Western blot of GCN2-Pi, GCN2, ATF4, and PHGDH, with VCL as a loading control in PhgdhcKO cells supplemented with downstream metabolites of serine. PhgdhWT cells were used a control for baseline ISR activation. Cells were supplemented with increasing levels of serine and glycine, from 0 to 7.5 mM. CM concentration is 0.3 mM, which is indicated by the black arrow. Numbers above rows reflect semi-quantitative densitometry normalized to PhgdhWT lysate in CM. Note: both blots from (A) and (B) are from the same gel, so the WT Phgdh lane is the same.
