ABSTRACT
Recognition of environmental stimuli plays a fundamental role for eukaryotic cells. Many different signals are received at the surface of cells and submitted by transmembrane signaling pathways to elicit specific responses such as the induction of gene transcription, protein phosphorylation, or cytoskeleton reorganization. The movement of signals can be simple, such as those associated with receptor molecules that constitute channels which upon ligand interaction, allow the signals to be passed in the form of small ion movements, either into or out of the cell. These ion movements result in changes in the electrical potential of the cells and these in turn propagate the signal along the cell. More complex signal transduction involves the coupling of ligand-receptor interactions to many intracellular events which include phosphorylations by different kinases. The receptors used are of different types: e.g., G-protein-coupled receptors, His-Asp phosphorelay sensors, etc. The eventual outcome is an alternation in cellular activity and changes in the program of genes expressed within the responding cells that results in a cellular response to the external environment. Eukaryotic cells predominantly use serine, threonine, and tyrosine phosphorylation in various intracellular signal transduction pathways, among which the MAPK cascade module is a key element in mediating the transduction to the nucleus of many signals generated at the cell surface (Banuett 1998). Fungal cells can also sense many different chemical and physical stimuli such as nutrients, osmolarity, pH, light, or surface hydrophobicity. Furthermore, fungi recognize the presence of mating partners by pheromones and pathogenic fungi respond to signals from their respective hosts. Besides the model organism Saccharomyces cerevisiae whose signaling cascades are quite well characterized (e.g., Banuett 1998; Gustin et al. 1998; Hunter and Plowman 1997; Thevelein 1994), the information about signal transduction pathways in filamentous fungi is fragmentary. Even though a significant percentage of the fungal genome encodes phosphorylationrelated proteins, little is understood regarding their function, the interacting molecules, and pathways in which they are involved. Investigation of the signal transduction cascades that regulate fungal development and virulence and the related signaling pathways that operate in yeast, human pathogens, plant pathogens, and in model filamentous fungi reveal a high degree of conservation.
