Nuclear transport is coordinated by nuclear Ran·GTP, which releases cargo from importin proteins and promotes cargo binding to exportin proteins. The role of Ran·GTP in cargo release is reasonably well understood. But, how does Ran·GTP function in nuclear protein export to promote cargo binding? Matsuura and Stewart now provide insights by describing the 2.0-Å resolution crystal structure of a nuclear export complex in Nature.

The nuclear export complex comprised Ran·GTP, Cse1 (a yeast exportin) and Kap60. Kap60 is an adaptor protein (yeast importin-α) that binds to nuclear localization signal (NLS)-containing cargo proteins and links them to Kap95 (yeast importin-β) in the cytoplasm. Following protein import, Kap60 is exported from the nucleus as Cse1 cargo.

A notable feature of the structure — which is different to that seen for nuclear import complexes — is that Cse1 surrounds Ran·GTP and Kap60. Cse1 interacts with Ran·GTP at two distinct sites, effectively locking it in the GTP-bound state. The Kap95-binding domain of Kap60 is also able to bind to the NLS-binding sites of Kap60 in an autoinhibitory manner and, in the nuclear export complex, this domain is clamped to the Kap60 NLS-binding sites by Ran·GTP and Cse1. The fact that this intramolecular interaction in Kap60 is required for Cse1 binding ensures that only cargo-free Kap60 is exported.

Mutagenesis studies confirmed the importance of Kap95-binding-domain interactions in nuclear-export-complex assembly and Kap60 export. In addition, the study of other mutant proteins indicated that free Cse1 adopts a different conformation to that seen in the nuclear export complex. It seems that Kap60 binding distorts Cse1 into a high-energy, strained conformation, such that the nuclear export complex is spring-loaded for disassembly following GTP hydrolysis in the cytoplasm.