Role of protein phosphorylation in phototropin receptor signalling in plants

Abstract

Abstract Phototropins are blue-light activated kinases that mediate responses contributing to optimization of growth in plants. Autophosphorylation, the activation mechanism for phototropin kinase activity, is the primary step in phototropin-signalling, however, molecular mechanisms of downstream signalling remain poorly understood. It is thought phosphorylation of phototropin-interacting proteins may play a regulatory role and this work aimed to identify new and conserved phosphorylation sites within phototropins (phot1, phot2) and the signalling components, Non-phototropic Hypocotyl 3 (NPH3), Root Phototropism 2 (RPT2), NRL protein for chloroplast movement 1 (NCH1), phytochrome kinase substrate (PKS) 1-4 for further insight into signal transduction mechanisms. Arabidopsis phosphoproteomic databases and the literature were interrogated for experimentally identified phosphorylation sites. Conservation of the sites was determined by multiple sequence alignment of orthologues of Arabidopsis in various plant species. 3-D structural modelling was used to visualize the molecular environment surrounding the phosphorylation sites with further analysis focused on phosphorylation sites conserved across all the plant species examined: S335, S352, S450, S185, T242, S849, S851 in phot1, and S121, S504, S761, S763 in phot2. Conserved phosphorylation sites, T208, S223, S239, S467, S744, were also identified in NPH3, however, no phosphorylation sites were found in the plant species for RPT2 or NCH1. Very low conservation of phosphorylation sites was detected in PKS1-4 proteins. The main findings here was successful in identifying conserved phosphorylation sites in phototropin signalling components, suggesting they might have an important regulatory role to play. Further functional analyses of these sites are required to elucidate their role in light signalling.