Selectivity Mechanism of the Nuclear Pore Complex Characterized by Single Cargo Tracking

The Nuclear Pore Complex (NPC) mediates all exchange between the cytoplasm and the nucleus. Small molecules can passively diffuse through the NPC, while larger cargos require transport receptors to translocate1. How the NPC facilitates the translocation of transport receptor/cargo complexes remains unclear. Here, we track single protein-functionalized Quantum Dot (QD) cargos as they translocate the NPC. Import proceeds by successive sub-steps comprising cargo capture, filtering and translocation, and release into the nucleus. The majority of QDs are rejected at one of these steps and return to the cytoplasm including very large cargos that abort at a size-selective barrier. Cargo movement in the central channel is subdiffusive and cargos that can bind more transport receptors diffuse more freely. Without Ran, cargos still explore the entire NPC, but have a markedly reduced probability of exit into the nucleus, suggesting that NPC entry and exit steps are not equivalent and that the pore is functionally asymmetric to importing cargos. The overall selectivity of the NPC appears to arise from the cumulative action of multiple reversible sub-steps and a final irreversible exit step.