Oxidizing side of the cyanobacterial photosystem I. Evidence for interaction between the electron donor proteins and a luminal surface helix of the PsaB subunit.
Photosystem I (PSI) interacts with plastocyanin or cytochrome c6 on the luminal side. To identify sites of interaction between plastocyanin/cytochrome c6 and the PSI core, site-directed mutations were generated in the luminal J loop of the PsaB protein from Synechocystis sp. PCC 6803. The eight mutant strains differed in their photoautotrophic growth. Western blotting with subunit-specific antibodies indicated that the mutations affected the PSI level in the thylakoid membranes. PSI proteins could not be detected in the S600R/G601C/N602I, N609K/S610C/T611I, and M614I/G615C/W616A mutant membranes. The other mutant strains contained different levels of PSI proteins. Among the mutant strains that contained PSI proteins, the H595C/L596I, Q627H/L628C/I629S, and N638C/N639S mutants showed similar levels of PSI-mediated electron transfer activity when either cytochrome c6 or an artificial electron donor was used. In contrast, cytochrome c6 could not function as an electron donor to the W622C/A623R mutant, even though the PSI activity mediated by an artificial electron donor was detected in this mutant. Thus, the W622C/A623R mutation affected the interaction of the PSI complex with cytochrome c6. Biotin-maleimide modification of the mutant PSI complexes indicated that His-595, Trp-622, Leu-628, Tyr-632, and Asn-638 in wild-type PsaB may be exposed on the surface of the PSI complex. The results presented here demonstrate the role of an extramembrane loop of a PSI core protein in the interaction with soluble electron donor proteins.
Sun
J
J
Department of Biochemistry and Biophysics, Iowa State University, Ames, Iowa 50011, USA.
Xu
W
W
Hervás
M
M
Navarro
J A
JA
Rosa
M A
MA
Chitnis
P R
PR
D000458:Cyanobacteria; D004269:DNA, Bacterial / Q000737:chemistry; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics / Q000378:metabolism*; D045331:Photosystem I Protein Complex; D015183:Restriction Mapping
D004269:DNA, Bacterial; D008565:Membrane Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D045331:Photosystem I Protein Complex; C089909:photosystem I, psaB subunit