C-terminal cleavage of the LH1 α-polypeptide in the Sr2+-cultured Thermochromatium tepidum

The light-harvesting 1 reaction center (LH1-RC) complex in the thermophilic purple sulfur bacterium Thermochromatium ( Tch .) tepidum binds Ca ions as cofactors, and Ca-binding is largely involved in its characteristic Q y absorption at 915 nm and enhanced thermostability. Ca 2+ can be biosynthetically replaced by Sr 2+ in growing cultures of Tch. tepidum . However, the resulting Sr 2+ -substituted LH1-RC complexes in such cells do not display the absorption maximum and thermostability of those from Ca 2+ -grown cells, signaling that inherent structural differences exist in the LH1 complexes between the Ca 2+ - and Sr 2+ -cultured cells. In this study, we examined the effects of the biosynthetic Sr 2+ -substitution and limited proteolysis on the spectral properties and thermostability of the Tch. tepidum LH1-RC complex. Preferential truncation of two consecutive, positively charged Lys residues at the C-terminus of the LH1 α -polypeptide was observed for the Sr 2+ -cultured cells. A proportion of the truncated LH1 α -polypeptide increased during repeated subculturing in the Sr 2+ -substituted medium. This result suggests that the truncation is a biochemical adaptation to reduce the electrostatic interactions and/or steric repulsion at the C-terminus when Sr 2+ substitutes for Ca 2+ in the LH1 complex. Limited proteolysis of the native Ca 2+ -LH1 complex with lysyl protease revealed selective truncations at the Lys residues in both C- and N-terminal extensions of the α - and β -polypeptides. The spectral properties and thermostability of the partially digested native LH1-RC complexes were similar to those of the biosynthetically Sr 2+ -substituted LH1-RC complexes in their Ca 2+ -bound forms. Based on these findings, we propose that the C-terminal domain of the LH1 α -polypeptide plays important roles in retaining proper structure and function of the LH1-RC complex in Tch. tepidum .