FCP1 inhibits capping. (A) Ternary +24 complexes were incubated alone (lane 1) or with 0.06, 0.12, and 0.6 ng of CE and GTP for 5 min (lanes 2-4). In parallel, +24 complexes were first incubated with 60 ng of FCP1 for 30 min at 30°C, washed with TB500, and then equilibrated with TB60. Finally, FCP1 treated complexes were incubated with 0.06, 0.12, and 0.6 ng of CE (lanes 5-7) and GTP for an additional 5 min. Lane 8: complexes treated with FCP1 only. (B) Quantitation of A. (C) Ternary +24 complexes were preincubated with FCP1 or catalytically inactive mutant FCP1a for 30 min at 30°C and then treated with 12 ng of CE and GTP for an additional 5 min.