Incorporation of light-harvesting complex I α and β polypeptides into the intracytoplasmic membrane of Rhodobacter capsulatus

Abstract

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 α and LHI β, encoded by the pufA (LHI α) and pufB (LHI β) genes. Pulse-labeling experiments showed that in the presence of the LHI α polypeptide, the LHI β polypeptide was inserted earlier into the intracytoplasmic membrane than was the LHI α polypeptide. Each of the pufA and pufB genes was deleted to test whether the LHI α and β 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 β polypeptide was inserted into the intracytoplasmic membrane with the same kinetics and in the same amounts regardless of whether the LHI α polypeptide was present. However, the LHI β polypeptide did not accumulate in the membrane in the absence of the LHI α protein but was degraded linearly within about 12 min. In contrast to the LHI β protein, only trace amounts of the LHI α polypeptide were inserted into or attached to the membrane if the LHI β polypeptide was not synthesized.