Nuclear, but not cytoplasmic, cyclin B1 alleles induce apoptosis.

(A) Cyclin B-5xE (labeled 5xE) localizes predominantly to the nucleus, whereas cyclin B-5xA (labeled 5xA) is predominantly cytoplasmic (both GFP tagged). Hoechst nuclear counterstain is indicated. (B) The level of apoptosis in cyclin B-5xE–transfected cells is substantially higher than that induced by cyclin B-5xA. The apoptosis induced by cyclin B-5xA is comparable to control pCDNA3 transfected cells. Apoptosis in pCDNA3 control transfected cells is likely due to the transfection and staining protocol. Percent apoptosis was measured by annexin V staining and flow cytometry analysis. Total DNA (5 μg) was constant in all transfections. Error bars represent the mean and SD of 3 independent transfections. (C) Cyclin B-5xE expression induces apoptotic bodies (arrows) not seen in untransfected cells (U) or cells transfected with cyclin B-5xA. Original magnification A,C, × 1000.