Hybrid Histidine Kinases Shape the Architecture of the High Osmolarity Glycerol (HOG) Pathway: From Saccharomyces cerevisiae to the Priority Pathogen Aspergillus fumigatus and other Filamentous Fungi

Abstract The HOG pathway enables adaptation to hyperosmotic and other environmental stresses. This review highlights key differences between the pathway in Saccharomyces cerevisiae and in filamentous fungi, focusing on the major pathogen Aspergillus fumigatus. In the multistep phosphorelay of the HOG pathway, stress signals are detected by hybrid histidine kinases (HHKs). According to the current model, HHKs function as kinases under non-stress conditions to maintain phosphorylation of downstream relay components. Upon stress exposure, their activity decreases or may switch to phosphatase activity, resulting in dephosphorylation of the relay. This change ultimately activates the downstream MAP kinase cascade, creating an inverse relationship between phosphorylation and signaling output in the two sections of the pathway. Fungi differ markedly in their HHK repertoire, ranging from a single HHK in S. cerevisiae to multiple functionally distinct HHKs in filamentous fungi. This review examines how this diversity has shaped the organization of the multistep phosphorelay and its integration into the HOG pathway. We summarize current knowledge of HHK function across fungal species, discuss why the multistep phosphorelay represents an Achilles’ heel for fungi, and consider how the spatial organization of HOG pathway components influences signaling and how HHK repertoires may have diversified across the fungal kingdom.