Chapter 5 : Signal Transduction Pathways

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Most filamentous fungi are exquisitely sensitive to changes in their environment. Sensing and integration of signals from multiple sources require a complex web of signal transduction pathways. This chapter covers major signal transduction pathways that have been characterized in multiple species of filamentous fungi. The signaling pathways included are monomeric and heterotrimeric GTP-binding proteins, mitogen-activated protein kinases (MAPKs), protein kinase A/cyclic AMP (PKA/cAMP) signaling, two-component regulatory systems, calcium signaling, target of rapamycin (Tor) pathways and pH regulatory mechanisms. With the exception of two-component systems, related pathways are found in animals, where they also play fundamental roles. In general, the elements of these systems are found in all fungal species that have been sequenced; however, the number of genes representing each signaling protein class often varies. In spite of this conservation, several interesting variations in how pathway components are arranged or regulated have also emerged. Furthermore, accumulating evidence indicates that multiple pathways often cooperate to regulate the same function in the same species, resulting in more complex signal transduction networks. The response regulators from two-component regulatory systems have also been shown to regulate Hog1p-like cascades in filamentous fungi. Two-component regulatory systems are major signal transduction pathways in filamentous fungi.

Citation: Park G, Jones C, Borkovich K. 2010. Signal Transduction Pathways, p 50-59. In Borkovich K, Ebbole D (ed), Cellular and Molecular Biology of Filamentous Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555816636.ch5

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Overview of signal transduction pathways in filamentous fungi. Environmental signals are sensed by GPCRs or other unknown proteins, leading to regulation of downstream pathways. Activation of heterotrimeric and small G proteins can result in modulation of cAMP levels and MAPK pathways. In response to environmental signals, free Ca transported into the cytoplasm from the external environment or from intracellular Ca stores binds to the Ca sensor calmodulin and other regulatory proteins. The Ca+-calmodulin complex regulates downstream targets, including the serine-threonine protein phosphatase calcineurin. Abbreviations: Gα, Gα subunit; Gβ, Gβ subunit; Gγ, Gγ subunit; g, small G proteins; PAK, p21-activated kinase; PKC, protein kinase C; AC, adenylyl cyclase; PKA-R, protein kinase A regulatory subunit; PKA-C, protein kinase A catalytic subunit; CaM, calmodulin.

Citation: Park G, Jones C, Borkovich K. 2010. Signal Transduction Pathways, p 50-59. In Borkovich K, Ebbole D (ed), Cellular and Molecular Biology of Filamentous Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555816636.ch5
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Image of FIGURE 2

Two-component regulatory systems. (A) Two-component signaling pathway. In simple two-component signaling pathways the two components are an HK and an RR protein. The sensor domain(s) located at the N terminus of the HK protein senses environmental signals. In response to such a signal, a histidine residue (H) in the HK domain autophosphorylates, using ATP. The same phosphate (P) is then transferred to an aspartate residue (D) on the RR protein. The phosphorylation status of the RR determines how it regulates downstream pathways, such as MAPK cascades. Alternatively, the RR may act directly as a transcription factor. (B) Multicomponent phosphorelays. Multicomponent phosphorelays contain HHKs, possessing both an HK and an RR domain within the same protein. The sensor domain(s) of the HHK located at the N terminus senses an environmental signal(s) to regulate autophosphorylation of the HHK on a histidine residue in the HK domain. The phosphate is then transferred intramolecularly from the HK domain to an aspartate residue in the RR domain of the same HHK protein. The phosphorelay continues with transfer of the phosphate from the RR of the HHK to a histidine residue on an HPT. This same phosphate is then passed onto an aspartate residue on a terminal RR protein.

Citation: Park G, Jones C, Borkovich K. 2010. Signal Transduction Pathways, p 50-59. In Borkovich K, Ebbole D (ed), Cellular and Molecular Biology of Filamentous Fungi. ASM Press, Washington, DC. doi: 10.1128/9781555816636.ch5
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