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Phytochrome structure and GTP-binding proteins
Abstract
Phytochromes are a family of photoreceptors that regulate plant growth and development. The relative extent of chromophore exposure of the red-absorbing (P$\sb{\rm r}$) and far-red-absorbing (P$\sb{\rm fr}$) forms of 124-kDa oat phytochrome and the secondary structure of the phytochrome apoprotein have been investigated using zinc-induced modification of the phytochrome chromophore. Circular dichroism (CD) analyses revealed that upon the addition of a 1:1 molar ratio of zinc ions to phytochrome, a dramatic change in the CD of the P$\sb{\rm fr}$ form is observed, while the CD spectrum of the P$\sb{\rm r}$ form is unaffected. Analysis of the bleached P$\sb{\rm fr}$ CD spectrum reveals that chelation with zinc ions significantly alters the secondary structure of the phytochrome molecule. It is proposed that chelation with zinc ions at the phytochrome chromophore cyclizes the semiextended chromophore, which reduces/eliminates the interactive forces between the chromophore and the polypeptide. As part of testing the hypothesis that phytochrome may interact with signal-transducing GTP-binding proteins, a 24 kDa GTP-binding protein was isolated and characterized from dark grown oat seedlings. The protein bound maximally 0.9 $\pm$ 0.1 mol of GTP$\gamma$S/mol of protein with an apparent K$\sb{\rm d}$ of 32 nM. Competition binding assays indicated that the protein specifically binds GDP and GTP. The 24 kDa protein hydrolyzed GTP with a rate of 0.06 mol P$\sb{\rm i}$/mol protein/min. In the presence of the P$\sb{\rm r}$ form of phytochrome, the GTPase activity of the protein is enhanced. Nucleoside diphosphate kinase, an enzyme that reportedly co-purifies with a variety of GTP-binding proteins and which has been implicated in cancer metastasis suppression, was also isolated and characterized from oat seedlings. N-terminal sequencing of the Avena protein revealed that 87% of the 23 amino acids sequenced were identical to the human nm23-H2 protein.
Subject Area
Biochemistry
Recommended Citation
Sommer, Debbie, "Phytochrome structure and GTP-binding proteins" (1993). ETD collection for University of Nebraska-Lincoln. AAI9415996.
https://digitalcommons.unl.edu/dissertations/AAI9415996