C-terminal cleavage of the LH1 [alpha]-polypeptide in the Sr.sup.2+-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.sub.y absorption at 915 nm and enhanced thermostability. Ca.sup.2+ can be biosynthetically replaced by Sr.sup.2+ in growing cultures of Tch. tepidum. However, the resulting Sr.sup.2+-substituted LH1-RC complexes in such cells do not display the absorption maximum and thermostability of those from Ca.sup.2+-grown cells, signaling that inherent structural differences exist in the LH1 complexes between the Ca.sup.2+- and Sr.sup.2+-cultured cells. In this study, we examined the effects of the biosynthetic Sr.sup.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 [alpha]-polypeptide was observed for the Sr.sup.2+-cultured cells. A proportion of the truncated LH1 [alpha]-polypeptide increased during repeated subculturing in the Sr.sup.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.sup.2+ substitutes for Ca.sup.2+ in the LH1 complex. Limited proteolysis of the native Ca.sup.2+-LH1 complex with lysyl protease revealed selective truncations at the Lys residues in both C- and N-terminal extensions of the [alpha]- and [beta]-polypeptides. The spectral properties and thermostability of the partially digested native LH1-RC complexes were similar to those of the biosynthetically Sr.sup.2+-substituted LH1-RC complexes in their Ca.sup.2+-bound forms. Based on these findings, we propose that the C-terminal domain of the LH1 [alpha]-polypeptide plays important roles in retaining proper structure and function of the LH1-RC complex in Tch. tepidum.