirmed by the use of the
fluorescent probe ANS. Tryptophan fluorescence experiments were used to study
the interactions between Mg(2+)-sCaM4 and CaM-binding peptides derived from
smooth muscle myosin light chain kinase and plant glutamate decarboxylase. These
results suggest that Mg(2+)-sCaM4 does not bind to Ca(2+)-CaM target peptides
and therefore is functionally similar to apo-mCaM. The Mg(2+)- and
apo-structures of the sCaM4-NT provide unique insights into the structure and
function of some plant calmodulins in resting cells.
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