Biochemical and structural characterization of recombinant copper-metallothionein from Saccharomyces cerevisiae

Resonant x-ray scattering and absorption for the global and local structures of Cu-modified metallothioneins in solution

With Cd and Zn metal ions removed from the native rabbit-liver metallothionein upon unfolding, Cu-modified metallothioneins (Cu-MTs) were obtained during refolding in solutions containing Cu(I) or Cu(II) ions. X-ray absorption near-edge spectroscopic results confirm the respectively assigned oxidation states of the copper ions in Cu(I)-MT and Cu(II)-MT. Global and local structures of the Cu-MTs were subsequently characterized by anomalous small-angle x-ray scattering (ASAXS) and extended x-ray absorption fine structure. Energy-dependent ASAXS results indicate that the morphology of Cu(II)-MT resembles that of the native MT, whereas Cu(I)-MT forms oligomers with a higher copper content. Both dummy-residue simulation and model-shape fitting of the ASAXS data reveal consistently rodlike morphology for Cu(II)-MT. Clearly identified Cu-S, Cu-O, and Cu-Cu contributions in the extended x-ray absorption fine structure analysis indicate that both Cu(I) and Cu(II) ions are bonded with O and S atoms of nearby amino acids in a four-coordination environment, forming metal clusters smaller than metal thiolate clusters in the native MT. It is demonstrated that a combination of resonant x-ray scattering and x-ray absorption can be particularly useful in revealing complementary global and local structures of metalloproteins due to the atom specific characteristics of the two techniques.

Conformational analysis of the leukocyte-specific EF-hand protein p65/L-plastin by X-ray scattering in solution

p65/L-Plastin is a leukocyte-specific EF-hand protein which plays a vital role in organizing the actin cytoskeleton. Since its overall structural information has been largely unknown, we employed the X-ray scattering technique to elucidate the structure. Kratky plots of p65/L-plastin showed one peak, indicating that the protein takes compact globular conformations. The radii of gyration (Rg) of the monomer p65/L-plastin estimated from Guinier plots were 27.5 +/- 0.5 A and 28.6 A in the absence and presence of Ca(2+), respectively. The distance distribution function P(r) gave single peaks at 31.5-32.3 A and 33 A in the absence and presence of Ca(2+), respectively. These indicate that p65/L-plastin becomes somewhat larger in the presence of Ca(2+). The molecular shape of p65/L-plastin reconstructed from X-ray scattering data using the DAMMIN program has provided the first view of the overall structure of full-length plastin/fimbrin family proteins: a compact horseshoe-like shape with a small projection, which also exhibits Ca(2+) -induced conformational changes.

Triticum durum Metallothionein

A novel gene sequence, with two exons and one intron, encoding a metallothionein (MT) has been identified in durum wheat Triticum durum cv. Balcali85 genomic DNA. Multiple alignment analyses on the cDNA and the translated protein sequences showed that T. durum MT (dMT) can be classified as a type 1 MT. dMT has three Cys-X-Cys motifs in each of the N- and C-terminal domains and a 42-residue-long hinge region devoid of cysteines. dMT was overexpressed in Escherichia coli as a fusion protein (GSTdMT), and bacteria expressing the fusion protein showed increased tolerance to cadmium in the growth medium compared with controls. Purified GSTdMT was characterized by SDS- and native-PAGE, size exclusion chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. It was shown that the recombinant protein binds 4 +/- 1 mol of cadmium/mol of protein and has a high tendency to form stable oligomeric structures. The structure of GSTdMT and dMT was investigated by synchrotron x-ray solution scattering and computational methods. X-ray scattering measurements indicated a strong tendency for GSTdMT to form dimers and trimers in solution and yielded structural models that were compatible with a stable dimeric form in which dMT had an extended conformation. Results of homology modeling and ab initio solution scattering approaches produced an elongated dMT structure with a long central hinge region. The predicted model and those obtained from x-ray scattering are in agreement and suggest that dMT may be involved in functions other than metal detoxification.

Solution structure of Cu6 metallothionein from the fungus Neurospora crassa

The 3D-solution structure of Neurospora crassa Cu(6)-metallothionein (NcMT) polypeptide backbone was determined using homonuclear, multidimensional (1)H-NMR spectroscopy. It represents a new metallothionein (MT) fold with a protein chain where the N-terminal half is left-handed and the C-terminal half right-handedly folded around a copper(I)-sulfur cluster. As seen with other MTs, the protein lacks definable secondary structural elements; however, the polypeptide fold is unique. The metal coordination and the cysteine spacing defines this unique fold. NcMT is only the second MT in the copper-bound form to be structurally characterized and the first containing the -CxCxxxxxCxC- motif. This motif is found in a variety of mammalian MTs and metalloregulatory proteins. The in vitro formation of the Cu(6)NcMT identical to the native Cu(6)NcMT was dependent upon the prior formation of the Zn(3)NcMT and its titration with Cu(I). The enhanced sensitivity and resolution of the 800 MHz (1)H-NMR spectral data permitted the 3D structure determination of the polypeptide backbone without the substitution and utilization of the NMR active spin 1/2 metals such as (113)Cd and (109)Ag. These restraints have been necessary to establish specific metal to cysteine restraints in 3D structural studies on this family of proteins when using lower field, less sensitive (1)H-NMR spectral data. The accuracy of the structure calculated without these constraints is, however, supported by the similarities of the 800 MHz structures of the alpha-domain of mouse MT1 compared to the one recalculated without metal-cysteine connectivities.

Addition of missing loops and domains to protein models by x-ray solution scattering

Inherent flexibility and conformational heterogeneity in proteins can often result in the absence of loops and even entire domains in structures determined by x-ray crystallographic or NMR methods. X-ray solution scattering offers the possibility of obtaining complementary information regarding the structures of these disordered protein regions. Methods are presented for adding missing loops or domains by fixing a known structure and building the unknown regions to fit the experimental scattering data obtained from the entire particle. Simulated annealing was used to minimize a scoring function containing the discrepancy between the experimental and calculated patterns and the relevant penalty terms. In low-resolution models where interface location between known and unknown parts is not available, a gas of dummy residues represents the missing domain. In high-resolution models where the interface is known, loops or domains are represented as interconnected chains (or ensembles of residues with spring forces between the C(alpha) atoms), attached to known position(s) in the available structure. Native-like folds of missing fragments can be obtained by imposing residue-specific constraints. After validation in simulated examples, the methods have been applied to add missing loops or domains to several proteins where partial structures were available.

Drosophila MTN: a metazoan copper-thionein related to fungal forms

Two Drosophila metallothioneins (MT) have been reported: MTN, a 40 residue peptide including 10 Cys, and MTO, a 43 residue peptide including 12 Cys. However, neither functional nor evolutionary analyses for either of the Drosophila MT are available. Here, heterologous expression of Mtn in Escherichia coli is reported. The metal binding abilities of the Cu- and Zn-MTN complexes conformed in vivo, as well as the features of the Cd- and Cu-aggregates produced by metal replacement in vitro, have been determined by atomic emission spectrometry, circular dichroism and electrospray ionization mass spectrometry. Primary structure relationships with other MT have been examined. The results indicate a close resemblance of MTN to fungal copper-thioneins.