In vivo analysis of human nucleoporin repeat domain interactions

The nuclear pore complex (NPC), assembled from similar to 30 proteins termed nucleoporins (Nups), mediates selective nucleocytoplasmic trafficking. A subset of nucleoporins bear a domain with multiple phenylalanine-glycine (FG) motifs. As binding sites for transport receptors, FG Nups are critical in translocation through the NPC. Certain FG Nups are believed to associate via low-affinity, cohesive interactions to form the permeability barrier of the pore, although the form and composition of this functional barrier are debated. We used green fluorescent protein-Nup98/HoxA9 constructs with various numbers of repeats and also substituted FG domains from other nucleoporins for the Nup98 domain to directly compare cohesive interactions in live cells by fluorescence recovery after photobleaching (FRAP). We find that cohesion is a function of both number and type of FG repeats. Glycine-leucine-FG (GLFG) repeat domains are the most cohesive. FG domains from several human nucleoporins showed no interactions in this assay; however, Nup214, with numerous VFG motifs, displayed measurable cohesion by FRAP. The cohesive nature of a human nucleoporin did not necessarily correlate with that of its yeast orthologue. The Nup98 GLFG domain also functions in pore targeting through binding to Nup93, positioning the GLFG domain in the center of the NPC and supporting a role for this nucleoporin in the permeability barrier.