The eight-amino acid internal loop of PSI-C mediates association of low molecular mass iron-sulfur proteins with the P700-FX 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-Ccore) and one where the loop replace the C-terminal extension (PSI-CcoreLc-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-FX cores lacking PSI-C, -D, and-E. Western blotting showed that PSI-CcoreLc-term binds to PS I, whereas PSI-Ccore does not. Without PSI-D the PSI-CcoreLc-term mutant accepts electrons from FX in contrast to PSI-C mutants without the loop. Flash photolysis of P700-FX cores reconstituted with C. pasteurianum ferredoxin showed that only the ferredoxin mutants with the loop accepted electrons from FX. 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.
Plant Biochemistry Laboratory, Department of Plant Biology, The Royal Veterinary and Agricultural University, 40 Thorvaldsensvej, DK 1871 Frederiksberg C, Copenhagen, Denmark.
H
H
Naver
M P
MP
Scott
J H
JH
Golbeck
C E
CE
Olsen
H V
HV
Scheller
D000595:Amino Acid Sequence; D015153:Blotting, Western; D002734:Chlorophyll / Q000378:metabolism*; D003013:Clostridium; D004578:Electron Spin Resonance Spectroscopy; D005288:Ferredoxins / Q000378:metabolism*; D001467:Hordeum; D045342:Light-Harvesting Protein Complexes; D046911:Macromolecular Substances; D008565:Membrane Proteins; D008969:Molecular Sequence Data; D008970:Molecular Weight; D009249:NADP / Q000737:chemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism*; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D011506:Proteins / Q000378:metabolism*; D013054:Spectrophotometry, Atomic
D005288:Ferredoxins; D045342:Light-Harvesting Protein Complexes; D046911:Macromolecular Substances; D008565:Membrane Proteins; C113690:PSI-C protein, Oryza sativa; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; D010940:Plant Proteins; D011506:Proteins; C089909:photosystem I, psaB subunit; D002734:Chlorophyll; D009249:NADP; C001785:chlorophyll P 700