The Structure and Function of the Nuclear Pore Complex Michael Rexach, MCD Biology The nuclear pore complex (NPC) is a large protein structure embedded in the double-membrane of a cell's nucleus. This porin structure regulates all transport between the nucleoplasm and the cytoplasm of the cell. Research in the Rexach lab focuses on fundamental aspects of nucleocytoplasmic transport and on the architecture of the nuclear pore complex.

Research in the Rexach lab focuses on two fundamental aspects of nucleocytoplasmic transport: 1) the mechanics of karyopherin movement across the NPC and 2) the structure of the NPC transport conduit. Rexach is addressing these topics using a combination of cell biological, biochemical, biophysical, structural, genetic, molecular modeling and proteomics techniques in the model eukaryote S. cerevisiae.

I. Mechanics of karyopherin-mediated transport across the nuclear pore complex

Many karyopherins and their cargo have been identified but a mechanistic description of how they are mobilized within the NPC is lacking. Each NPC contains more than 200 potential docking sites for karyopherins (provided by nucleoporins that contain FG repeats), so movement of karyopherin-cargo complexes across the NPC is envisioned to be a stochastic process that operates via repeated association-dissociation reactions of karyopherins with FG nucleoporins. Currently, the Rexach lab is charting the path of transport used by karyopherins within the NPC through the identification of nucleoporins that physically contact them in situ within the NPC transport conduit. They are also addressing the kinetics of karyopherin transport across the NPC by characterizing the dynamics of association and dissociation between karyopherins, cargos and nucleoporins, and by identifying factors (KaRFs) that function to accelerate the dissociation rate of the most stable, long-lived intermediate complexes in the transport processes.

II. Architecture of the protein diffusion barrier in the nuclear pore complex

Although the mechanics of karyopherin-mediated transport across the NPC are still poorly understood, it is clear that interactions between karyopherins and FG Nups are central to the translocation process.  Thus, knowledge of the structural characteristics of FG nucleoporins may explain how karyopherin-cargo complexes of different shapes and sizes can translocate across the NPC while its permeability barrier remains intact. To that end, they are characterizing the structure of individual FG nucleoporins using biophysical, structural and molecular modeling techniques. The group has found that FG repeat regions of Nups are largely devoid of secondary structure and are mostly random coils 200-700 amino acids in length. Thus, the ~200 FG Nups present in each NPC could in principle form a flexible and highly amorphous meshwork of filaments at its center, which captures and ‘engulfs’ karyopherin-cargo complexes of different shapes and sizes as they move across the NPC. Indeed, Rexach's group recently discovered that a discrete subset of FG nups self-assemble in vitro and in vivo into a meshwork of filaments held together by weak hydrophobic interactions between FG repeats. The group is currently focusing its attention on elucidating the architecture and dynamics of this meshwork.