Phosphorylation has been observed in a wide variety of plant proteins and is described to play a role in different aspects of protein regulation. An important process that depends on phosphorylation in plants is phytochrome signaling. Phytochromes are proteins mediating biochemical processes in response to light. They are produced in an inactivated form and are activated upon light adsorption followed by a conforma-tional change. Signals are then transduced either indirectly or directly by binding of the activated phytochrome to transcriptional activators. Three phosphorylation sites have been identified on phytochrome using different methods including radioactive labeling, Edman degradation, mutation analysis, and MS [67, 111-113]. Phosphory-lation of the activated form is described to attenuate light signaling processes, while dephosphorylation leads to enhanced photoresponsiveness [114]. Phosphorylation is reported to inhibit PPIs, between phytochrome and its putative transducer proteins NDPK2 and PIF3 (a basic transcription factor) [115], thereby interrupting the signal transduction process.

Another protein class widely involved in signal transduction processes is formed by the CDPKs. These proteins are composed of an N-terminal domain, a catalytic domain, a junction (j) domain that can autoinhibit the kinase, and a CaM-like domain responsible for calcium binding. The binding of calcium to the CaM-like domain exerts the release of the autoinhibitory function of the j domain. Recently, a comprehensive study of CDPK phosphorylation was published by Hegeman et al. [116]. In this study, several CDPKs were overexpressed in vitro and their capability of autophosphoryla-tion was examined by MS. In total, 31 new phosphorylation sites were mapped on 7 CDPKs and 2 CDPK-related kinases (CRKs), thus suggesting the importance of CDPK multisite phosphorylation. Other signal transduction processes are also directly linked to phosphorylation. BRs, which regulate plant growth and development, are reported to lead to multiple phosphorylations of BRI1 and BAK1 as was suggested by IMAC coupled to LC-MS/MS and partly supported by mutation analysis [117]. Phosphorylation of these proteins seems to be important for their kinase activity and might be linked to heterodimerization.

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