Trimeric structure and other properties of the chloroplast reductase binding protein

A complex between the ferredoxin-NADP + reductase and a 17.5 kDa polypeptide was isolated from lettuce chloroplasts by the same procedure used previously in spinach [(1984) J. Biol. Chem. 259, 8048-8051]. The stoichiometry of the complex from both sources was determined by quantification of stained protein bands after gel electrophoresis and, in the case of the spinach complex, by rocket inmunolectrophoresis. The results suggest that the binding protein is a trimer of the 17.5 kDa polypeptide. The amino arid composition of the spinach reductase binding protein and partial sequencing of its tryptic peptides clearly show that it is not identical to the 17 kDa polypeptide of the cytochrome b 6-ƒ complex.