The localization of pigment-binding polypeptides in membranes of Rhodopseudomonas viridis

Abstract Inside-out and right-side out vesicles were isolated from the intracytoplasmic membrane system of the photosynthetic bacterium Rhodopseudomonas viridis and treated with proteinase K. Afterwards the pigment-binding proteins of the photosynthetic apparatus were extracted from the membrane, purified and the N- and C-terminal amino acyl sequences determined.
Forty-eight amino acids were found to be removed from the N-terminal domain of the M-subunit and twenty-eight amino acids split off the L-subunit of reaction center when inside-out vesicles were digested with proteinase K.
Six amino acids of the N-terminal region of the beta polypeptide of the light-harvesting complex B1020 were removed when inside-out vesicles were treated with proteinase K. The N-terminal domains of alpha and gamma polypeptides of the antenna complex B1020 were not cleaved by proteinase K either in right-side out or in inside-out vesicles. It is concluded that the N-terminal domains of M-, L- and β-subunits are exposed and accessible to proteinase K on the cytoplasmic surface of the membrane. This is in agreement with results obtained with other photosynthetic bacteria. The orientation of the other light-harvesting polypeptides is discussed.