Single-molecule studies of nucleocytoplasmic transport: from one dimension to three dimensions

In eukaryotic cells, the bidirectional trafficking of proteins and genetic materials across the double-membrane nuclear envelope is mediated by nuclear pore complexes (NPCs). A highly selective barrier formed by the phenylalanine-glycine (FG)-nucleoporin (Nup) in the NPC allows for two transport modes: passive diffusion and transport receptor-facilitated translocation. Strict regulation of nucleocytoplasmic transport is crucial for cell survival, differentiation, growth and other essential activities. However, due to the limited knowledge of the native configuration of the FG-Nup barrier and the interactions between the transiting molecules and the barrier in the NPC, the precise nucleocytoplasmic transport mechanism remains unresolved. To refine the transport mechanism, single-molecule fluorescence microscopy methods have been employed to obtain the transport kinetics of individual fluorescent molecules through the NPC and to map the interactions between transiting molecules and the FG-Nup barrier. Important characteristics of nucleocytoplasmic transport, such as transport time, transport efficiency and spatial distribution of single transiting molecules in the NPC, have been obtained that could not be measured by either ensemble average methods or conventional electron microscopy. In this critical review, we discuss the development of various single-molecule techniques and their application to nucleocytoplasmic transport in vitro and in vivo. In particular, we highlight a recent advance from one-dimensional to three-dimensional single-molecule characterization of transport through the NPC and present a comprehensive understanding of the nucleocytoplasmic transport mechanism obtained by this new technical development (105 references).