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Further Characterization of the in vitro Binding of Phytochrome to a Membrane Fraction Enriched for Mitochondria

Thomas E. Cedel and Stanley J. Roux
Plant Physiology
Vol. 66, No. 4 (Oct., 1980), pp. 696-703
Stable URL: http://www.jstor.org/stable/4266469
Page Count: 8
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Further Characterization of the in vitro Binding of Phytochrome to a Membrane Fraction Enriched for Mitochondria
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Abstract

This study employs 125I-labeled phytochrome (125I-P) from oats to quantitate the binding of phytochrome to a membrane fraction from oats that is highly enriched for mitochondria, and it examines several parameters that influence this attachment. The binding of 125I-Pfr to the mitochondrial fraction of unirradiated oat seedlings is significantly higher than that of 125I-Pr. However, 125I-Pfr and 125I-Pr bind in equal quantities to mitochondrial preparations isolated from light-exposed seedlings. Maximum 125I-Pfr binding to membranes from light-exposed plants occurs within 30 seconds and is optimized in a reaction buffer containing 5 millimolar MgCl2 at pH 6.8. Scatchard plots of the binding data for Pfr indicate a single high-affinity site with an affinity constant of 1.79 × 1011 per molar. When optimal binding conditions are used, over 20% of the 125I-P added is bound and a stoichiometry of about 100 molecules per mitochondrion is attained. When the specificity of binding is tested using competition experiments with a 15-fold excess of unlabeled phytochrome, 125I-Pfr shows no specific binding to rat liver mitochondria.

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