Cross-kingdom analyses of transmembrane protein kinases show their functional diversity and distinct origins in protists.

Transmembrane kinases (TMKs) are important mediators of cellular signaling cascades. The kinase domains of most metazoan and plant TMKs belong to the serine/threonine/tyrosine kinase (S/T/Y-kinase) superfamily. They share a common origin with prokaryotic kinases and have diversified into distinct subfamilies. Diverse members of the eukaryotic crown radiation such as amoebae, ciliates, and red and brown algae (grouped here under the umbrella term "protists") have long diverged from higher eukaryotes since their ancient common ancestry, making them ideal organisms for studying TMK evolution. Here, we developed an accurate and high-throughput pipeline to predict TMKomes in cellular organisms. Cross-kingdom analyses revealed distinct features of TMKomes in each grouping. Two-transmembrane histidine kinases constitute the main TMKomes of bacteria, while metazoans, plants, and most protists have a large proportion of single-pass TM S/T/Y-kinases. Phylogenetic analyses classified most protist S/T/Y-kinases into three clades, with clades II and III specifically expanded in amoebae and oomycetes, respectively. In contrast, clade I kinases were widespread in all protists examined here, and likely shared a common origin with other eukaryotic S/T/Y-kinases. Functional annotation further showed that most non-kinase domains were grouping-specific, suggesting that their recombination with the more conserved kinase domains led to the divergence of S/T/Y-kinases. However, we also found that protist leucine-rich repeat (LRR)- and G-protein-coupled receptor (GPCR)-type TMKs shared similar sensory domain architectures with respective plant and animal TMKs, despite that they belong to distinct kinase subfamilies. Collectively, our study revealed the functional diversity of TMKomes and the distinct origins of S/T/Y-kinases in protists.