Scientific background and research focus of the Research Unit

Signaling requires messengers whose concentration varies in time and space. Ca2+ ions and phosphate ions have come to dominate cellular signaling. Ca2+ binding triggers changes in protein shape and charge. Similarly, phosphorylation imparts a negative charge, modulating protein conformations and protein interactions (Hunter, 1995; Soderling, 1999; Clapham, 2007). Ca2+-dependent kinases and protein kinases regulated by interaction with Ca2+ binding proteins functionally combine these two major cellular currencies of signal transduction and allow for the perception and transmission of Ca2+-signatures directly into phosphorylation events that orchestrate downstream signaling responses (Harmon et al., 2000; Batistič and Kudla, 2004, Kudla et al., 2010). This step of information processing and biological regulation represents the central and interconnecting scientific focus of this Research Unit. Plants possess an extensive repertoire of Ca2+-dependent protein kinases, which are represented by the three families of CCaMKs (Ca2+-calmodulin-dependent kinases), CDPKs (Ca2+-dependent protein kinases) and CIPKs (CBL-interacting protein kinases) (Sanders et al., 2002; Batistič and Kudla, 2004; Gleason et al., 2006; Kudla et al., 2010). While CDPKs and CCaMKs (genes for the later appear not to exist in the Arabidopsis genome) represent typical sensor responders, the CIPKs are targets of Calcineurin B-like (CBL) sensor-relay proteins. These kinases and Ca2+ binding proteins form an intricate cellular network for decoding Ca2+ signals in diverse cellular processes (Batistič and Kudla, 2009; Weinl and Kudla, 2009; Kudla et al., 2010). The investigation and characterization of these Ca2+-regulated signaling components and especially their role in governing the regulation of ion transport processes, organogenesis and transcriptional regulation is investigated by joined projects of the members of this research consortium

The identification of true ABA receptors that was reported by the groups of Erwin Grill and Sean Cutler during the last funding period of this RU is of special relevance for the current and future scientific orientation of this RU. Both groups independently identified a family of soluble, cytoplasmic proteins, designated as RCAR/PYR/PYL proteins (that we in the following for simplification will refer to as RCAR proteins) as ABA-binding proteins that, upon ABA binding, interact with protein phosphatases of the PP2C family (like ABI1, ABI2 and HAB1) and upon interaction inhibit the activity of these phosphatases (Ma et al., 2009; Park et al., 2009). The families of RCARs and PP2Cs appear to form an intricate interaction network in that differential expression, differential ABA-affinity of the co-receptors, differential RCAR-PP2C interaction, differential ABA-dependence of RCAR-PP2C interaction and differential PP2C inhibition all contribute to a faithful decoding of ABA signals (Nishimura et al., 2009a; Santiago et al., 2009a; Raghavendra et al., 2010; Szostkiewicz et al, 2010). Together with the most recent results from several crystallization studies of RCAR proteins or RCAR-PP2C complexes (Miyazono et al., 2009; Melcher et al., 2009; Nishimura et al., 2009b; Santiago et al., 2009b; Yin et al., 2009), the first in vitro reconstitution of an ABA signaling module (Fujji et al., 2009) and the extensive analysis of the function and regulation of SnRK2 kinases (Vlad et al., 2009; Umezawa et al., 2009) all these data support a model in that ABA binding to RCARs triggers RCAR-PP2C complex formation leading to inhibition of otherwise active PP2Cs. This ABA-induced inhibition of the phosphatases releases the inhibition of the SnRK2s that thereby become ABA-dependently activated and subsequently mediate ABA-induced transcriptional responses and ion fluxes:

Although this current model is most exciting and intuitive, the scientific findings that were obtained during the last funding period by the groups of this RU and that aresummarized below, point to a much more complex network of protein interactions inearly ABA perception. The accumulating evidence resulting from the work of thisResearch Unit and laboratories world wide suggest the potential existence of supra-molecular complexes potentially involving ABA receptors, PP2Cs, SnRK2s, CIPKs, CDPKs and WNKs that mediate ABA-regulated transcriptional responses and ionchannel regulation. Such a dynamic complex would represent a convergence point that would allow for Ca2+-dependent modulation of ABA signaling and at the same time would enable ABA-dependent modulation of Ca2+ signaling. However, the dynamics of protein interactions in such a complex, the consequences of these protein interactions and their impact on substrate regulation remain unknown. In addition, important functional principles of this elaborate signaling network, like the biochemical regulation of these proteins and the influence of cellular Ca2+ transients on complex formation and on signal processing remain enigmatic. Consequently, we have decided that during the requested funding period we will deemphasize attempts to model the dynamics of primary Ca2+ release events and signatures that were originally proposed as a goal with our first funding application. However, in this regard we will still continue to analyze Ca2+ signatures and to advance such analyses to novel qualities in tissue-specific and sub-cellular resolution. This approach should thereby enable modeling analysis of Ca2+ signatures in the future. Considering our most interesting and promising findings during the last funding period it will be a novel and major focus of this RU to address the interconnection of Ca2+-regulated signaling systems with ABA signaling in the regulation of transcription and ion transport.

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