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Plant Physiology 83:628-632 (1987)
© 1987 American Society of Plant Biologists

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Metabolism and Enzymology

Protein Modulase Appears to Be a Complex of Ferredoxin, Ferredoxin/Thioredoxin Reductase, and Thioredoxin 1

Duane Merlin Ford2, Peter P. Jablonski3, A. Habib Mohamed4 and Louise E. Anderson

Department of Biological Sciences, University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680

Protein modulase and ferredoxin/thioredoxin reductase are soluble proteins that have been suggested to catalyze the light-dependent modulation of enzyme activity in the stromal compartment of the chloroplast. Protein modulase is active in vitro without additional ferredoxin and thioredoxin, whereas ferredoxin/thioredoxin reductase requires additional ferredoxin and thioredoxin. We hypothesize that protein modulase is a complex protein composed of ferredoxin/thioredoxin reductase, ferredoxin, and thioredoxin. In reconstituted chloroplast systems, antiserum directed against ferredoxin, at concentrations sufficient to inhibit the photoreduction of NADP, had no effect on light modulation. Antiserum directed against thioredoxin gave variable results: one batch of polyclonal antibodies inhibited light modulation, another was stimulatory, and another was without effect. These results suggest that the ferredoxin and thioredoxin active in light modulation are not free in solution. Furthermore, molecular sieve chromatography of stromal proteins results in the elution of four species that catalyze light modulation. Based on whether or not ferredoxin and/or thioredoxin must be added for activity, these four species have been tentatively identified as protein modulase, a complex of ferredoxin/thioredoxin reductase and ferredoxin, a complex of ferredoxin/thioredoxin reductase and thioredoxin, and ferredoxin/thioredoxin reductase. That is, the four correspond to all the possible combinations of ferredoxin, ferredoxin/thioredoxin reductase, and thioredoxin. We suggest that buffer ionic strength affects the interactions among these proteins and in part determines the fate of the protein modulase complex in vitro.

2 Present address: Northeast Missouri State University, 162 Barnett Hall, Kirksville, MO 63501.

3 Present address: Department of Developmental Biology, Research School of Biological Sciences, The Australian National University, Box 475, P.O. Canberra City, A.C.T. 2601, Australia.

4 Present address: Department of Biochemistry, Baylor College of Medicine, 1200 Moursund Avenue, Houston, TX 77030.

1 Supported by National Science Foundation Grant DBM 84 17081.






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Copyright © 1987 by the American Society of Plant Biologists