Characterization of ROP GTPase-activated Arabidopsis receptor-like cytoplasmic kinases (RLCK class VI_A)

Plants have to respond and adapt to a variety of continuously changing environmental factors in order to establish an appropriate developmental strategy to ensure survival. There are ample data showing that protein phosphorylation/dephosphorylation plays a central role in cellular signal transduction in all organisms (Herrmann et al. 2006; Stone and Walker 1995). Interestingly, plants have a similar system as mammals to detect and transfer signals across the cell membrane into the nucleus where adaptations could be initiated. For the detection and transfer of an external signal, mammalian systems have receptor protein kinases. The proteins contain a single hydrophobic transmembrane domain, an extracellular domain and protein kinase domain. The majority of animal receptor kinases are phosphorylated on tyrosine residues within the kinase domain (receptor tyrosin kinases or RTKs; Ullrich and Schlessinger 1990), but a few were discovered which are phosphorylated on serine and threonine residues (Lin et al. 1992). In plants, two different types of transmembrane receptor kinases are known, including receptor-like serine/threonine (Ser/Thr) kinases (receptor-like kinases RLKs; Shiu and Bleecker 2001, 2003; Shiu et al. 2004; Walker 1994), structurally similar to mammalian RTKs, and receptor histidine (His) kinases (Grefen and Harter 2004; Mizuno 2005; Urao et al. 2000). Since the first RLK-encoding gene family was found in Zea mays (Walker and Zhang 1990), thousands of RLK genes have been identified from many different plant species. The Arabidopsis genome contains more than 600 members, representing nearly 2.5% of the annotated protein-coding genes; and more than 1000 members were annotated in the rice genome (Shiu et al. 2004). Approximately 25% of the Arabidopsis RLKs contain only a kinase domain with no apparent signal sequence or transmembrane region and thus were collectively named as receptor like cytoplasmic kinases (RLCKs). Arabidopsis RLCKs can be subdivided into 12 classes with 193 protein coding genes all together. Concerning the function of plant RLCKs, at the present only few members have been characterized and it is very likely that they play major role in the perception and 93 transmission of external signals perceived by RLKs (Zhou et al. 1995; Murase et al. 2004). Recently, our group as well as a group in Germany reported a direct interaction of plant ROP GTPases with receptor-like cytoplasmic kinases (RLCK class VI) from Arabidopsis (Molendijk et al. 2008) and alfalfa (Dorjgotov et al. 2009). Moreover, we provide evidences that kinases belonging to the RLCK Class VI family of Medicago truncatula and Arabidopsis thaliana can be specifically activated by GTP-bound ROP GTPases in vitro further supporting the view that plant Rho (ROP) G-proteins may directly regulate downstream kinase signaling. A further kinase designated as cysteine-rich receptor kinase (NCRK) belonging to a distinct kinase family has also been shown to interact with ROPs (Molendijk et al. 2008). None of these plant specific ROP-interacting kinases has any characteristic domain or motif that could be correlated with their ability to bind ROP GTPases. Plant specific ROP GTPases are versatile molecular switches in many processes during plant growth, development and responses to the environment and thus a possible implication of RLCKs in these ROP-dependent signal transduction pathways is in discussion. Our general aim was to characterize the members of the Arabidopsis thaliana RLCK Class VI family of protein kinases. Despite of their potential significance in ROP GTPase mediated signaling, hardly any functional information was available until now about the fourteen Arabidopsis RLCK Class VI members. Sequence comparison and phylogenetic analysis revealed that gene duplication played a significant role in the formation of this kinase family and allowed the separation of the 14 RCLK VI kinases into two groups with seven members each (A1 to A7 and B1 to B7). The proteins are highly homologous to each other, especially in the kinase domain, but are divergent from the related kinase families. It was established that, several members have an N-terminal UspA (“universal stress protein”) domain (group B members) or an N-terminal serine-rich region (group A members). In order to formulate a possible biological role of AtRLCK_VI kinases, real-time quantitative reverse transcription-polymerase reaction (qRT-PCR) was used to determine relative transcript levels in the various organs (root, rosette leaves, cauline leaves, 94 inflorescence stem, flower buds, open flowers, siliques. exponentially dividing cultured cells) of the Arabidopsis plant as well as under a series of abiotic stress/hormone (osmotic, sugar, salt stress, oxidative stress, cold and hormone treatment) treatments in seedlings. AtRLCK VI genes exhibited diverse expression patterns in the various plant organs as well as in response to stress/hormone treatments.