The function of Mss4p is essential for Slt2p activation in ptc1Δ mutant cells. Yeast lysates were extracted separately from strains SFNY1890 (lane 1), SFNY1891 (lane 3), SFNY1892 (lane 2), or SFNY1893 (lane 4). Sec10p was used as a loading control. To quantify the relative Slt2p activation, the density of the phospho-Slt2p band was divided by the density of Sec10p, and the normalized phospho-Slt2p value of each strain was divided by that of the ptc1Δ mutant. Error bars, SEM from three independent experiments.