Examination of the putative Ca2+-binding site in the light-harvesting complex 1 of thermophilic purple sulfur bacterium Thermochromatium tepidum

Abstract

The core light-harvesting complex (LH1) of purple sulfur photosynthetic bacterium Thermochromatium tepidum exhibits an unusual absorption maximum at 915 nm for the Q (y) transition, and is highly stable when copurified with reaction center (RC) in a LH1-RC complex form. In previous studies, we demonstrated that the calcium ions are involved in both the large red shift and the enhanced thermal stability, and possible Ca(2+)-binding sites were proposed. In this study, we further examine the putative binding sites in the LH1 polypeptides using purified chromatophores. Incubation of the chromatophores in the presence of EDTA revealed no substantial change in the absorption maximum of LH1 Q (y) transition, whereas further addition of detergents to the chromatophores-EDTA solution resulted in a blue-shift for the LH1 Q (y) peak with the final position at 892 nm. The change of the LH1 Q (y) peak to shorter wavelengths was relatively slow compared to that of the purified LH1-RC complex. The blue-shifted LH1 Q (y) transition in chromatophores can be restored to its original position by addition of Ca(2+) ions. The results suggest that the Ca(2+)-binding site is exposed on the inner surface of chromatophores, corresponding to the C-terminal region of LH1. An Asp-rich fragment in the LH1 α-polypeptide is considered to form a crucial part of the binding network. The slow response of LH1 Q (y) transition upon exposure to EDTA is discussed in terms of the membrane environment in the chromatophores.

Ca2+-binding reduces conformational flexibility of RC–LH1 core complex from thermophile Thermochromatium tepidum

“…acidophila as template (McDermott et al 1995), the specific coordination of Ca 2? at the C-terminus of the a-subunit of LH1 has been proposed (Yu et al 2010). Amongst the amino acid atoms to bind the Ca 2?…”

“…Chromatophores were prepared as described previously (Yu et al 2010). In essence, the harvested cells were suspended in Tris-HCl (20 mM, pH 8.5) buffer and exposed to sonic oscillation.…”

Characterization of the quinones in purple sulfur bacterium Thermochromatium tepidum

“…tepidum cells were cultured for seven days. Chromatophores were prepared as previously reported [24]. The chromatophores were suspended in 0.5 mL of 20 mM Tris–HCl (pH 7.5) at concentration of A 850 = 120, and were then lyophilized.…”

A Spectroscopic Variant of the Light-Harvesting 1 Core Complex from the Thermophilic Purple Sulfur Bacterium Thermochromatium tepidum

“…35 Recently, it was revealed that the former is likely, based on a topological study using tepidum chromatophores and metal chelators. 54 The blue shift of the Q y bands due to Ca 2þ depletion by chelators was not induced when the native membranes were completely retained, indicating that the outside of the chromatophore does not include the metal binding site. However, once the membranes were destroyed by detergent treatments, the chelators invaded the inside of the chromatophore where the C-terminus of the LH1 R-polypeptide exists, resulting in a blue shift of the Q y bands and deterioration of the thermal stability.…”