Suppressor of K+ transport growth defect 1 (SKD1) interacts with RING-type ubiquitin ligase and sucrose non-fermenting 1-related protein kinase (SnRK1) in the halophyte ice plant

Abstract

SKD1 (suppressor of K+ transport growth defect 1) is an AAA-type ATPase that functions as a molecular motor. It was previously shown that SKD1 accumulates in epidermal bladder cells of the halophyte Mesembryanthemum crystallinum. SKD1 knock-down Arabidopsis mutants showed an imbalanced Na+/K+ ratio under salt stress. Two enzymes involved in protein post-translational modifications that physically interacted with McSKD1 were identified. McCPN1 (copine 1), a RING-type ubiquitin ligase, has an N-terminal myristoylation site that links to the plasma membrane, a central copine domain that interacts with McSKD1, and a C-terminal RING domain that catalyses protein ubiquitination. In vitro ubiquitination assay demonstrated that McCPN1 was capable of mediating ubiquitination of McSKD1. McSnRK1 (sucrose non-fermenting 1-related protein kinase) is a Ser/Thr protein kinase that contains an N-terminal STKc catalytic domain to phosphorylate McSKD1, and C-terminal UBA and KA1 domains to interact with McSKD1. The transcript and protein levels of McSnRK1 increased as NaCl concentrations increased. The formation of an SKD1–SnRK1–CPN1 ternary complex was demonstrated by yeast three-hybrid and bimolecular fluorescence complementation. It was found that McSKD1 preferentially interacts with McSnRK1 in the cytosol, and salt induced the re-distribution of McSKD1 and McSnRK1 towards the plasma membrane via the microtubule cytoskeleton and subsequently interacted with RING-type E3 McCPN1. The potential effects of ubiquitination and phosphorylation on McSKD1, such as changes in the ATPase activity and cellular localization, and how they relate to the functions of SKD1 in the maintenance of Na+/K+ homeostasis under salt stress, are discussed.

Identification of Ice Plant (Mesembryanthemum crystallinum L.) MicroRNAs Using RNA-Seq and Their Putative Roles in High Salinity Responses in Seedlings

“…Three-day-old dark-grown ice plant seedlings were immersed in water (control) or 200 mM NaCl (salt) for 6 h. These conditions were selected because it was possible to obtain high-quality, active primary roots. Furthermore, previous studies had shown that under these conditions salt-induced changes in subcellular localization of proteins could be documented (Chiang et al, 2013 ). Total RNA was extracted using TRIzol® (Invitrogen) according to the manufacturer's instructions.…”

Tocopherol deficiency reduces sucrose export from salt-stressed potato leaves independently of oxidative stress and symplastic obstruction by callose

“…This either indicates that SnRK1 signalling does not play a role in salt-stressed potato leaves, or that SnRK1 targets may differ between leaf and tubers (see Debast et al , 2011 ). In support of a role of SnRK1 signalling during salt stress, it has been shown that SnRK1 is engaged in the regulation of vacuolar potassium transport during salt challenge in the ice plant Mesembryanthemum crystallinum ( Chiang et al , 2013 ), that the SnRK1 target transcription factor bZIP11 is induced in salt-stressed Arabidopsis leaves ( Weltmeier et al , 2009 ), and that SnRK1 is activated in other abiotic stress conditions like drought, cold, or hypoxia (see O’Hara et al , 2013 for a compilation), which also influence the cellular osmotic potential. While the accumulation of sugars and the signalling metabolite T6P declined in SXD:RNAi leaves upon salt stress, AsnS1 , SuSy2 , and UDPglcE were increased about 2-fold more in the transgenic plants than in the wild type.…”

Transport, signaling, and homeostasis of potassium and sodium in plants

“…SKD1 is considered to function in protein sorting and to be a positive regulator of salinity tolerance via maintenance of K + /Na + homeostasis in both ice plant and Arabidopsis (Jou et al ; Ho et al ). A recent report has proposed a potential regulatory mechanism of SKD1 (Chiang et al ); however, the mechanism of SKD1‐dependent salt tolerance awaits further elucidation. Ethylene has also been suggested to contribute to the mechanism for soil‐salinity tolerance.…”