| Autor: |
Chitnis, Vaishali P., Jung, Yean-Sung, Albee, Lee, Golbeck, John H., Chitnis, Parag R. |
| Zdroj: |
Journal of Biological Chemistry; May 1996, Vol. 271 Issue: 20 p11772-11780, 9p |
| Abstrakt: |
The ADC4 mutant of the cyanobacterium Synechocystissp. PCC 6803 was studied to determine the structural and functional consequences of the absence of PsaD in photosystem I. Isolated ADC4 membranes were shown to be deficient in ferredoxin-mediated NADP25reduction, even though charge separation between P700 and FA/FBoccurred with high efficiency. Unlike the wild type, FBbecame preferentially photoreduced when ADC4 membranes were illuminated at 15 K, and the EPR line shapes were relatively broad. Membrane fragments oriented in two dimensions on thin mylar films showed that the gtensor axes of FA-and FB-were identical in the ADC4 and wild type strains, implying that PsaC is oriented similarly on the reaction center. PsaC and the FA/FBiron-sulfur clusters are lost more readily from the ADC4 membranes after treatment with Triton X-100 or chaotropic agents, implying a stabilizing role for PsaD. The specific role of Lys106of PsaD, which can be cross-linked to Glu93of ferredoxin (Lelong et al.(1994) J. Biol. Chem.269, 10034-10039), was probed by site-directed mutagenesis. Chemical cross-linking and protease treatment experiments did not reveal any drastic alterations in the conformation of the mutant PsaD proteins. The EPR spectra of FAand FBin membranes of the Lys106mutants were similar to those of the wild type. Membranes of all Lys106mutants showed wild type rates of flavodoxin reduction and flavodoxin-mediated NADP25reduction, but had 10-54% decrease in the ferredoxin-mediated NADP25reduction rates. This implies that Lys106is a dispensable component of the docking site on the reducing side of photosystem I and an ionic interaction between Lys106of PsaD and Glu93of ferredoxin is not essential for electron transfer to ferredoxin. These results demonstrate that PsaD serves distinct roles in modulating the EPR spectral characteristics of FAand FB, in stabilizing PsaC on the reaction center, and in facilitating ferredoxin-mediated NADP25photoreduction on the reducing side of photosystem I. |
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