Nuclear Pore Transport Mechanisms Revealed by NMR Study

In eukaryotic cells, DNA is stored in the nucleus, separated from the rest of the cell by a double membrane called the nuclear envelope. Yet, many large molecules such as proteins and RNA must cross this barrier to carry out gene expression and other critical activities. They do so via regulated transport through nuclear pore complexes, large structures composed of at least 30 different types of nucleoporin proteins. To pass through these pores, large molecules must be coupled to proteins called transport factors, which interact with string-like proteins called phenylalanyl-glycyl repeat rich nucleoporins (FG Nups) that fill the interior of nuclear pores. This transport is specific (not all macromolecules are allowed through the pore) and at the same time very fast (a few milliseconds). So far, only theories had been postulated for how this is achieved.

To shed light on how nuclear transport can be both rapid and selective, researchers from the New York Structural Biology Center (NYSBC), Albert Einstein College of Medicine, and Rockefeller University used a technique called nuclear magnetic resonance (NMR) spectroscopy to understand the interactions between FG Nups and transport factors in a study published in eLife on September 15 (Hough et al. 2015). FG Nups are intrinsically disordered proteins (IDPs), which are characterized by the absence of any secondary structure (alpha helices, beta sheets) and are highly flexible, making them difficult to study. NMR is ideal for such studies since it is uniquely suited to provide information about protein dynamics/movement.

NMR spectroscopy is ideal to study highly flexible proteins such as FG nucleoporins (green, blue) that line the interior of nuclear pores. These proteins make many, transient contacts with transport factors (purple), allowing them to quickly pass through the pore while other proteins (light blue) are kept out.

Whereas most other studies of FG Nups have been performed in buffer solutions, Hough et al. studied their function and dynamic properties inside cells or cellular extracts. This difference was crucial in revealing the highly dynamic nature of these proteins when placed in more natural conditions. This approach also revealed that FG Nups remain mostly disordered even when bound to transport factors and interact with these proteins via only 2-4 amino acid-long regions (the FG repeats), all of which suggest a very transient interaction. These properties likely explain how rapid transport through the nuclear pore is possible. The fact that FG Nups remain disordered/flexible even when bound to transport factors might also explain why the nuclear pore continues to act as a selective barrier even when many macromolecules are passing through it.

This study highlights the versatility of NMR spectroscopy and its power to provide structural information about very challenging proteins. It is the first time that the dynamic, disordered nature of a protein is proposed to be a necessary part of its function. The data published by Hough et al., most of which required the multiple fields and high performance NMR at NYSBC, will lead to a better understanding of the nuclear pore and, consequently, of the diseases caused by dysfunctional nuclear pore complexes, such as triple A syndrome, ALS, and various leukemias, as well as of viral mechanisms of infection.

Research article: Hough et al. 2015, eLife;10.7554/eLife.10027