BPS2026 – Structural stability of peripheral light-harvesting complexes of purple photosynthetic bacteria

Purple photosynthetic bacteria have LH2 as a peripheral light-harvesting protein. In this protein, BChl α and carotenoid are circularly organized in the scaffold of polypeptides. LH2 possesses two types of bacteriochlorophyll (BChl) α , termed B800 and B850, which exhibit Q y absorption bands at around 800 and 850 nm, respectively. Light energy captured by B800 BChl α is efficiently transferred to B850 BChl α in LH2. However, little information is available about effects of pigments and polypeptides on the structural stability of LH2. In this study, we analyzed an LH2 variant denoted as B800-free LH2, in which B800 BChl α was removed from native LH2, in order to clarify effects of B800 BChl α on the thermal stability of LH2 structure. Furthermore, we compared LH2 with another peripheral light-harvesting protein LH3, which has different carotenoids and amino acid sequences of polypeptides from LH2, in order to clarify effects of these components on the structural stability. Native LH2 and LH3 were isolated from the cultured cells of purple photosynthetic bacteria. B800-free LH2 was obtained by incubation of native LH2 under acidic pH conditions. B800-free LH2 exhibited unique spectral changes under neutral pH conditions, in which the Q y absorption band of B800 BChl α was automatically recovered. This spectral change accompanied a decrease of the Q y absorption band at 850 nm and tended to stop when the absorbance of B800/B850 ratio was close to that of native LH2. B800-recovered LH2 was purified and its structural and spectroscopic features were characterized. As a result, the features of B800-recovered LH2 were quite similar to those of native LH2. These results indicate that BChl α released by decomposition of B800-free LH2 inserts into the B800 cavity of survived B800-free LH2.