CHROMOPHORE - APOPROTEIN INTERACTIONS IN PHYTOCHROME-A

Phytochromes are molecular light switches by virtue of their photochro mic red/far-red reversibility, The His-323 residue next to the chromop hore-linked Cys-323 plays a critical role in conferring photochromism to the tetrapyrrole chromophore in native phytochrome A, The chromopho re appears to be enclosed between the amphiphilic alpha-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 mm, resp ectively, 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 alpha-helix of the amino termi nal chain, which occurs in 113 ms after picosecond photoisomerization of the Pr form, Compared to alpha-helical folding, unfolding of the al pha-helix occurs faster in about 310 mu s upon phototransformation of the Pfr form of phytochrome A. The photochromic transformation of phyt ochrome A modulates protein kinase-catalysed phosphorylation sites in vivo and in vitro, but only a subtle local change in conformation is d etectable in the phosphorylated phytochromes. This suggests that the p ost-translational modification serves as a surface label, rather than a transducer-activating trigger, for the recognition of a putative phy tochrome receptor.