The adaptive filter of the yeast galactose pathway.
Serge SMIDTAS, Vincent Schachter, Francois Kepes
BioInformatic, Genoscope, 2 rue Gaston Cremieu, Evry, 91000, France
In the yeast Saccharomyces cerevisiae, the interplay between galactose, Gal3p, Gal80p and Gal4p determines the transcriptional status of the genes required for galactose utilization. After a step increase, galactose binds onto Gal3p. This conducts throw Gal80p to the activation of Gal4p which then activates GAL3 and GAL80 gene transcription. Here we propose a qualitative dynamical model and that these are the two first stages of a functional feedback loop that closes with the capture of activation of Gal4p by newly synthesized Gal3p and Gal80p, thus decreasing transcriptional activation and reconstituting the proteins that make the signaling cascade from galactose to Gal4p and that can bind incoming galactose molecules. Based on the differential time scales of fast protein interactions versus slower biosynthetic steps, this feedback loop functions as a derivation filter where galactose is the input step signal and released Gal4p is the output derivative signal. One advantage of such a derivation filter is that GAL genes are expressed in proportion to the cellular requirement. Furthermore, this filter adaptively protects the cellular receptors from saturation by galactose, allowing cells to remain sensitive to variations in galactose concentrations rather than to absolute concentrations. Finally, this feedback loop, by allowing phosphorylation of some active Gal4p, may be essential to initiate the subsequent long term response.