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Phosphate sensing for activation of the protein kinase A pathway in yeast


A central problem in modern cell biology is to elucidate how the cell perceives signals from the surrounding environment and how the signals are transmitted to affect metabolic pathways and gene expression. Increasing evidence indicates that eukaryotic cells also have many specific sensing mechanisms for extra-cellular nutrients, but a major challenge is to distinguish between true signalling and effects due to metabolism. The yeast Saccharomyces cerevisiae is one of the best model systems for eukaryotic cell re search because of its genetic tractability and powerful genomic approaches. Its protein kinase A pathway plays a major role in cellularregulation but its control by specific nutrients is not well understood.

The plasma membrane phosphate carriers Pho84 and P ho87 appear to act as phosphate sensors controlling this pathway. The first objective of this proposal is to identify the domains of these proteins involved in phosphate sensing through construction of chimeric proteins with non-sensing carriers. The second objective is to identify downstream factors of the regulatory cascade.

This will be done by transposon mutagenesis and multicopy-suppressor isolation in a strain expressing recently isolated constitutively activating ale of Pho84 causing reduced viability under stress conditions. Recent work has also indicated possible involvement of Pho85, a central cyclin-dependent protein kinase in phosphate regulatory pathways. Hence, the effect of Pho85 deletion on phosphate control of protein kinase A targets will also be determined.

Pho85 is a homologue of human Cdk5 kinase, which has been implicated in several metabolic and developmental processes, as well as in neuro degenerative and other diseases. Since the function of cyclin-dependent protein kinases is conserved in evolution, the role ofPho85 in co-ordination of phosphate and PKA signalling in yeast might reveal important novel features.

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