Degradation of Saccharomyces cervisiae Rck2 upon exposure of cells to high levels of zinc is dependent on Pep4

In undisturbed cells, the MAPK-activated protein kinase Rck2 of Saccharomyces cerevisiae is a stable protein with a turnover time exceeding 60 min. However, we have found that Rck2 is subject to intracellular degradation after exposure of cells to Zn²⁺ concentrations of 5 mM or more. In high-zinc medium, most of the Rck2 pool is degraded within 5 min. This degradation is blocked by inhibiting the vacuolar proteolytic pathway with the protease inhibitor phenyl methyl sulphonyl fluoride or by mutation of the PEP4 gene. By contrast, blocking the proteasomal pathway with the inhibitor MG132 does not prevent Rck2 degradation upon addition of Zn²⁺, nor is degradation inhibited in the proteasomal mutations pre1 pre2, cim3, or cim5. The stability of Rck2 is not affected by any of the other stress conditions examined, or by growth rate. Possible mechanisms of the degradation of Rck2 under high zinc conditions, and its physiological significance, are discussed.