Abstract
The mitogen-activated protein kinase (MAPK) pathway is a three-tier signaling cascade that transmits cellular information from the plasma membrane to the cytoplasm where it triggers downstream responses. The MAPKs represent the last step in this cascade and are activated when both tyrosine and threonine residues in a conserved TxY motif are phosphorylated by MAPK kinases, which in turn are themselves activated by phosphorylation by MAPK kinase kinases. To understand the molecular evolution of MAPKs in the plant kingdom, we systematically conducted a Hidden-Markov-Model based screen to identify MAPKs in 13 completely sequenced plant genomes. In this analysis, we included green algae, bryophytes, lycophytes, and several mono- and eudicotyledonous species covering >800 million years of evolution. The phylogenetic relationships of the 204 identified MAPKs based on Bayesian inference facilitated the retraction of the sequence of emergence of the four major clades that are characterized by the presence of a TDY or TEY-A/TEY-B/TEY-C type kinase activation loop. We present evidence that after the split of TDY- and TEY-type MAPKs, initially the TEY-C clade emerged. This was followed by the TEY-B clade in early land plants until the TEY-A clade finally emerged in flowering plants. In addition to these well characterized clades, we identified another highly conserved clade of 45 MAPK-likes, members of which were previously described as Mak-homologous kinases. In agreement with their essential functions, molecular population genetic analysis of MAPK genes in Arabidopsis thaliana accessions reveal that purifying selection drove the evolution of the MAPK family, implying strong functional constraints on MAPK genes. Closely related MAPKs most likely subfunctionalized, a process in which differential transcriptional regulation of duplicates may be involved.
Highlights
To facilitate responses to developmental and environmental cues, a significant share of plant genomes evolved to equip the plant with a large variety of intracellular signaling pathways
An initial genomic survey based on the complete sequence of Arabidopsis thaliana (AT) and expressed sequence tags (ESTs) of additional plant species revealed a considerable expansion of the three different levels of the mitogen-activated protein kinase (MAPK) signaling cascade relative to yeast or metazoan model organisms (MAPK-Group et al, 2002)
We identified a total of 204 canonical MAPKs with the conserved TxY signature motif in the 13 investigated genomes (Table 1)
Summary
To facilitate responses to developmental and environmental cues, a significant share of plant genomes evolved to equip the plant with a large variety of intracellular signaling pathways. An important mode of signal transduction is protein modification such as phosphorylation. One highly conserved pathway of this category involves a tripartite mitogen-activated protein kinase (MAPK) signaling cascade that relays and amplifies stimuli that trigger intracellular responses. A membrane bound receptor is activated usually by an extracellular stress or developmental stimulus and relays the signal to a MAP kinase kinase kinase (MAP3K), which in turn activates a MAP kinase kinase (MAP2K). The MAPK phosphorylates specific substrate proteins such as other kinases, transcription factors, or enzymes (Khokhlatchev et al, 1998), which subsequently trigger the cellular response. The formation of MAPK cascades is thought to be mediated by scaffold proteins, which enable close proximity of the different players (Whitmarsh and Davis, 1998)
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