Molecular analysis of phytochrome-mediated signal transmission in etiolated pea seedlings

Phytochrome-mediated light signals are transduced to nucleoside diphosphate kinase in Pisum sativum L. cv. Alaska

To clarify the molecular mechanism for the transduction of light signals in plants, we have established an in vitro system that uses crude membrane and soluble fractions of stem sections of etiolated Pisum sativum L. cv. Alaska after irradiation by red light, or sequential application of red and far-red light to the stem section. In a previous report (T. Hamada et al., J. Photochem. Photobiol. B: Biol. 33 (1996) 143-151) the labelling of proteins in membrane fraction by [gamma-32P] ATP at 0 degree C for 15 s and subsequent separation of proteins by two-dimensional electrophoresis allowed unambiguous identification of a heavily phosphorylated protein spot at 18 kDa (p18). In the present study we have confirmed the former results in the membrane fraction, and obtained the result that an increase in the phosphorylation of p18 by red-light irradiation is observed in the soluble fraction. Further, we have provided evidence that the p18 in the soluble fraction is purified and identified as nucleoside diphosphate (NDP) kinase by Western blotting, immuno-precipitation, amino acid sequencing and cDNA analysis. Purified p18 shows autophosphorylation activity and strong phosphorylating activity against myelin basic protein (MBP), a substrate of MAP (mitogen activated protein) kinase. The results show that phytochrome-mediated light signals are transduced to NDP kinase, which may elicit signals by providing high concentrations of, for example, GTP from GDT and ATP, by the autophosphorylation and by the protein kinase activity similar to MAP kinase.

Phytochrome regulates phosphorylation of a protein with characteristics of a nucleoside diphosphate kinase in the crude membrane fraction from stem sections of etiolated pea seedlings

The molecular mechanism of light signal perception via phytochrome was analysed using the third internodes of etiolated pea seedlings irradiated with red or red followed by far-red light. A crude membrane fraction prepared from the tissue was labelled by [gamma-32P]ATP at 4 x 10(-8) M for 15 s at 0 degree C, and the proteins were separated by two-dimensional gel electrophoresis. The phosphorylation of a protein with a molecular mass of about 15 kDa in the crude membrane fraction increased with an increase in the intensity of red light irradiation (10, 50 and 100 mumol m-2 s-1) for 20 s. Successive irradiation with red light (100 mumol m-2 s-1 for 20 s) and subsequent irradiation with far-red light reduced the phosphorylation of the protein, depending on the intensity of the far-red light (from 0.1 to 8 mumol m-2 s-1 for 180 s). A plasma membrane purified from the crude membrane fraction from red light irradiated tissue showed a rapid phosphorylation of the 15 kDa protein by 4 x 10(-8) M [gamma-32P]ATP at 0 degree C for 7 s, and subsequent addition of ATP, GTP, ADP or GDP at 10(-5) or 10(-6) M efficiently removed the phosphoryl group of the 15 kDa protein. The 15 kDa protein was autophosphorylated in the gel following separation by sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis. The partially purified 15 kDa protein included nucleoside diphosphate kinase (NDP kinase) activity, as well as cross-reactivity with an antibody specific to rat NDP kinase as assayed by immunostaining and crossreactivity with an antibody specific to ricet NDP kinase as assayed by immunoprecipitation.