Import of proteins (green) into the nucleus is slower than expected, revealing that karyopherins are choosy about their cargo.

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. achieved a first: measuring how quickly labeled molecules entered the nucleus in individual yeast cells.

Theoretical calculations based on variables such as diffusion rate and the time needed to pass through a nuclear pore suggest that a karyopherin should take 1/10th of a second to find a cargo and ferry it to its destination. But the process actually requires about 10 seconds, the scientists found. They hypothesized that the slowdown occurs because karyopherins try out numerous potential cargoes, including those lacking an NLS, before settling on one.

To test the idea, the researchers incubated purified karyopherins and NLSs with increasing concentrations of bacterial cytosol. Even though bacteria contain no proteins with an NLS, the cytosol still inhibited binding between karyopherins and the recognition sequences. Thus, instead of homing in on a specific target, karyopherins grope their way through the crowd of molecules in the cell until they find one with a corresponding NLS. Although this search strategy seems inefficient, it might be the price cells pay for having karyopherins that must haul an assortment of molecules.