Cell Biology

Keep the Exits Clear

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Science  07 May 2010:
Vol. 328, Issue 5979, pp. 669
DOI: 10.1126/science.328.5979.669-b

Open any cell biology textbook and you will find a transmission electron micrograph of a cross section through a cell nucleus. The dense staining of heterochromatin, which is largely inactive and highly condensed, will be clearly visible as dark patches, often lining the inner surface of the nuclear membrane. However, there will be periodic gaps in these heterochromatic territories at peripheral sites where the nuclear pore complexes (NPCs) are situated. NPCs are macromolecular structures that enable the transit of nucleic acids and proteins across the nuclear envelope. Although the existence of these NPC-associated heterochromatin-free regions has been known for over 50 years, how they are generated and what their function is have remained unknown.

Krull et al. have discovered a role for the NPC-binding protein Tpr in establishing NPC-associated zones free of heterochromatin. In a cell culture model where heterochromatin condensation and heterochromatin exclusion zones around the NPCs could be induced vigorously, they found that depletion of Tpr resulted in dramatic nuclear reorganization as heterochromatin became evenly spread around the nuclear periphery and NPC-associated exclusion zones were lost. Keeping the NPC portals clear of heterochromatin might facilitate passage of the 1000 molecules that are transported through a single NPC per second in human cells. That a single protein can generate these exclusion zones is somewhat surprising and an important first step.

EMBO J. 29, 10.1038/emboj.2010.54 (2010).

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