Effect of methionine80 heme coordination on domain swapping of cytochrome c.

Cytochrome c (cyt c) forms oligomers by domain swapping. It exchanges the C-terminal α-helical region between protomers, and the Met80‒heme iron bond is perturbed significantly in domain-swapped oligomers. The peroxidase activity of cyt c increases by Met80 dissociation from the heme iron, which may trigger apoptosis. This study elucidates the effect of the Met80 heme coordination on cyt c domain swapping by obtaining oligomers for both wild-type (WT) and M80A human cyt c by an addition of ethanol to their monomers, followed by lyophilization and dissolution to buffer, and investigating their dimer properties. The absorption and circular dichroism spectra of WT and M80A cyt c exhibited similar changes upon dimerization, indicating that Met80 does not affect the oligomerization process significantly. According to differential scanning calorimetric measurements, Met80 coordination to the heme iron had an effect on the stabilization of the monomer (ΔH = 16 kcal/mol), whereas no large difference was observed between the dimer-to-monomer dissociation temperatures of WT and M80A cyt c (61.0 °C). The activation enthalpy values were similar and relatively large for the dissociation of both WT and M80A cyt c dimers (WT, 120 ± 10 kcal/mol; M80A, 110 ± 10 kcal/mol), indicating that the dimers suffered large structural changes upon dissociation to monomers independent of the Met80 coordination to the heme iron. These results indicate that cyt c domain swapping may occur regardless of the Met80 coordination, whereas the monomer is stabilized by Met80 but the domain-swapped dimer structure and stability are less affected by the Met80 coordination.

Rice RCN1/OsABCG5 mutation alters accumulation of essential and nonessential minerals and causes a high Na/K ratio, resulting in a salt-sensitive phenotype.

Mineral balance and salt stress are major factors affecting plant growth and yield. Here, we characterized the effects of rice (Oryza sativa L.) reduced culm number1 (rcn1), encoding a G subfamily ABC transporter (OsABCG5) involved in accumulation of essential and nonessential minerals, the Na/K ratio, and salt tolerance. Reduced potassium and elevated sodium in field-grown plants were evident in rcn1 compared to original line 'Shiokari' and four independent rcn mutants, rcn2, rcn4, rcn5 and rcn6. A high Na/K ratio was evident in the shoots and roots of rcn1 under K starvation and salt stress in hydroponically cultured plants. Downregulation of SKC1/OsHKT1;5 in rcn1 shoots under salt stress demonstrated that normal function of RCN1/OsABCG5 is essential for upregulation of SKC1/OsHKT1;5 under salt stress. The accumulation of various minerals in shoots and roots was also altered in the rcn1 mutant compared to 'Shiokari' under control conditions, potassium starvation, and salt and d-sorbitol treatments. The rcn1 mutation resulted in a salt-sensitive phenotype. We concluded that RCN1/OsABCG5 is a salt tolerance factor that acts via Na/K homeostasis, at least partly by regulation of SKC1/OsHKT1;5 in shoots.