From the trap to the basket: getting to the bottom of the nuclear pore complex

Abstract

Nuclear pore complexes (NPCs) are large supramolecular assemblies that perforate the doublemembraned nuclear envelope and serve as the sole gateways of molecular exchange between the cytoplasm and the nucleus in interphase cells. Combining novel specimen preparation regimes with innovative use of highresolution scanning electron microscopy, Hans Ris produced in the late eighties stereo images of the NPC with unparalleled clarity and structural detail, thereby setting new standards in the field. Since that time, efforts undertaken to resolve the molecular structure and architecture, and the numerous interactions that occur between NPC proteins (nucleoporins), soluble transport receptors, and the small GTPase Ran, have led to a deeper understanding of the functional role of NPCs in nucleocytoplasmic transport. In spite of these breakthroughs, getting to the bottom of the actual cargo translocation mechanism through the NPC remains elusive and controversial. Here, we review recent insights into NPC function by correlating structural findings with biochemical data. By introducing new experimental and computational results, we reexamine how NPCs can discriminate between receptor-mediated and passive cargo to promote vectorial translocation in a highly regulated manner. Moreover, we comment on the importance and potential benefits of identifying and experimenting with individual key components implicated in the translocation mechanism. We conclude by dwelling on questions that we feel are pertinent to a more rational understanding of the physical aspects governing NPC mechanics. Last but not least, we substantiate these uncertainties by boldly suggesting a new direction in NPC research as a means to verify such novel concepts, for example, a de novo designed 'minimalist' NPC.

Rapid Evolution Exposes the Boundaries of Domain Structure and Function in Natively Unfolded FG Nucleoporins

“…Their unusually high content of charged and polar AAs (Table I and Supplemental Table S3) is a hallmark of disordered proteins in general (11). Biophysical experiments will be necessary to test conclusively the structural characteristics of each FG Nup in higher eukaryotes, but recent studies support our prediction that their FG domains also exist in a natively unfolded state (43)(44)(45). The apparent conservation of structural disorder in FG domains of Nups throughout Eukaryotae indicates that their inherent structural flexibility plays an important role in NPC function possibly for efficient capture and release of karyopherin-cargo complexes during transport across the NPC or as architectural elements of a size-selective permeability barrier.…”

Managing Free-energy Barriers in Nuclear Pore Transport

“…Given the simplicity of this structure, we expect that Nature has adopted it for other active transport and filtration purposes involving macromolecules. Furthermore, we expect that a model system of NPC, which can provide active transport easily can be realized experimentally with modern surface and polymer grafting techniques [4,26].…”

“…It is striking that a third of the NPC mass is constituted by multiple copies of 13 different Nups, which are rich in phenylaline and glycine. These FG-Nups lack secondary structure [4], with Stokes' radii in solution up to 10 nm [5] and act as binding sites for the cargo-carrier complex, reviewed in [6]. Also, it is noteworthy that no molecular motors or other enzymes associated with transport are located in NPC.…”

Nuclear Membrane Disassembly and Rupture

“…The NPC has an eightfold rotational symmetry and comprises multiple copies of ~ 30 or more (6,7) nucleoporins. The structural framework of the NPC, consists of the spoke-ring complex, the central transporter (8), cytoplasmic and nucleoplasmic rings, attached cytoplasmic filaments and the nuclear basket, (9)(10)(11)(12)(13)(14).…”