Spatial relationships in pH signalling in a model filamentous fungus: roles of MVB pathway and plasma membrane components

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A vitally important feature of the environment of an organism is the degree of acidity or alkalinity, i.e. the pH. Fungi tailor the combinations of the enzymes they secrete, the nutrient transporters they synthesise and the products (such as toxins and antibiotics) they export to the pH of their environment. The ability of fungi to adapt to a range of environmental pH is crucial to their ability to infect both plants and animals (including humans) and to their ability to produce toxins such as aflatoxin and antibiotics such as penicillin and cephalosporin. Our work is directed towards determining how a signal is triggered by environmental alkalinity, ultimately leading to the processing of a DNA-binding protein such that it is able to 'turn on' fungal genes whose action is appropriate to alkaline environments and 'turn off' those appropriate to acidic environments. Previous work has shown that seven proteins are exclusively involved of which one is the DNA-binding protein and the other six are involved in transmitting the pH signal. These six signalling proteins include two located at the cell surface, one of which facilitates localisation of the other which is probably the pH sensor, a protein interacting with one of the surface proteins, two proteins interacting with a component of the cellular protein-sorting machinery of which one also interacts with the DNA-binding protein and a protein-cutting enzyme involved in activating the DNA-binding protein. In addition, such signalling of environmental pH involves some components of intracellular structures whose other job is the sorting of certain proteins towards an appropriate location within the cell or towards destruction. In this project we will use the model filamentous fungus Aspergillus nidulans, a relatively benign organism but related to the serious opportunistic pathogen Aspergillus fumigatus, the aflatoxin-producing agricultural scourge Aspergillus flavus and the filamentous fungus used in the fermentation to produce sake Aspergillus oryzae. We shall determine which of the protein sorting components are involved in signalling environmental pH and examine the consequences of mutations affecting those components. In some cases these mutations seriously impair growth of the fungus but other mutations can abrogate that impairment. We shall determine the mechanism of abrogation. We shall determine whether the demands of pH signalling result in a shortage of those components also needed for protein sorting and whether the sorting machinery is able to attenuate pH signalling. Finally we will investigate how the localisation of the pH sensor at the cell surface is assisted by the other pH signalling cell surface protein.