Modulation of KSR activity in Caenorhabditis elegans by Zn ions, PAR-1 kinase and PP2A phosphatase

Vulval differentiation in Caenorhabditis elegans is controlled by a conserved signal transduction pathway mediated by Ras and a kinase cascade that includes Raf, Mek and MAPK. Activation of this cascade is positively regulated by a number of proteins such as KSR ( kinase suppressor of Ras), SUR-8/SOC-2, SUR-6/PP2A-B and CDF-1. We describe the functional characterization of sur-7 and several genes that regulate signaling downstream of ras. We identified sur-7 by isolating a mutation that suppresses an activated ras allele, and showed that SUR-7 is a divergent member of the cation diffusion facilitator family of heavy metal ion transporters that is probably localized to the endoplosmic recticulum membrane and regulates cellular Zn2+ concentrations. Genetic double mutant analyses suggest that the SUR-7-mediated effect is not a general toxic response. Instead, Zn2+ ions target a specific step of the pathway, probably regulation of the scaffolding protein KSR. Biochemical analysis in mammalian cells indicates that high Zn2+ concentration causes a dramatic increase of KSR phosphorylation. Genetic analysis also indicates that PP2A phosphatase and PAR-1 kinase act downstream of Raf to positively and negatively regulate KSR activity, respectively.