Activation-dependent
degradation of the MEKK Ste11p.
Nicolas Dard (1),
Frank van Drogen (2), Matthias Peter (1)
(1) Institute of Biochemistry, ETH Honggerberg, Building HPM, Zurich, 8093,
Switzerland (nicolas.dard@bc.biol.ethz.ch); (2) The Scripps Research Institute,
Department of Molecular Biology, La Jolla, CA 92037 USA
Ste11p is involved in three different MAPK cascades in the yeast Saccharomyces cerevisiae: the mating pathway (Fus3p/Kss1p), the high osmolarity glycerol pathway (Hog1p), and the pseudohyphal growth pathway (Kss1p). However, the activity of Ste11p remains pathway-specific and cross-activation of these cascades does not occur in wild type cells. Scaffold proteins most likely play a central role in mediating pathway specificity by providing local activation platforms and insulating the MAPK modules. However, negative feedback loops may also represent an efficient mechanism to restrict MAPK cascades. Indeed, our recent data suggest that Ste11p is targeted for degradation through a MAPK-dependent mechanism after its activation: i) Ste11p was shown to be unstable after pheromone treatment; ii) in contrast to all other members of the MAPK module, Ste11p-GFP cannot be detected at the shmoo tip; iii) activated forms of Ste11p are very unstable in vivo, while an inactive mutant is stable; iv) the activity of the three downstream MAPKs is involved in this process since the stability of these active Ste11p mutants is completely restored in fus3Δkss1Δhog1Δ strains. Our goal is to identify the machinery responsible for the degradation of Ste11p. Interestingly, the mating-specific scaffold protein Ste5p contains a ring finger domain, and this type of domain is present in all known ubiquitin E3-ligases. We are currently investigating its potential role in Ste11p degradation.