Western blotting was performed as previously described [49 (link)]. Primary antibodies were: rat anti-ATE1 (Merck Millipore; clone 6F11), mouse anti-GAPDH (Abcam; ab8245), mouse anti-actin (08691001, MP Biomedicals), mouse anti-tubulin (T9026, Merck), rabbit anti-ERK (#4695), rabbit anti-phosphorylated ERK (#4370), rabbit anti-phosphorylated S6K (Thr421/Ser424, #9204), rabbit anti-V5 (#13202) and rabbit anti-AXIN1 (#2087) (all from Cell Signaling Technology). HRP-conjugated secondary antibodies were purchased from Jackson ImmunoResearch and proteins were visualized by chemiluminescence (Image-Quant LAS400, GE Healthcare).