Isolation and identification of a fourth subunit in the membrane part of the chloroplast ATP-synthase

The subunit composition of highly active purified ATP-synthase from chloroplasts, CF₀F₁, was investigated by SDS gel electrophoresis. An additional subunit of CF₀, was detected with an apparent molecular mass of 20 kDa. It is stained weakly with Coomassie blue but very strongly with silver. This subunit was isolated on a preparative SDS gel and the N-terminal amino acid sequence analyzed. It shows that the 20 kDa protein is identical with the protein encoded by the spinach chloroplast gene atpI, called subunit IV [(1986) Mol. Genet. 203, 117-128]. However, in comparison to the gene-derived sequence, the first 18 amino acids are missing, indicating N-terminal processing.