In Vitro Synthesis and Membrane Assembly of Photosynthetic Polypeptides from Rhodobacter Capsulatus

Abstract

We have used the facultatively photoheterotrophic bacterium Rhodobacter capsulatus to study the biogenesis of polymeric membrane proteins. Upon lowering the oxygen tension or the light intensity the synthesis of the membrane-located photosynthetic apparatus is induced. This is accompanied by a drastic increase in the surface of the plasma membrane leading to the development of the so-called intracytoplasmic membranes (ICM) (for a recent review see Drews, 1985; Kiley and Kaplan, 1988). The photosynthetic apparatus is organized into two light-harvesting complexes, B870 (LH-I) and B800-850 (LH-II), and a reaction center (RC). Each LH-complex consists of two pigment-binding proteins in a 1:1 stoichiometry: B870 α and α (Mr = 12 and 7 kDa), and B800-850 α and β (Mr = 10 and 8 kDa). The B800-850 complex contains in addition the non-pigment-binding protein γ (Mr = 14 kDa). Each of these α and β peptides has one α-helical transmembrane domain of approximately 20 amino acids with an N-terminus facing the cytoplasm and a C-terminus located in the periplasmic space (Tadros et al. 1984; 1985; 1987). The RC consists of the two pigment-binding proteins L and M (Mr = 20.5 and 24 kDa) each with five membrane-spanning domains, and the non-pigment-binding protein H (Mr = 28 kDa), which is anchored within the membrane by a single hydrophobic stretch. The three RC-proteins are found in the ICM in a 1:1:1 stoichiometry with their N-termini located in the cytoplasm (Tadros et al. 1987).