Living target of Ce(III) action on horseradish cells: proteins on/in cell membrane.

Positive and negative effects of rare earth elements (REEs) in life have been reported in many papers, but the cellular mechanisms have not been answered, especially the action sites of REEs on plasma membrane are unknown. Proteins on/in the plasma membrane perform main functions of the plasma membrane. Cerium (Ce) is the richest REEs in crust. Thus, the interaction between Ce(III) and the proteins on/in the plasma membrane, the morphology of protoplast, and the contents of nutrient elements in protoplast of horseradish were investigated using the optimized combination of the fluorescence microscopy, fluorescence spectroscopy, circular dichroism, scanning electron microscopy, and X-ray energy dispersive spectroscopy. It was found that Ce(III) at the low concentrations (10, 30 µM) could interact with proteins on/in the plasma membrane of horseradish, leading to the improvement in the structure of membrane proteins and the plasma membrane, which accelerated the intra-/extra-cellular substance exchange and further promoted the development of cells. When horseradish was treated with Ce(III) at the high concentrations (60, 80 µM), Ce(III) also could interact with the proteins on/in the plasma membrane of horseradish, leading to the destruction in the structure of membrane proteins and the plasma membrane. These effects decelerated the intra-/extra-cellular substance exchange and further inhibited the development of cells. Thus, the interaction between Ce(III) and proteins on/in the plasma membrane in plants was an important reason of the positive and negative effects of Ce(III) on plants. The results would provide some references for understanding the cellular effect mechanisms of REEs on plants.

Interaction between La(III) and proteins on the plasma membrane of horseradish.

Lanthanum (La) is an important rare earth element in the ecological environment of plant. The proteins on the plasma membrane control the transport of molecules into and out of cell. It is very important to investigate the effect of La(III) on the proteins on the plasma membrane in the plant cell. In the present work, the interaction between La(III) and proteins on the plasma membrane of horseradish was investigated using optimization of the fluorescence microscopy and fluorescence spectroscopy. It is found that the fluorescence of the complex system of protoplasts and 1-aniline Kenai-8-sulfonic acid in horseradish treated with the low concentration of La(III) is increased compared with that of the control horseradish. The opposite effect is observed in horseradish treated with the high concentration of La(III). These results indicated that the low concentration of La(III) can interact with the proteins on the plasma membrane of horseradish, causing the improvement in the structure of proteins on the plasma membrane. The high concentration of La(III) can also interact with the proteins on the plasma membrane of horseradish, leading to the destruction of the structure of proteins on the plasma membrane. We demonstrate that the proteins on the plasma membrane are the targets of La(III) action on plant cell.

Structural basis for the decrease in the outward potassium channel current induced by lanthanum.

The current of the outward K+ channel in the cell of horseradish treated with La3+ and the direct interaction between La3+ and the K+ channel protein were investigated using the whole-cell patch-clamp technique, molecular dynamics simulation, and quantum chemistry calculation methods. It was found for the first time that La3+ decreases the current of the K+ channel in the horseradish mesophyll cell. The decrease results from the formation of a coordination bond and hydrogen bond between La3+ and the K+ channel protein in the plasma membrane. The direct interaction destroys the native structure of the K+ channel protein, disturbing the function of the K+ channel protein in the cells. The results can provide the theoretical foundation for understanding the interaction between metal ions (especially high-valence metal ions) and the channel protein in organisms, including animal and plant cells.

Molecular and cellular mechanism of the effect of La(III) on horseradish peroxidase.

Horseradish is an important economic crop. It contains horseradish peroxidase (HRP) and lots of nutrients, and has specific pungency. Lanthanum is one of the heavy metals in the environment. It can transfer through the food chain to humans. In this paper, the molecular and cellular mechanism of the toxic effects of La(III) on HRP in vivo was investigated with an optimized combination of biophysical, biochemical, and cytobiological methods. It was found that La(III) could interact with O and/or N atoms in the backbone/side chains of the HRP molecule in the cell membrane of horseradish treated with 80 microM La(III), leading to the formation of a new complex of La and HRP (La-HRP). The formation of the La-HRP complex causes the redistribution of the electron densities of atoms in the HRP molecule, especially the decrease in the electron density of the active center, Fe(III), in the heme group of the La-HRP molecule compared with the native HRP molecule in vivo. Therefore, the electron transfer and the activity of HRP in horseradish treated with 80 microM La(III) are obviously decreased compared with those of the native HRP in vivo. This is a possible molecular and cellular mechanism for the toxic effect of La(III) on HRP in vivo. It is suggested that the accumulation of La in the environment, especially the formation of the La-HRP complex in vivo, is harmful to organisms.