Non-Canonical Inter-Protein Interactions of Key Proteins Belonging to Cytokinin Signaling Pathways

Abstract

The multistep phosphorelay (MSP) is a conserved signaling system that allows plants to sense and respond to a variety of cues under rapidly changing environmental conditions. The MSP system comprises three main protein types: sensor histidine kinases, phosphotransmitters, and response regulators. There are numerous signaling pathways that use, in whole or in part, this set of proteins to transduce diverse signals. Among them, the cytokinin signal transduction system is the best-studied pathway, which utilizes the entire MSP cascade. Focusing on this system, we review here protein-protein interaction of MSP components that are not directly related to cytokinin signaling. These interactions are likely to play an essential role in hormonal crosstalk and may be promising targets for fine-tuning plant development. In addition, in light of recent advances in the study of cytokinin signaling, we discuss new insights into the putative molecular mechanisms that mediate the pleiotropic action of cytokinins and provide specificity for distinct MSP signals. A detailed network of known non-canonical protein-protein interactions related to cytokinin signaling was demonstrated.

Keywords: cytokinin signaling; dimerization; multistep phosphorelay; signaling specificity.

Figure 1

Integrative scheme of multistep phosphorelay in plants. AHK—Arabidopsis Histidine Kinase; AHK5/CKI2—Arabidopsis Histidine Kinase 5/Cytokinin Independent 2; CHARK-CHASE domain receptor serine/threonine kinase, cytokinin receptor; CKI1—Cytokinin Independent 1, histidine kinase; CRFs—Cytokinin Response Factors; ER—endoplasmic reticulum; ETR1—Ethylene Response 1, ethylene receptor, histidine kinase; HPts—phosphotransfer proteins; PHPs—pseudo-phosphotransfer proteins; PhyB—phytochrome B; OsRAC3—GTP binding protein of GTPase Rac/Rop family; RRA—type-A response regulator; RRB—type-B response regulator; RRC—type-C response regulator; SnRK2s—Sucrose non-fermenting-1-Related Protein Kinases 2; TFs—transcription factors; w/o RD—without receiver domain. Active phosphate is labeled as “P” in the circle. Shaded area—canonical CK signaling pathway.