Karyopherins play the field

Proteins that shuttle molecules into the nucleus face the same problem as customers of a dating service. To find the right partner, they have to sort through a lot of bad matches, as Timney et al. show on page 579. The time required to search out a target provides the biggest limitation on how fast cells can import materials into the nucleus, the results indicate.

The transport proteins in question are karyopherins, which latch onto molecules that carry a nuclear localization sequence (NLS) and shepherd them through nuclear pore complexes. Researchers have studied the process in groups of cells by halting transport with a poison that blocks ATP production. Removing the poison allows import to resume within seconds. By applying quantitative microscopy, Timney et al....