Posttranslational Regulation

Recent studies have gained insights into AtAKTl regulation. By analyzing mutants sensitive to low K+ stress, a CBL-interacting protein kinase, CIPK23, turned out to be essential in the activation of AtAKTl, therefore permitting Arabidopsis plants to grow under low K+ conditions (Xu et al. 2006). Two positive regulators of CIPK23, CBL1, and CBL9, which are two Calcineurin B-like proteins, were also found. Both CBL's could phos-phorylate CIPK23 which became active after this phosphorylation (Fig. 4.2). Activation of the CBL's depended on Ca2+ as demonstrated by patch-clamp recordings (Li et al. 2006). Later on, it was shown that the network was more complex, being CIPK6 and CIPK16, in addition to CIPK23, able to interact with CBL1, CBL2 and CBL3 and CBL9 (Lee et al. 2007) . Furthermore, a PP2C phosphatase (AIP1) inactivated AKT1 by dephos-phorylating the latter. AIP1 bound AKT1 through AKT1's ankyrin domain. In addition, formation of heterotetramers of AKT1 subunits with AtKC1 also serves as another mode of regulation in which such heterotetramers display different

Fig. 4.2 Regulation of the activity of AtAKTl by the CBL-CIPK complex and by the phosphatase AIP1. (a) Low K+ stress provokes an increase in ROS levels that can possibly be translated into a cytosolic Ca2+ wave. This wave mainly activates CBL1, CBL9, and CIPK23. The activated CBL/CIPK complex phosphorylates the AtAKTl channel, resulting in its activation (channel open). (b) At high-external K+, the AIP1 phosphatase is thought to dephosphorylate AtAKTl, resulting in the inactivation of the channel (channel closed)

Fig. 4.2 Regulation of the activity of AtAKTl by the CBL-CIPK complex and by the phosphatase AIP1. (a) Low K+ stress provokes an increase in ROS levels that can possibly be translated into a cytosolic Ca2+ wave. This wave mainly activates CBL1, CBL9, and CIPK23. The activated CBL/CIPK complex phosphorylates the AtAKTl channel, resulting in its activation (channel open). (b) At high-external K+, the AIP1 phosphatase is thought to dephosphorylate AtAKTl, resulting in the inactivation of the channel (channel closed)

voltage dependence and sensitivity to external K+ (Duby et al. 2008; Geiger et al. 2009).

The only report in which the regulation of HAKl-like K+ transporters at the protein level was discussed revealed that HvHAKl-mediated Rb+ uptake in yeast cells was modulated by the HAL4/5 kinases and the PPZ1 phosphatase (Fulgenzi et al. 2008+. Both types of enzymes seemed to negatively regulate HvHAKl activity in K+-starved yeast cells.

changes in the K+ status or to direct/indirect regulation of K+ uptake, suggesting a role in this process. In some cases, significant advances in the mechanisms by which some of these molecules regulate K+ nutrition have been reported (Fig. 4.3).

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