Signalling through kinase-defective domains: the prevalence of atypical receptor-like kinases in plants

The structure of plant receptor-like kinases (RLKs) is similar to that of animal receptor tyrosine kinases (RTKs), and consists of an extracellular domain, a transmembrane span, and a cytoplasmic domain containing the conserved kinase domain. The mechanism by which animal RTKs, and probably plant RLKs, signal includes the dimerization of the receptor, their intermolecular phosphorylation, and the phosphorylation of downstream signalling proteins. However, atypical RTKs with a kinase-dead domain that signal through phosphorylation-independent mechanisms have also been described in animals. In the last few years, some atypical RLKs have also been reported in plants. Here these examples and their possible signalling mechanisms are reviewed. Plant genomes contain a much larger number of genes coding for receptor kinases than other organisms. The prevalence of atypical RLKs in plants is analysed here. A sequence analysis of the Arabidopsis kinome revealed that 13% of the kinase genes do not retain some of the residues that are considered as invariant within kinase catalytic domains, and are thus putatively kinase-defective. This percentage rises to close to 20% when analysing RLKs, suggesting that phosphorylation-independent mechanisms mediated by atypical RLKs are particularly important for signal transduction in plants.