Phytochrome kinase

Fig. 8.2 Schematic illustration of the interdomain signal transmission in a dimeric oat phytochrome. Q Quail box. PAS Per-Arnt-Sim motif. Q and PAS constitute the regulatory core region. HD Histidine kinase-related domain. PKSl Phytochrome kinase...

The phosphorylation at Ser-598 exerts an accelerating effect on the association of PIF3 and NDPK2 and the phosphorylation of PKSl (phytochrome kinase substrate 1). The latter is a protein that is complexed to the Pr state of the phytochrome and is released from the photoactivated Pjj. state after phosphorylation to give downstream signals through a kinase cascade [39]. Recall that a kinase is an enzyme that catalyzes the phosphorylation of a substrate, here a protein. 

Until recently, the hypothesis that the phytochrome is a kinase has been controversial. The finding that the Cphl has histidine kinase activity triggered the reexamination of plant phytochrome kinase activity and confirmed that plant phytochromes are Ser/Thr kinases. The Ser-7 of Phytochrome A is phosphorylated in vivo in Pr and Pfr forms, but the Ser-17 is phosphorylated in vitro in the Pr... 

Phytochrome is found not only in higher plants but also in algae, where it controls the movement of chloroplasts,611 and also in cyanobacteria.623 54 Cyano-bacterial phytochromes contain histidine kinase domains, which may function in a two-component system with a response regulator similar to protein CheY of the chemotaxis system in E. coli (Fig. 19-5).624/625 Some nonphotosynthetic bacteria also use bacteriophytochromes for light sensing. In some cases biliverdin (Fig. 24-24) is the chromophore.6253... 

In Arabidopsis genome, there are in total 16 different histidine kinases. Besides the above mentioned cytokinin receptors, they function also as the receptors of phytohormone ethylene and light stimuli - phytochromes [216]. 

In plants, reports that protein kinase activities are associated with regulatory signals are appearing [24], and the identification of protein kinases is just emerging [32, 36]. The phosphorylation of phytochrome itself may be a component of regulatory systems [34]. 

Collectively, these various lines of evidence suggest that in plants, regulatory actions by hormones and phytochrome may involve attachment to a receptor, activation of second messengers (Ca, and possibly PI), and subsequent activation of protein kinases, which then serve to regulate specific enzymes bringing about the regulatory action. 

Wong, Y.S., McMichael, R.W.J., and Lagarias, J.C., Properties of a polycation-stimulated protein kinase associated with purified Avena phytochrome, Plant Physiol, 91, 709, 1989. 

Yeh, K.C. and Lagarias, J.C., Eukaryotic phytochromes hght-regulated serine/threonine protein kinases with histidine kinase ancestry, Proc. Natl Acad. Sci USA, 95,13976,1998. 

Reed, J.W, Phytochromes are Pr-specific kinases, Curr. Opin. Plant Biol, 2, 393, 1999. 

FIGURE 130.2 Domain structures of prokaryotic and plant phytochromes. Phytochromes have two functional domains, the JM-terminal photosensory domain and the C-terminal regulatory domain. CBD, chromophore-hearing domain HKRD, histidine kinase-related domain HKD, histidine kinase domain PAS, Pas (Per/Arndt/Sim) domain SS, serine-rich domain PYP, photoactive yellow protein domain similar to PAS domain , bilin-chromophore. (Modified from Frankhauser, C., Phytochromes as light-modulated protein kinases. Figure 1, Seminars in Cell and Developmental Biology, 11, 468, 2000. With permission from Elsevier Science.)... 

Soon after the discovery of phytochrome, the BeltsviUe group (H. Borthwick, S.B. Hendricks, and M.W. Parker) proposed that phytochrome might be an enzyme, however, determining the identity of this enzymatic activity has proven difficult Finally, it was demonstrated that phytochrome acts as a serine kinase and can autophosphorylate itself or transphosphorylate its partner proteins. Even the subceUular localization of phytochrome appears to be affected by light... 

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