The SOS3 Family of Calcium Sensors and SOS2 Family of Protein Kinases in Arabidopsis

Abstract

During the course of its lifecycle, a plant may experience extremes of temperature and moisture, excesses and deficiencies of minerals, and challenges from herbivores and pathogens, all of which provide specific signals for altered plant growth and development. Ca2+ has been widely implicated as an intracellular messenger of physiologically and environmentally induced signaling pathways in plants (Trewavas and Malho, 1998). Because cellular Ca2+ levels are tightly regulated, small changes in intracellular Ca2+ can provide information for the modification of enzyme activity and gene expression needed for subsequent responses. Signals may trigger changes in the cellular Ca2+ signature (for example, an oscillation in the cytosolic free Ca2+ concentration), which is then perceived by various intracellular Ca2+ sensors/binding proteins to regulate a series of signaling cascades (Snedden and Fromm, 1998; Zielinski, 1998). Ca2+ sensors can be classified into sensor responders and sensor relays (Sanders et al., 2002). Upon binding Ca2+, sensor responders change their conformation and modulate their own activity or function through intramolecular interactions. In plants, the best-characterized sensor responders are the Ca2+-dependent protein kinases (Harmon et al., 2001). Ca2+-dependent protein kinases have protein kinase and calmodulin-like Ca2+- binding domains in a single protein, which allows their direct activation by Ca2+ (for review, see Cheng et al., 2002; Hrabak et al., 2003). In contrast, sensor relays such as calmodulin communicate the changed conformation to interacting partner(s) such as protein kinases, resulting in a change in kinase activity. A number of mechanisms have been postulated to account for the wide-ranging role of Ca2+ in signaling in plants as well as for the specificity resulting from the Ca2+ signal. These include qualitative and quantitative information resident in the Ca2+ signal itself and in the Ca2+ sensors that perceive and translate the Ca2+ signal. The potential for different sensors to contribute to functional specificity is further magnified by the presence of families of interacting proteins such as protein kinases. This review focuses on recent progress in the characterization of the salt overly sensitive (SOS) 3 family of Ca2+ sensors and their associated SOS2 family of protein kinases. Recent genetic, molecular, and biochemical studies provide evidence that these two families of proteins interact differentially to form complexes that function in signaling pathways in Arabidopsis during growth and development and in response to abiotic stresses. In a genetic screen designed to identify components of the mechanisms controlling salt tolerance in Arabidopsis, several SOS genes were identified. One of these genes, SOS3, encodes a novel EF-hand Ca2+ sensor (Liu and Zhu, 1998). SOS3 shares significant sequence similarity with the regulatory calcineurin B subunit from yeast (Saccharomyces cerevisiae) and neuronal Ca2+ sensors from animals (Klee et al., 1988). In spite of this similarity with calcineurin B at the primary sequence level, it is now clear that Arabidopsis does not have calcineurin and that the SOS3 Ca2+- sensing protein activates a protein kinase and not a protein phosphatase. SOS3 is a small myristoylated protein that appears to have no enzymatic activity by itself; Ca2+ binding and myristoylation are required for SOS3 function in salt tolerance (Ishitani et al., 2000). The SOS2 gene encodes a novel Ser/Thr protein kinase that also functions in salt tolerance in Arabidopsis (Liu et al., 2000). SOS2 contains an N-terminal kinase catalytic domain similar to that found in the Suc nonfermenting1 (SNF1) and AMP-activated (AMPK) kinases (Hardie, 1999) and a novel C-terminal regulatory domain. SOS3 has been shown to interact physically with SOS2 in yeast two-hybrid assays as well as in vitro (Halfter et al., 2000). Moreover, SOS3 activates SOS2 kinase activity in a Ca2+- dependent manner and sos3/sos2 double-mutant analysis also indicates that SOS3 and SOS2 function in the same pathway (Halfter et al., 2000). The first target of the SOS3-SOS2 regulatory pathway to be identified is the plasma membrane Na+/H+ exchanger (antiporter) encoded by the SOS1 gene. SOS1 gene expression during salt stress is partially controlled by SOS3 and SOS2 (Shi et al., 2000), and activation of SOS1 Na+/H+ antiport activity requires SOS3 and SOS2 (Qiu et al., 2002). Recent studies using yeast have provided additional evidence for the interaction between SOS3 and SOS2 and their regulation of SOS1. Coexpression of SOS1, 2, and 3 dramatically enhanced the salt tolerance of a yeast mutant in which all endogenous Na+ transporters had been removed (Quintero et al., 2002). The SOS3-SOS2 kinase complex phosphorylates and activates SOS1 expressed in yeast, enhancing Na+ exclusion and increasing NaCl tolerance (Quintero et al., 2002). Expression of a constitutively activated SOS2 mutant also increased salt