INCORPORATION OF LIGHT-HARVESTING COMPLEX-I ALPHA-POLYPEPTIDE AND BETA-POLYPEPTIDE INTO THE INTRACYTOPLASMIC MEMBRANE OF RHODOBACTER-CAPSULATUS

The light-harvesting complex I (LHI) of Rhodobacter capsulatus is an oligomer of basic subunits each consisting of the two different pigment-binding polypeptides LHI-alpha and LHI-beta, encoded by the pufA (LHI-alpha) and pufB (LHI-beta) genes. Pulse-labeling experiments showed that in the presence of the LHI-alpha polypeptide, the LHI-beta polypeptide was inserted earlier into the intracytoplasmic membrane than was the LHI-alpha polypeptide. Each of the pufA and pufB genes was deleted to test whether the LHI-alpha and beta-polypeptides, respectively, are inserted into the intracytoplasmic membrane independently of the LHI partner polypeptide. Neither deletion mutant strain formed the LHI antenna, but a functional reaction center complex was present. Pulse-labeling experiments indicated that the LHI-beta polypeptide was inserted into the intracytoplasmic membrane with the same kinetics and in the same amounts regardless of whether the LHI-alpha polypeptide was present. However, the LHI-beta polypeptide did not accumulate in the membrane in the absence of the LHI-alpha protein but was degraded linearly within about 12 min. In contrast to the LHI-beta protein, only trace amounts of the LHI-alpha polypeptide were inserted into or attached to the membrane if the LHI-beta polypeptide was not synthesized.