An hour or two after mitosis, chromosomes appear to be locked down, as only short-range, tethered diffusion-like movements had been seen during interphase. Yet several groups had noted that certain genes localize at the nuclear periphery when repressed but in the interior when active.
Chuang and colleagues visualized the movements accompanying this repositioning. They used an engineered locus that they could turn on through inducible targeting of a transcriptional activator. Upon transcription factor targeting, the group observed rapid, directed movements of the locus from the nuclear envelope toward the interior. The linear paths taken, often approximately perpendicular to the envelope, discount simple diffusion. During this motion, the locus moved at speeds approaching anaphase chromosome separation.
Although the existence of a nuclear actin network is still controversial, the group found a dependence of this long-range motion on actin and myosin. If the DNA is indeed transported on actin filaments, the preferential inward movements suggest that the intranuclear actin network is polarized. The authors are now testing whether the locus returns as directly to the periphery when the transcriptional activator is removed.