Metallothioneins with unusual residues: histidines as modulators of zinc affinity and reactivity

The two Caenorhabditis elegans metallothioneins (CeMT-1 and CeMT-2) discriminate between essential zinc and toxic cadmium

The nematode Caenorhabditis elegans expresses two metallothioneins (MTs), CeMT-1 and CeMT-2, that are believed to be key players in the protection against metal toxicity. In this study, both isoforms were expressed in vitro in the presence of either Zn(II) or Cd(II). Metal binding stoichiometries and affinities were determined by ESI-MS and NMR, respectively. Both isoforms had equal zinc binding ability, but differed in their cadmium binding behaviour, with higher affinity found for CeMT-2. In addition, wild-type C. elegans, single MT knockouts and a double MT knockout allele were exposed to zinc (340 microm) or cadmium (25 microm) to investigate effects in vivo. Zinc levels were significantly increased in all knockout strains, but were most pronounced in the CeMT-1 knockout, mtl-1 (tm1770), while cadmium accumulation was highest in the CeMT-2 knockout, mtl-2 (gk125) and the double knockout mtl-1;mtl-2 (zs1). In addition, metal speciation was assessed by X-ray absorption fine-structure spectroscopy. This showed that O-donating, probably phosphate-rich, ligands play a dominant role in maintaining the physiological concentration of zinc, independently of metallothionein status. In contrast, cadmium was shown to coordinate with thiol groups, and the cadmium speciation of the wild-type and the CeMT-2 knockout strain was distinctly different to the CeMT-1 and double knockouts. Taken together, and supported by a simple model calculation, these findings show for the first time that the two MT isoforms have differential affinities towards Cd(II) and Zn(II) at a cellular level, and this is reflected at the protein level. This suggests that the two MT isoforms have distinct in vivo roles.

Metallothioneins: unparalleled diversity in structures and functions for metal ion homeostasis and more

Metallothioneins have been the subject of intense study for five decades, and have greatly inspired the development of bio-analytical methodologies including multi-dimensional and multi-nuclear NMR.With further advancements in molecular biology, protein science, and instrumental techniques, recent years have seen a renaissance of research into metallothioneins. The current report focuses on in vitro studies of so-called class II metallothioneins from a variety of phyla, highlighting the diversity of metallothioneins in terms of structure, biological functions, and molecular functions such as metal ion specificity, thermodynamic stabilities, and kinetic reactivity. We are still far from being able to predict any of these properties, and further efforts will be required to generate the knowledge that will enable a better understanding of what governs the biological and chemical properties of these unusual and intriguing small proteins.

Caenorhabditis elegans metallothionein isoform specificity--metal binding abilities and the role of histidine in CeMT1 and CeMT2

Two metallothionein (MT) isoforms have been identified in the model nematode Caenorhabditis elegans: CeMT1 and CeMT2, comprising two polypeptides that are 75 and 63 residues in length, respectively. Both isoforms encompass a conserved cysteine pattern (19 in CeMT1 and 18 in CeMT2) and, most significantly, as a result of their coordinative potential, CeMT1 includes four histidines, whereas CeMT2 has only one. In the present study, we present a comprehensive and comparative analysis of the metal [Zn(II), Cd(II) and Cu(I)] binding abilities of CeMT1 and CeMT2, performed through spectroscopic and spectrometric characterization of the recombinant metal-MT complexes synthesized for wild-type isoforms (CeMT1 and CeMT2), their separate N- and C-terminal moieties (NtCeMT1, CtCeMT1, NtCeMT2 and CtCeMT2) and a DeltaHisCeMT2 mutant. The corresponding in vitro Zn/Cd- and Zn/Cu-replacement and acidification/renaturalization processes have also been studied, as well as protein modification strategies that make it possible to identify and quantify the contribution of the histidine residues to metal coordination. Overall, the data obtained in the present study are consistent with a scenario where both isoforms exhibit a clear preference for divalent metal ion binding, rather than for Cu coordination, although this preference is more pronounced towards cadmium for CeMT2, whereas it is markedly clearer towards Zn for CeMT1. The presence of histidines in these MTs is revealed to be decisive for their coordination performance. In CeMT1, they contribute to the binding of a seventh Zn(II) ion in relation to the M(II)(6)-CeMT2 complexes, both when synthesized in the presence of supplemented Zn(II) or Cd(II). In CeMT2, the unique C-terminal histidine abolishes the Cu-thionein character that this isoform would otherwise exhibit.

Myosin binding protein C positioned to play a key role in regulation of muscle contraction: structure and interactions of domain C1

Myosin binding protein C (MyBP-C) is a thick filament protein involved in the regulation of muscle contraction. Mutations in the gene for MyBP-C are the second most frequent cause of hypertrophic cardiomyopathy. MyBP-C binds to myosin with two binding sites, one at its C-terminus and another at its N-terminus. The N-terminal binding site, consisting of immunoglobulin domains C1 and C2 connected by a flexible linker, interacts with the S2 segment of myosin in a phosphorylation-regulated manner. It is assumed that the function of MyBP-C is to act as a tether that fixes the S1 heads in a resting position and that phosphorylation releases the S1 heads into an active state. Here, we report the structure and binding properties of domain C1. Using a combination of site-directed mutagenesis and NMR interaction experiments, we identified the binding site of domain C1 in the immediate vicinity of the S1–S2 hinge, very close to the light chains. In addition, we identified a zinc binding site on domain C1 in close proximity to the S2 binding site. Its zinc binding affinity (K_d of approximately 10–20 μM) might not be sufficient for a physiological effect. However, the familial hypertrophic cardiomyopathy-related mutation of one of the zinc ligands, glutamine 210 to histidine, will significantly increase the binding affinity, suggesting that this mutation may affect S2 binding. The close proximity of the C1 binding site to the hinge, the light chains and the S1 heads also provides an explanation for recent observations that (a) shorter fragments of MyBP-C unable to act as a tether still have an effect on the actomyosin ATPase and (b) as to why the myosin head positions in phosphorylated wild-type mice and MyBP-C knockout mice are so different: Domain C1 bound to the S1–S2 hinge is able to manipulate S1 head positions, thus influencing force generation without tether. The potentially extensive extra interactions of C1 are expected to keep it in place, while phosphorylation dislodges the C1–C2 linker and domain C2. As a result, the myosin heads would always be attached to a tether that has phosphorylation-dependent length regulation.