It is known from animal and yeast systems that kinases of the AGC family are targets of the 3-phosphoinositide-dependent-protein kinase PDK1. PDK1 contains a lipid binding pleckstrin homology (PH) domain and binds to PtdIns(3,4,5)P3 and PtdIns(3,4)P2 in animals (Bogre et al. 2003), and phosphatidic acid, PtdIns3P and PtdIns(3,4)P2, among others, in Arabidopsis (Deak et al. 1999). These lipid molecules are generated in response to signals from phosphorylation events or by cleavage of their inositol groups by phosphoinositide kinases, phosphoinositide phosphatases or phospholipases (Mueller-Roeber and Pical 2002; Meijer and Munnik 2003) and participate in many signal transduction pathways and cellular processes in various organisms (Vanhaesebroeck et al. 2001).
In animals, the acknowledged targets of PDK1 are members of the AGC family, for example PKA, PKB, PKC, p70 ribosomal S6 kinase (S6K), p90 S6 kinases (RSKs) and the serum and glucocorticoid-inducible kinase (SGK) (Storz and Toker 2002; Bogre et al. 2003). Being the predicted orthologs of animal PKAs and PKCs, the plant-specific AGCVIII kinases like PID, and the WAG kinases are also predicted targets of PDK1, and indeed recent data have partly confirmed this. For example, Anthony and co-workers (Anthony et al. 2004) showed that the kinase AGC2-1, which probably participates in cell growth and division induced by auxin and cytokinin, interacts with PDK1 and is activated by it in a process mediated by phosphatidic acid. In another study, it was demonstrated that PID interacts with and is activated by PDK1, both through in vitro and in semi in vivo experiments using protein extracts from flowers or seedlings shoots, organs where PID is naturally more active (Zegzouti et al. 2006a). Finally, it has been shown that several other members of the AGCVIII subfamily are also direct targets of PDK1 (Zegzouti et al. 2006b). The in planta relevance of PDK1-mediated phosphorylation of the AGCVIII kinases, however, remains to be addressed.
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