Chromophore: apoprotein interactions in phytochrome A*

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Abstract
Phytochromes are molecular light switches by virtue of their photochromic red/far‐red reversibility. The His‐324 residue next to the chromophore‐linked 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 α‐helical chains in a hydrophobic 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 α‐helix of the amino terminal chain, which occurs in 113 ms after picosecond photoisomerization of the Pr form. Compared to α‐helical folding, unfolding of the α‐helix occurs faster in about 310 μs 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‐translational modification serves as a surface label, rather than a transducer‐activating trigger, for the recognition of a putative phytochrome receptor.