Mitotic segregation of viral and cellular acentric extrachromosomal molecules by chromosome tethering.

Mitotic chromosome segregation is mediated by spindle microtubules attached to centromeres. Recent studies, however, revealed that acentric DNA molecules, such as viral replicons and double minute chromosomes, can efficiently segregate into daughter cells by associating with mitotic chromosomes. Based on this similarity between viral and cellular acentric molecules, we introduced Epstein-Barr virus vectors into cells harboring double minute chromosomes and compared their mitotic behaviors. We added lac operator repeats to an Epstein-Barr virus vector, which enabled us to readily identify the transgene in cells expressing a fusion protein between the lac repressor and green fluorescent protein. Unexpectedly, we found that Epstein-Barr virus vectors integrated into the acentric double minute chromosomes, but not into normal chromosomes, in all of the six stably transfected clones examined. While transiently transfected Epstein-Barr virus vectors randomly associated with wheel-shaped prometaphase chromosome rosettes, the chimeras of double minute chromosomes and Epstein-Barr virus vectors in stably transfected clones always attached to the periphery of chromosome rosettes. These chimeric acentric molecules faithfully represented the behavior of native double minute chromosomes, providing a tool for analyzing their behavior in living cells throughout the cell cycle. Further detailed analyses, including real-time observations, revealed that double minute chromosomes appeared to be repelled from the spindle poles at the same time that they attached to the chromosome periphery, while centromeric regions were pulled poleward by the attached microtubules. Disrupting microtubule organization eliminated such peripheral localization of double minute chromosomes, but it did not affect their association with chromosomes. The results suggest a model in which double minute chromosomes, but not Epstein-Barr virus vectors, are subject to the microtubule-mediated antipolar force, while they both employ chromosome tethering strategies to increase their segregation to daughter cells.