Interleukin-3 induces the phosphorylation of a distinct fraction of Bcl-2

Bcl-2-related proteins (i.e. Bcl-2 and Bax) regulate the effector stage of apoptosis and can modulate the entry of quiescent cells into the cell cycle . Phosphorylation of Bcl-2 is presumed to modify its apoptosis-inhibitory f unction. By utilizing an interleukin-3 (IL-3)-dependent hematopoietic cell line, we examined the structural requirements of Bcl-2 phosphorylation and the correlation of this post-translational modification with its function. In the presence of IL-3, constitutively expressed Bcl-2 was phosphorylated on serine residue(s), and phospho rylated Bcl-2 lost its capacity to hetero dimerize with Bax. Whereas the majority of Bcl-2 resided in mitochondria, p hosphorylation only affected a minor pool of total Bcl-2 that selectively p artitioned into a soluble fraction. Cytosolic targeting of Bcl-2 greatly in creased its ratio of phosphorylation. Bcl-2 phosphorylation was reduced dur ing IL-3 deprivation, and its phosphorylation was also delayed after transi ent cytokine deprivation. This pattern of phosphorylation temporally correl ated with the accelerated exit and delayed reentry of Bcl-2-expressing cell s into the cell cycle upon transient IL-3 deprivation and subsequent cytoki ne restimulation. Thus, IL-3-induced phosphorylation of a distinct pool of Bcl-2 may contribute to the inactivation of its antiproliferative function.