Ligase activity-deficient COP1 mutants fail to induce degradation of C/EBPα and block granulocytic differentiation. (A) Schematic representation of ligase activity-deficient COP1 mutants. (B) Lysates from GFP-positive 32D cells infected with viruses containing GFP, GFP-COP1 (WT and mutants [CS and ΔRING]) were analyzed by western blotting using antibodies against COP1, C/EBPα, and γ-tubulin. (C) 32D transfectants (GFP, COP1 WT and mutants) prepared in B were maintained in IL3, transferred to G-CSF for 4 days, and analyzed for expression of Mac-1 by flow cytometer. Results shown in B and C are representative of 2 independent experiments. CS, an abbreviation of C138S/C141S.