In situ analysis of spatial relationships between proteins of the nuclear pore complex.

Macromolecular transport between the nucleus and cytoplasm occurs through the nuclear pore complexes (NPCs). The NPC in the budding yeast Saccharomyces cerevisiae is a 60-MDa structure embedded in the nuclear envelope and composed of ~30 proteins, termed nucleoporins or nups. Here we present a large-scale analysis of spatial relationships between nucleoporins using fluorescence resonance energy transfer (FRET) in living yeast cells. Energy transfer was measured in a panel of strains, each of which coexpresses the enhanced cyan and yellow fluorescent proteins as fusions to distinct nucleoporins. With this approach, we have determined 13 nucleoporin pairs yielding FRET signals. Independent experiments are consistent with the FRET results: Nup120 localization is perturbed in the nic96-1 mutant, as is Nup82 localization in the nup116Delta mutant. To better understand the spatial relationship represented by an in vivo FRET signal, we have investigated the requirements of these signals. We demonstrate that in one case FRET signal is lost upon insertion of a short spacer between the nucleoporin and its enhanced yellow fluorescent protein label. We also show that the Nup120 FRET signals depend on whether the fluorescent moiety is fused to the N- or C-terminus of Nup120. Combined with existing data on NPC structure, the FRET pairs identified in this study allow us to propose a refined molecular model of the NPC. We suggest that the approach may serve as a prototype for the in situ study of other large macromolecular complexes.