Multistep phosphorelay in fungi: the enigma of multiple signals and a limited number of signaling pathways

Current knowledge of fungal ecology depends on the understanding of the perception and processing of biotic and abiotic factors facilitating environmental interactions. Ongoing research regarding fungal signaling mechanisms has not only increased our appreciation for multistep phosphorelay signaling systems as an asset, but also sheds light on the role that these upgrades of the simple two-component signaling systems play in fungal biology, stress response, differentiation processes, metabolism, and pathogenicity. In contrast to two-component signaling systems, multistep phosphorelay systems comprise a phosphotransfer protein enabling modular phosphate transfer from the sensor hybrid histidine kinase to the response regulator protein. Histidine kinases were found to perceive a lot of environmental signals and integrate them into a number of signaling pathways. The major question is how multistep phosphorelay signal transduction takes place in detail, since most fungi possess only one single histidine-containing phosphotransfer protein enabling phosphate transfer. One single phosphotransfer protein routes various signals in a specific way and coordinates the phosphates precisely to different target locations within the cell. This mini review opens the door and presents three hypotheses as the basis for further investigations to unravel the complex signaling mechanisms in fungal multistep phosphorelay systems. Scaffold proteins, frequency-based signaling, and alternative splicing are discussed as putative candidates to implement fungal signaling in multistep phosphorelay systems having only one phosphotransfer protein for coordinated phosphate transfer.