Phosphorylation at Tyr380 by Lyn results in the formation of an inactive procasp8 dimer. (A) The elution profile of procasp8, as such or phosphorylated by Lyn and subsequently incubated in the absence or presence of 0.7 M sodium citrate, from a Superdex 200 HR column was determined by Wb analysis with anti-p-procasp8 and anti-procasp8 antibodies. (B) Equivalent protein amounts of procasp8, as such or phosphorylated by Lyn and subsequently incubated in the absence or presence of 0.7 M sodium citrate, were assayed for in vitro casp8 activity as described in the supplemental Data.