REDOX-CONTROLLED, INVIVO AND INVITRO PHOSPHORYLATION OF THE ALPHA-SUBUNIT OF THE LIGHT-HARVESTING COMPLEX-I IN RHODOBACTER-CAPSULATUS

Labelling of Rhodobacter capsulatus cells with (P-32)Pi in a phototrophic culture results in phosphorylation of a membrane-bound polypeptide identified as the alpha subunit of the LHI antenna complex of the photosynthetic apparatus. Phosphorylation of the same polypeptide was also observed by incubation of chromatophores with (P-32)ATP or under conditions of photophosphorylation with ADP and (P-32)Pi. The identity of the phosphorylated LHI-alpha subunit was demonstrated by N-terminal protein sequencing of the phosphorylated polypeptide and by failure of labelling in LHI-defective mutants. Pre-aeration of the samples or addition of the oxidant potassium ferricyanide stimulated the kinase activity whereas the presence of soluble cytoplasmic proteins impaired phosphorylation in an in vitro assay. No effect resulted from addition of reductants to the assay medium. The results indicate the presence of a membrane-bound protein kinase in R. capsulatus that phosphorylates the alpha subunit of the LHI antenna complex under redox control.