The proapoptotic BH3-only protein bim is expressed in hematopoietic, epithelial, neuronal, and germ cells

Proapoptotic Bcl-2 family members activate cell death by neutralizing their anti-apoptotic relatives, which in turn maintain cell viability by regulat ing the activation of the cell death effecters, the caspases, Bim belongs t o a distinct subgroup of proapoptotic proteins that only resemble other Bcl -2 family members within the short BH3 domain. Gene targeting experiments i n mice have shown that him is essential for the execution of some but not a ll apoptotic stimuli, for hematopoietic cell homeostasis, and as a barrier against autoimmunity. There are three Bim isoforms, Bim(S), Bim(L), and Bim (EL), which have different proapoptotic potencies due at least in part to d ifferences in interaction with the dynein motor complex. The expression pat tern of Bim was investigated by immunohistochemical staining, immunoprecipi tation followed by Western blotting, and in situ hybridization, him was fou nd in hematopoietic, epithelial, neuronal, and germ cells. Bim(L) and Bim(E L) were coexpressed at similar levels In many cell types, but Bim(S) was no t detected. Microscopic examination revealed a punctate pattern of Bim(L) a nd Bim(EL) immunostaining, indicating association with cytoplasmic structur es. These results are discussed in the context of the phenotype of Bim-defi cient mice and the post-translational regulation of Bim's pro-apoptotic act ivity.