Protein kinase C activators and microtubule-damaging drugs stimulate BCL2 phosphorylation, which has been associated with either enhancement or inhibition of cell viability. In a Burkitt lymphoma cell line, both types of agents likewise stimulated phosphorylation of myeloid cell leukemia 1 (MCL1), another viability-promoting BCL2 family member. However, while MCL1 phosphorylation induced by the protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), did not affect its electrophoretic mobility, microtubuledamaging agents, such as taxol, induced MCL1 phosphorylation associated with 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 occurred rapidly and was not associated with decreased viability, while the taxol-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 distinct types of phosphorylation: (i) TPAinduced, ERK-associated phosphorylation, which does not alter the electrophoretic mobility of MCL1, and (ii) ERK-independent phosphorylation, which results in an MCL1 band shift and is induced by events in G 2 /M or protein phosphatase 1/2A inhibitors.Both anti-and proapoptotic BCL2 family members undergo phosphorylation, as is seen with BCL2, BCLX, and BAD. The role of this post-translational modification has been well defined for the proapoptotic family member, BAD, where phosphorylation promotes association with a 14-3-3 protein instead of BCLX, thereby freeing BCLX to exert its antiapoptotic activity (1-6). However, the role of phosphorylation in the case of BCL2 and other antiapoptotic family members is not yet completely understood. BCL2 phosphorylation is induced by several types of agents. These include growth factors and protein kinase C activators, such as erythropoietin, interleukin-3, and bryostatin-1 (7-9). They also include microtubule-directed agents, such as taxol and nocodazole, as well as the protein phosphatase 1/2A inhibitor, okadaic acid (10 -18). BCL2 phosphorylation has been associated with viability promotion in some cases and with cell death in others (8, 18). These varying results could relate to the fact that BCL2 phosphorylation has been studied by several investigators using different agents in varied cell lines.Our studies focus on myeloid cell leukemia 1 (MCL1), 1 an antiapoptotic BCL2 family member identified by its rapid up-regulation during 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced differentiation of myeloid ML-1 cells (19). Previous studies on MCL1 regulation indicated that TPA increases MCL1 expression within 3 h through an extracellular signal-regulated kinase 1/2 (ERK1/2)-dependent signal transduction pathway, which activates a serum response factor-Elk-1 transcription factor complex (20, 21). Microtubule-disrupting agents such...