Posttranslational modification of NirA nuclear export signal

Rational: We have previously identified the nuclear accumulation of the nitrate-specific transcription factor NirA as a checkpoint in nitrate-dependent activation of this protein. Accumulation in the nucleus is a consequence of inhibition of attachment of nuclear export proteins to NirA when nitrate is present in the cell. By proteomic analysis of NirA we found a specific modification of an amino acid which forms part of the attachment site for the NirA export protein. In this project we plan to study in detail the underlying signalling pathway. Description: The common soil fungus Aspergillus nidulans has proven to be one of the most advanced model organism to study the molecular basis of eukaryotic nitrate assimilation and a wealth of information is available on the main regulators governing the expression of genes responsible for the uptake and subsequent reduction of nitrate to ammonia. In this project we plan to characterize in detail a recently discovered molecular switch which signals the presence or absence of nitrate by a post-translational modification of a specific amino acid in NirA. This amino acid is located in a small domain of NirA (called nuclear export signal, or NES) which is required and sufficient to interact with the export complex that transports NirA out of the nucleus under nitrate-deficient conditions. Interestingly, nirA gain-of-function mutants isolated following a genetic screen show alterations in the NES and in a neighboring region. We plan to study the interaction of different NirA-NES forms with the export complex and characterize putative signalling pathways which may add or remove the NES-specific post-translational modification of NirA in response to nitrate. The results from our studies will possibly unravel details about a new post-translational modification regulating the subcellular localization and activity of a eukaryotic transcription factor. Additionally, the work may result in new insights in the signalling pathway underlying fungal, and possibly also plant nitrate assimilation.

Rational: We have previously identified the nuclear accumulation of the nitrate-specific transcription factor NirA as a checkpoint in nitrate-dependent activation of this protein. Accumulation in the nucleus is a consequence of inhibition of attachment of nuclear export proteins to NirA when nitrate is present in the cell. By proteomic analysis of NirA we found a specific modification of an amino acid which forms part of the attachment site for the NirA export protein. In this project we plan to study in detail the underlying signalling pathway. Description: The common soil fungus Aspergillus nidulans has proven to be one of the most advanced model organism to study the molecular basis of eukaryotic nitrate assimilation and a wealth of information is available on the main regulators governing the expression of genes responsible for the uptake and subsequent reduction of nitrate to ammonia. In this project we plan to characterize in detail a recently discovered molecular switch which signals the presence or absence of nitrate by a post-translational modification of a specific amino acid in NirA. This amino acid is located in a small domain of NirA (called nuclear export signal, or NES) which is required and sufficient to interact with the export complex that transports NirA out of the nucleus under nitrate-deficient conditions. Interestingly, nirA gain-of-function mutants isolated following a genetic screen show alterations in the NES and in a neighboring region. We plan to study the interaction of different NirA-NES forms with the export complex and characterize putative signalling pathways which may add or remove the NES-specific post-translational modification of NirA in response to nitrate. The results from our studies will possibly unravel details about a new post-translational modification regulating the subcellular localization and activity of a eukaryotic transcription factor. Additionally, the work may result in new insights in the signalling pathway underlying fungal, and possibly also plant nitrate assimilation.