A photoreversible phytochrome affinity column chromatography for putative phytochrome receptor studies

Protein kinase A-catalyzed phosphorylation and its effect on conformation in phytochrome A

Phytochromes are ubiquitous red/far-red wavelength-sensitive photoreceptors in plants. Oat phytochrome A is a phosphoprotein. Phytochrome A (phyA) possesses two spatially different sites for phosphorylation with cAMP-dependent protein kinase (PKA) [McMichael & Lagarias (1990) Biochemistry 29, 3872-3878]. To assess the modulation of protein conformation by phosphorylation/dephosphorylation and its possible implication in phytochrome-mediated signal transduction, the conformations of phytochrome have been probed by PKA catalyzed phosphorylation. The phosphorylated species were purified and analyzed, along with untreated phytochrome, by limited proteolysis, circular dichroism (CD) and fluorescence quenching measurements. No significant changes in secondary structure of the phyA molecule after its phosphorylation were observed by CD. However, a subtle topographic and/or electrostatic effect of the phytochrome phosphorylation was detected by the time-resolved fluorescence quenching of Trp residues with Cs+ ions. N-Terminal phosphorylation at Ser17 was unique to the Pr form, but both Pr and Pfr phytochromes were phosphorylated at the hinge region to some extent. Phosphorylation at the hinge region resulted in noticeable changes in the proteolytic patterns, inhibiting cleavage near the phosphorylation site and favoring tryptic digestion of the Lys536-Asn537 peptide bond. Phosphorylation at the N-terminus did not cause observable changes in the helical structure of this region, but had an inhibitory effect on proteinase V8 accessibility at a site near the chromophore attachment. The functional relevance of protein phosphorylation of phyA is also discussed.

Mechanism of native oat phytochrome photoreversion: a time-resolved absorption investigation

The regulation of plant photomorphogenesis is mediated by the thermal reactions that follow light absorption by the phytochrome photoreceptor. Phytochromes are tetrapyrrolic chromoproteins that exist in two photochromically interconvertible forms, a red light absorbing species, Pr, and a far-red light absorbing form, Pfr. Upon irradiation with 670 nm light, the inactive, red light sensing Pr form is converted to the active Pfr form. Although the forward phototransformation has been studied extensively by several groups using various techniques, the Pfr-->Pr photoreversion reaction that occurs upon irradiation with 730 nm light is not as thoroughly characterized. In this study, time-resolved absorption (TROD) spectroscopy is used to examine the intermediate species involved in the phytochrome photoreversion mechanism at 10 degrees C. Analysis of the TROD data identifies three species with lifetimes of 320 ns, 265 microseconds, and 5.5 ms. TROD results are described in terms of the simplest parallel and sequential kinetic models. Comparison of intermediate spectra from these mechanisms with those of previously reported species from flash photoreversion and low-temperature studies indicates that Pfr photoreversion follows a sequential pathway that does not share any intermediates with the Pr phototransformation pathway.