The eight-amino acid internal loop of PSI-C mediates association of low molecular mass iron-sulfur proteins with the P700-F(X) core in photosystem I

The PSI-C subunit of photosystem I (PS I) shows similarity to soluble 2[4Fe-4S] ferredoxins. PSI-C contains an eight residue internal loop and a 15 residue C-terminal extension which are absent in the ferredoxins. The eight- residue loop has been shown to interact with PSI-A/PSI-B (Naver, H., Scott, M. P., Golbeck, J. H., Moller, B. L., and Scheller, H. V. (1996) J. Biol. Chem. 271, 8996-9001). Four mutant proteins were constructed. Two were modified barley PSI-C proteins, one lacking the loop and the C terminus (PSI- C(core)) and one where the loop replace the C-terminal extension (PSI- C(core)L(c-term)). Two were modified Clostridium pasteurianum ferredoxins, one with the loop of barley PSI-C and one with both the loop and the C terminus of PSI-C. Wild-type proteins and the mutants were used to reconstitute barley P700-F(X) cores lacking PSI-C, -D, and -E. Western blotting showed that PSI-C(core)L(c-term) binds to PS I, whereas PSI-C(core) does not. Without PSI-D the PSI-C(core)L(c-term) mutant accepts electrons from F(X) in contrast to PSI-C mutants without the loop. Flash photolysis of P700-F(X) cores reconstituted with C. pasteurianum ferredoxin showed that only the ferredoxin mutants with the loop accepted electrons from F(X). From this, it is concluded that the loop of PSI-C is necessary and sufficient for the association between PS I and PSI-C, and that the loop is functional as an interaction domain even when positioned at the C terminus of PSI-C or on a low molecular mass, soluble ferredoxin.