The peripheral light-harvesting complexes from purple sulfur bacteria have different ‘ring’ sizes

The integral membrane light-harvesting (LH) proteins from purple photosynthetic bacteria form circular oligomers of an elementary unit that is composed of two very hydrophobic polypeptides, termed α and β. These apoprotein dimers are known to associate into closed circular arrays of 8, 9 and 16 α/β-mers. We report the existence of peripheral LH proteins purified from Allochromatium vinosum with two intermediate ring sizes and postulate that one is a 13 α/β-mer. This shows that LH proteins are able to form membrane rings of continuously increasing diameter from 68 to 115 Å. The presence of these new ring sizes warrants further study, as it will help to further validate the structure–function models of LH proteins currently found in the literature.