Chromophore: apoprotein interactions in phytochrome A*

Abstract

Pbytocbromes are molecular ligbt switcbes by virtue of their pbotocbromic red/far-red reversibility. The His-324 residue next to the chromophore-Iinked Cys-323 plays a critical role in conferring photochromism to the tetrapyrrole chromophore in native phytochrome A. The chromophore appears to be enclosed between the amphiphilic a-helical chains in a hydrophobie pocket. The absorbance maxima of both the Pr and the Pfr forms of pea phytochrome A are blue-shifted by 10 and 20 nm, respectively, upon C-terminal truncation. We speculate that the quaternary structure of the phytochrome A molecule involves some interactions of the C-terminal half with the chromophore domain. The Pfr conformation of phytochrome includes an amphiphilic ahelix of the amino terminal chain, which occurs in 113 ms after picosecond photoisomerization of the Pr form. Compared to a-helical folding, unfolding of the a-helix occurs faster in about 310 fis upon phototransformation of the Pfr form of phytochrome A. The photochromic transformation of phytochrome A modulates protein kinase-catalysed phosphorylation sites in vivo and in vitro, but only a subtle local change in conformation is detectable in the phosphorylated phytochromes. This suggests that the post-transiational modification serves as a surface label, rather than a transducer-activating trigger, for the recognition of a putative phytochrome receptor.

In vitro assembly of phytochrome B apoprotein with synthetic analogs of the phytochrome chromophore

“…Hence, characterization of the molecular interaction between the chromophore and the apoprotein is crucial for understanding the functional mechanism(s) of phytochromes. Several studies have addressed this interaction by using in vitro assembly of PHYA (5,28,29,47), PHYB (35,37,48), PHYC and PHYE (43), or apoprotein mutants (30)(31)(32). However, in contrast to the vertebrate photoreceptor rhodopsin (49), in the phytochrome field, little has been done to examine the relationships among chromophore structure, its assembly to apoprotein, and photochromism of the holoprotein, because of the difficulty of synthesizing the chromophore and its structural analogs.…”

“…This unique photochromic property of phytochrome results from the interaction of the chromophore with apoprotein of phytochrome. The two main approaches to elucidate apoproteinchromophore interaction are site-directed mutagenesis (5) and chromophore modification by chemical synthesis (6,7), because the crystal structure of holoprotein has not yet been determined.…”

An emerging molecular map of the phytochromes

“…1993;Vierstra 1993;Cherry & Vierstra 1994;Wagner et al 1996a). In a more detailed sitedirected mutational analysis of five conserved residues (positions 360, 369, 372, 375 and 377) surrounding the chromophore attachment residue (the cysteine at position 374), Song and colleagues have obtained evidence that none is essential for chromophore lyase activity, suggesting a structural rather than a catalytic role (Deforce, Furuya & Song 1993;Song, Park & Furuya 1997).…”

Genetic identification of FIN2, a far red light‐specific signaling component ofArabidopsis thaliana

“…In contrast to the extensive characterization of the function and structure of PhyA (Casal et al, 1997;Cherry et al, 1993;Quail, 1997;Song et al, 1997), only a few reports on downstream signaling components of PhyA are available. Previous experiments employing a microinjection technique identified several biochemical elements in the PhyAsignal transduction pathway (Bowler et al, 1994;Neuhaus et al, 1993).…”