Difference in the degree of exposure of chromophores in the Pr and Pfr forms of phytochrome

Interactions between native oat phytochrome and tetrapyrroles

The suggestion, that the increase in the far-UV CD signal of the 124 kDa oat phytochrome upon phototransformation of the Pr to Pfr form is possibly due to the chromophore interaction with the N-terminus segment of the phytochrome protein in the Pfr from (Chai, Y.G., Song, P.S., Cordonnier, M.-M. and Pratt, L.H. (1987) Biochemistry 26, 4947-4952), has been investigated by measuring the circular dichroism in the absence of exogenous tetrapyrrolic chromophores (bilirubin, biliverdin, chlorophyllin and hemin). Open tetrapyrrolic chromophores (bilirubin and biliverdin) did not have any significant effect on the phototransformability of the far-UV CD signal of the phytochrome, whereas closed tetrapyrroles (chlorophyllin and hemin) almost completely blocked the increase in the far-UV CD signal upon Pr to Pfr phototransformation. However, closed tetrapyrroles had no effect on the decrease in the CD signal upon Pfr to Pr photoconversion. Secondary structure analysis showed that the alpha-helix content of both Pr and Pfr forms of phytochrome (with 53 and 56% alpha-helical content, respectively) increased to 62% when a 50-fold molar excess of chlorophyllin was added to them separately. Spectral phototransformation of phytochrome was not affected in the presence of tetrapyrroles, except in the case of hemin. A 50-fold molar mass of hemin caused a significant bleaching of the Pfr form of phytochrome but not that of the Pr form. These results suggest that the chromophore-protein interaction is significantly altered during the phototransformation of phytochrome.

Use of bilirubin oxidase for probing chromophore topography in tetrapyrrole proteins

Bilirubin oxidase has been used to probe the surface topography of phycocyanins (C-phycocyanin and phycocyanin-645), peridinin-chlorophyll a-protein and phytochrome. The enzyme catalyzes oxidation of the tetrapyrrolic chromophores in these proteins. Relative rates of oxidation were 78.0 X 10(-6) s-1 (monitored at 617 nm) and 58.0 X 10(-6) s-1 (592 nm) for C-phycocyanin, 43.0 X 10(-6) s-1 for phycocyanin-645, 0.3 X 10(-6) s-1 (at 671 nm) and 1.3 X 10(-6) s-1 (at 480 nm) for peridinin-chlorophyll a-protein. The relative rate of free chlorophyllin a was 2.8 X 10(4) s-1 whereas upon binding to human serum albumin its rate of oxidation was reduced to 3.3 X 10(-3) s-1. Relative rates for the oxidation of Pr and Pfr forms of phytochrome were 2.9 and 19.5 s-1, respectively, which are consistent with earlier finding [( 1984) Plant Physiol. 74, 755-758] that indicated a preferential exposure of tetrapyrrolic chromophore in the Pfr form. In general, kcat/Km values derived from the Lineweaver-Burk plots followed the same trend as the relative rates of oxidation. For example, the kcat/Km for the free chlorophyllin a was 2.8 X 10(6) M-1 s-1 but it was only 1.1 M-1s-1 for the chlorophyll a in peridinin-chlorophyll a-protein where the chlorophyll is shielded by protein. These results reflect varying degrees of protection of the tetrapyrrolic chromophores from the enzymatic oxidation and prove that bilirubin oxidase can be generally used as a probe for deducing the topography of tetrapyrrolic chromophores.

Binding of phytochrome to liposomes and protoplasts

The physiologically active form of oat phytochrome, Pfr, increases its binding to egg lecithin unilamellar liposomes with increasing ionic strength of the medium while the binding of Pr is almost constant. The preferential binding of Pfr is as much as twice that of Pr at KCl concentrations of above 0.2 M, in 0.1 M phosphate buffer (pH 7.2) at 27 degrees C. The binding of phytochrome to liposomes is also enhanced by approximately 80% at 27 degrees C compared to that at 3 degrees C. Thus, it appears that the binding between Pfr phytochrome and liposomes is hydrophobic in nature whereas the binding of Pr is not predominantly through hydrophobic interactions. The binding of both Pfr and Pr to multilamellar liposomes increases with increasing cholesterol content in the liposomes. The extent of phytochrome's binding is higher in the neutral pH region than above pH 7.5. It takes several hours to reach an equilibrium of binding. The photoreversion of liposome-bound Pfr is inhibited by 40% compared to that of free Pfr, while the phototransformation of liposome-bound Pr to Pfr is promoted by 30%. The rate of dithionite-accelerated dark reversion of liposome-bound Pfr is lower by 50% than that of the free form. These results are consistent with the proposal that the hydrophobic binding site involved results from a vacancy produced by the reorientation or displacement of the Pfr chromophore from the protein. Upon binding to phytochrome, unilamellar liposomes undergo fusion to form larger diameter liposomes. No preferential binding of the Pfr form was found with intact oat protoplasts in vitro.

Hydrophobic properties of phytochrome as probed by 8-anilinonaphthalene-1-sulfonate fluorescence

8-Anilinonaphthalene-1-sulfonate (ANS) complexes with phytochrome, exhibiting a higher affinity for the Pfr form of phytochrome than for the Pr form. ANS fluorescence is enhanced by the additional binding of ANS to Pfr upon transformation of phytochrome from Pr to Pfr. The specific site of ANS binding appears to be the hydrophobic surface area of the protein, which becomes at least partially exposed in the Pfr form. An exposed, hydrophobic surface area in the Pfr phytochrome has been confirmed by the effects of ANS on the phototransformation of phytochrome. ANS accelerates the Pr leads to Pfr phototransformation, and it inhibits Pfr leads to Pr photoreversion and dark reversion. These effects are interpretable in terms of competitive binding of ANS to the chromophore binding site. Binding of ANS results in a drastic bleaching of the chromophore's absorption bands at 660 and 730 nm, particularly of the latter. This can be attributed to the exposed chromophore, which tends to resume a cyclic conformation with concomitant blue shift and hypochromism of the Qy bands. Sodium dithionite counteracts the inhibitory effects of ANS on the dark reversion of Pfr to Pr, and its effect on the biphasic kinetics of the reversion has been discussed in terms of the Pfr model proposed.