Abstract
Thermochromatium tepidum is a thermophilic purple sulfur photosynthetic bacterium collected from the Mammoth Hot Springs, Yellowstone National Park. A previous study showed that the light-harvesting-reaction center core complex (LH1-RC) purified from this bacterium is highly stable at room temperature (Suzuki, H., Hirano, Y., Kimura, Y., Takaichi, S., Kobayashi, M., Miki, K., and Wang, Z.-Y. (2007) Biochim. Biophys. Acta 1767, 1057-1063). In this work, we demonstrate that thermal stability of the Tch. tepidum LH1-RC is much higher than that of its mesophilic counterparts, and the enhanced thermal stability requires Ca2+ as a cofactor. Removal of the Ca2+ from Tch. tepidum LH1-RC resulted in a complex with the same degree of thermal stability as that of the LH1-RCs purified from mesophilic bacteria. The enhanced thermal stability can be restored by addition of Ca2+ to the Ca2+-depleted LH1-RC, and this process is fully reversible. Interchange of the thermal stability between the two forms is accompanied by a shift of the LH1 Qy transition between 915 nm for the native and 880 nm for the Ca2+-depleted LH1-RC. Differential scanning calorimetry measurements reveal that degradation temperature of the native LH1-RC is 15 degrees C higher and the enthalpy change is about 28% larger than the Ca2+-depleted LH1-RC. Substitution of the Ca2+ with other metal cations caused a decrease in thermal stability of an extent depending on the properties of the cations. These results indicate that Ca2+ ions play a dual role in stabilizing the structure of the pigment-membrane protein complex and in altering its spectroscopic properties, and hence provide insight into the adaptive strategy of this photosynthetic organism to survive in extreme environments using natural resources.
Highlights
Lowstone National Park and can grow anaerobically at optimum temperatures of 48 –50 °C with an upper limit of 58 °C [1]
We reported in a previous work that strong interaction exists between the Tch. tepidum reaction center (RC) and its surrounding core lightharvesting 1 (LH1) complex [13], providing evidence for a subsequent proposal that such interaction could enhance the stability of the so-called LH1-RC core complex [12]
The 879-nm band decreased, and a new band at 775.5 nm appeared with an isosbestic point at 802 nm. This indicates that the Ca2ϩ-depleted B880 complex is thermally more unstable than the B915, and the LH1 complex was directly decomposed into monomeric forms of the BChl a-bound ␣- and -polypeptides upon the degradation
Summary
Lowstone National Park and can grow anaerobically at optimum temperatures of 48 –50 °C with an upper limit of 58 °C [1]. There are three arginine residues in the Tch. tepidum RC [10], which are not present in the RCs of other mesophiles These basic residues are located at the membrane interface, as revealed by the high resolution crystal structure [11], and were postulated to contribute to the RC stability. Ca2؉-binding Enhances Thermostability of Tch. tepidum LH1-RC the strong interaction with the LH1 complex [14]. We present experimental evidence that the Ca2ϩ is required for the enhanced thermal stability of the Tch. tepidum LH1-RC complex. The roles of Ca2ϩ in the thermal stability of the Tch. tepidum LH1-RC complex are discussed in relation to several known factors responsible for the enhanced stability of proteins from thermophiles. The results of this study will provide insight into how the LH1-RC complex of this thermophilic organism was adapted to the growing environment at elevated temperatures by utilizing natural resources
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