Myeloid cell leukemia 1 is phosphorylated through two distinct pathways, one associated with extracellular signal-regulated kinase activation and theother with G(2)/M accumulation or protein phosphatase 1/2A inhibition

Protein kinase C activators and microtubule-damaging drugs stimulate BCL2 p hosphorylation, which has been associated with either enhancement or inhibi tion of cell viability. In a Burkitt lymphoma cell line, both types of agen ts likewise stimulated phosphorylation of myeloid cell leukemia 1 (MCL1), a nother viability-promoting BCL2 family member. However, while MCL1 phosphor ylation induced by the protein kinase C activator, 12-O-tetradecanoylphorbo l-13-acetate (TPA), did not affect its electrophoretic mobility, microtubul e-damaging agents, such as taxol, induced MCL1 phosphorylation associated w ith a band shift to decreased mobility. Inhibitors of extracellular signal- regulated kinase (ERK) activation blocked TPA-induced MCL1 phosphorylation but not the taxol-induced band shift, TPA-induced MCL1 phosphorylation occu rred rapidly and was not associated with decreased viability, while the tax ol-induced band shift occurred upon extended exposure as cells accumulated in G(2)/M followed by cell death. Protein phosphatase 1/2A inhibitors also induced the MCL1 band shift/phosphorylation. Thus, MCL1 undergoes two disti nct types of phosphorylation: (i) TPA-induced, ERK-associated phosphorylati on, which does not alter the electrophoretic mobility of MCL1, and (ii) ERK -independent phosphorylation, which results in an MCL1 band shift and is in duced by events in G(2)/M or protein phosphatase 1/2A inhibitors.