Popping out of the nucleus

摘要

Previous structural snapshots have provided insight into how proteins are imported into the cell nucleus. The structure of an export complex now completes the molecular picture of the nuclear transport cycle. A notable stage in evolution was the compartmentalization of cells, producing the eukaryotic cell — the cell type found in organisms from fungi to humans. During this process, the genetic information became enclosed inside a nucleus, spatially separated from the rest of the cell. The only way that macromolecules can be transported into and out of the nucleus is through large protein assemblies termed nuclear pore complexes, which are located in circular openings in the nuclear membrane, or 'envelope'. Mobile proteins that ferry cargo are also required for import and export, as is another protein, Ran, whose conformational state alters the affinity of the carriers for their cargo. On page 872 of this issue, Matsuura and Stewart reveal the structure of one nuclear export protein complex. Comparing this snapshot with structures of nuclear import complexes sheds light on why Ran has different roles depending on whether cargo is being directed into or out of the nucleus.