Purification and characterization of the polypeptides of core light-harvesting complexes from purple sulfur bacteria

Although the polypeptides of core light-harvesting complexes (LH1) from many purple nonsulfur bacteria have been well characterized, little information is available on the polypeptides of LH1 from purple sulfur photosynthetic organisms. We present here the results of isolation and characterization of LH1 polypeptides from two purple sulfur bacteria, Thermochromatium (Tch.) tepidum and Allochromatium (Ach.) vinosum. Native LH1 complexes were extracted and purified in a reaction center (RC)-associated form with the Qy absorption at 914 nm and 889 nm for Tch. tepidum and Ach. vinosum, respectively. Three components were confirmed from reverse-phase HPLC for the LH1 apopolypeptides of Tch. tepidum. The beta-polypeptide was found to be methylated at N-terminus, and two alpha-polypeptides were identified with one of them being modified by a formyl group at the N-terminal methionine residue. Two alpha- and two beta-polypeptides were confirmed for the LH1 complex of Ach. vinosum, and their primary structures were precisely determined. Homologous and hybrid reconstitution abilities were examined using bacteriochlorophyll a and separated alpha- and beta-polypeptides. The beta-polypeptide from Tch. tepidum was capable of forming uniform structural subunit not only with the alpha-polypeptide of Tch. tepidum but also with the alpha-polypeptide from a nonsulfur bacterium Rhodospirillum rubrum. The alpha-polypeptide alone or beta-polypeptide alone appeared only to result in incomplete subunits in the reconstitution experiments.