Large-Scale Multigenome-Wide Study Predicts the Existence of Transmembrane Phosphotransfer Proteins in Plant MSP Signaling Pathway

A new class of plant phosphotransfer proteins belonging to multistep phosphorelay (MSP) system was discovered by using large-scale bioinformatics methods. Unlike the canonical soluble nucleo- cytosolic forms, these proteins were predicted to have transmembrane (TM) domains and, apparently, should be localized on some kind of cell membranes. To date, 95 predicted TM-containing phosphotransmitter (TM-HPt) homologs were found in 61 plant species belonging to different clades, taxa and groups: bryophytes, gymnosperms, mono- and dicotyledons. The conserved HPt motif with phosphorylatable histidine was preserved in the most of TM-HPts under study that allowed us to consider these proteins potentially active in MSP signaling. For the found TM-HPts, a Bayesian analysis at the DNA level was performed and a relevant phylogenetic tree was constructed. According to evolutionary relationships, plant TM-HPts were divided into two main groups corresponding to Arabidopsis AHP1-3,5,6 and AHP4 orthologs. Transcriptomic analysis confirmed the expression of the most of the investigated TM-HPt-encoding genes. Their moderate-to-low overall transcription rate may be a consequence of inducible and/or tissue-specific expression. Using molecular modeling methods, a variety of potential spatial organizations of several such proteins are demonstrated. Possible roles of TM-HPts as modulators of MSP signaling pathway and corresponding putative mechanisms of their action are suggested.