Characterization of an isolated chloroplast membrane FeS protein and its identification as the photosystem I FeS_A/FeS_B binding protein

An iron-sulfur protein of approx. 9 kDa has been isolated from spinach chloroplast membranes. Based on iron and sulfide content (∼900 nmol Fe and S^2- per mg protein), optical absorbance and low-temperature EPR spectra, it appears that this protein contains 8Fe and 8S^2- in two [4Fe4S] clusters. The protein cross-reacts with an antibody raised against a 9 kDa PS I subunit previously identified as the psaC (frxA) gene product. This identity was confirmed by the N-terminal amino acid sequence of the isolated FeS protein. It is concluded that the isolated FeS protein binds PS I centers FeS_A and FeS_B and that each of these centers contains 4Fe and 4S^2-.