Calcium ions are involved in the unusual red shift of the light-harvesting 1 Q_y transition of the core complex in thermophilic purple sulfur bacterium Thermochromatium tepidum

Thermophilic purple sulfur bacterium, Thermochromatium tepidum, can grow at temperatures up to 58 °C and exhibits an unusual Q_y absorption at 915 nm for the core light-harvesting complex (LH1), an ∼35-nm red shift from those of its mesophilic counterparts. We demonstrate in this study, using a highly purified LH1-reaction center complex, that the LH1 Q_y transition is strongly dependent on metal cations and Ca²⁺ is involved in the unusual red shift. Removal of the Ca²⁺ resulted in formation of a species with the LH1 Q_y absorption at 880 nm, and addition of the Ca²⁺ to the 880-nm species recovered the native 915-nm form. Interchange between the two forms is fully reversible. Based on spectroscopic and isothermal titration calorimetry analyses, the Ca²⁺ binding to the LH1 complex was estimated to occur in a stoichiometric ratio of Ca²⁺/αβ-subunit = 1:1 and the binding constant was in 10⁵ M^-1 order of magnitude, which is comparable with those for EF-hand Ca²⁺-binding proteins. Despite the high affinity, conformational changes in the LH1 complex upon Ca²⁺ binding were small and occurred slowly, with a typical time constant of ∼6 min. Replacement of the Ca²⁺ with other metal cations caused blue shifts of the Q_y bands depending on the property of the cations, indicating that the binding site is highly selective. Based on the amino acid sequences of the LH1 complex, possible Ca²⁺-binding sites are proposed that consist of several acidic amino acid residues near the membrane interfaces of the C-terminal region of the α-polypeptide and the N-terminal region of the β-polypeptide.