Analysis of the role of Mg²⁺ on conformational change and target recognition by ciliate Euplotes octocarinatus centrin

Distinct Calcium Binding and Structural Properties of Two Centrin Isoforms from Toxoplasma gondii

Centrins are calcium (Ca2+)-binding proteins that have been implicated in several regulatory functions. In the protozoan parasite Toxoplasma gondii, the causative agent of toxoplasmosis, three isoforms of centrin have been identified. While increasing information is now available that links the function of centrins with defined parasite biological processes, knowledge is still limited on the metal-binding and structural properties of these proteins. Herein, using biophysical and structural approaches, we explored the Ca2+ binding abilities and the subsequent effects of Ca2+ on the structure of a conserved (TgCEN1) and a more divergent (TgCEN2) centrin isoform from T. gondii. Our data showed that TgCEN1 and TgCEN2 possess diverse molecular features, suggesting that they play nonredundant roles in parasite physiology. TgCEN1 binds two Ca2+ ions with high/medium affinity, while TgCEN2 binds one Ca2+ with low affinity. TgCEN1 undergoes significant Ca2+-dependent conformational changes that expose hydrophobic patches, supporting a role as a Ca2+ sensor in toxoplasma. In contrast, Ca2+ binding has a subtle influence on conformational features of TgCEN2 without resulting in hydrophobic exposure, suggesting a different Ca2+ relay mode for this isoform. Furthermore, TgCEN1 displays a Ca2+-dependent ability to self-assemble, while TgCEN2 did not. We discuss our findings in the context of Ca2+ signaling in toxoplasma.

Crystal structure of the trimeric N-terminal domain of ciliate Euplotes octocarinatus centrin binding with calcium ions

Centrin is a member of the EF-hand superfamily of calcium-binding proteins, a highly conserved eukaryotic protein that binds to Ca²⁺ . Its self-assembly plays a causative role in the fiber contraction that is associated with the cell division cycle and ciliogenesis. In this study, the crystal structure of N-terminal domain of ciliate Euplotes octocarinatus centrin (N-EoCen) was determined by using the selenomethionine single-wavelength anomalous dispersion method. The protein molecules formed homotrimers. Every protomer had two putative Ca²⁺ ion-binding sites I and II, protomer A, and C bound one Ca²⁺ ion, while protomer B bound two Ca²⁺ ions. A novel binding site III was observed and the Ca²⁺ ion was located at the center of the homotrimer. Several hydrogen bonds, electrostatic, and hydrophobic interactions between the protomers contributed to the formation of the oligomer. Structural studies provided insight into the foundation for centrin aggregation and the roles of calcium ions.

Modulation of XPC peptide on binding Tb3+ to Euplotes octocarinatus centrin

Centrins are Ca²⁺-binding proteins found throughout eukaryotic organisms. Xeroderma pigmentosum group C protein (XPC), a dominant component of the nuclear excision repair (NER) pathway, is a critical target protein of centrins. A 22-residue peptide (K842-R863) from XPC was used to investigate the effect of metal ions (Ca²⁺ and Tb³⁺) on the peptide binding of Euplotes octocarinatus centrin (EoCen) by isothermal titration calorimetry (ITC) and fluorescence spectroscopy. ITC and tryptophan spectrofluorimetric titrations revealed that metal ions (Ca²⁺ and Tb³⁺) could enhance the affinity between EoCen and the XPC peptide, and the enhanced effects were closely related to the ion potential of metal ions. Since the ion potential of Tb³⁺ (e/r = 0.0325) is larger than that of Ca²⁺ (e/r = 0.0202), the conformational change in the protein induced by Tb³⁺ is larger than that induced by Ca²⁺, and the enhanced affinity of Tb³⁺ is stronger than that of Ca²⁺. This interaction was driven by enthalpy in the presence of EDTA and enthalpy and entropy in the presence of Ca²⁺ or Tb³⁺. Similar to that observed in the presence of EDTA, the N-terminal domain did not participate in the interaction with the XPC peptide even in the presence of metal ions. Resonance light scattering (RLS) and the band shift in native polyacrylamide gel electrophoresis (PAGE) suggested that peptide binding resulted in the dissociation of EoCen aggregates and complex formation via the monomer-peptide form. Tb³⁺-Sensitized emission suggested that peptide binding in turn also had an impact on the Tb³⁺ binding of the protein: the C-terminal domain was slightly strengthened and the N-terminal domain was weakened about 225 fold. RLS and native PAGE indicated that the self-assembly induced by Tb³⁺ binding to the N-terminal domain of EoCen was inhibited in the presence of the XPC peptide. This study elucidates the molecular mechanism of EoCen function in the cellular context.