Although scientists appreciate that the nuclear pore complex (NPC) is more than a complex structure, and that it is, in fact, an intricate machine, they are still trying to work out all the mechanistic details. For example, they are looking for the interlocking gears that drive NPC functions, such as barrier selectivity, the transport of cargo complexes, and cargo release from cargo complexes, once the cargo has been imported into the nucleus.
New research on nuclear transport regulation might overthrow established models, which tend to emphasize the role of the nuclear pore itself, rather than its accessories, which include protein factors called karyopherins (Kaps). Indeed, one new study, out of the University of Basel, suggests that Kaps known as importin alpha (or Kapα) and importin beta (or Kapβ) are at the center of NPC control.
In contrast to prevailing views, a research team led by Roderick Lim, Ph.D., Argovia Professor at the Biozentrum and the Swiss Nanoscience Institute of the University of Basel, has now demonstrated that the NPC does not work like a simple filter that regulates the nuclear transport process. Rather, Kapα and Kapβ cooperate to continuously open and close the pore like a revolving door.
Details appeared September 1 in the Journal of Cell Biology, in an article entitled “Karyopherins Regulate Nuclear Pore Complex Barrier and Transport Function.” The article focuses on how nucleocytoplasmic transport is sustained by Kaps and a Ran guanosine triphosphate (RanGTP) gradient that imports nuclear localization signal (NLS)–specific cargoes (NLS-cargoes) into the nucleus.
“Kapα facilitates Kapα turnover and occupancy at the NPC in a RanGTP-dependent manner that is directly coupled to NLS-cargo release and NPC barrier function,” wrote the article’s authors. “This is underpinned by the binding affinity of Kapβ1 to phenylalanine–glycine nucleoporins (FG Nups), which is comparable with RanGTP·Kapβ1, but stronger for Kapα·Kapβ1.”
For a long time, scientists have reasoned that a molecular filter within the NPC prevents or enables the passage of molecules into the nucleus. Prof. Lim's current study now shows that this filter alone is not sufficient for barrier function but provides only the basic infrastructure for establishing one. Instead, cargo-carrying importins function as bona fide components that regulate the NPC transport barrier.
Prof. Lim and colleagues show how the shuttling of Kap importins is coupled to their barrier function—Kapβ1 promotes cargo access into the pore, whereas Kapα determines the cargo that can enter the nucleus.
Surprisingly, the team has now discovered that importin Kapα acts as a molecular switch that helps to release or retain importin beta to open or close the pore. In the absence of Kapα, importin beta loses its ability to shuttle through the nuclear pore channel.
“RanGTP is ineffective at releasing standalone Kapβ1 from NPCs,” the authors explained. “Depleting Kapα·Kapβ1 by RanGTP further abrogates NPC barrier function, whereas adding back Kapβ1 rescues it while Kapβ1 turnover softens it. Therefore, the FG Nups are necessary but insufficient for NPC barrier function.”
The insights provided by the study also have implications for the understanding of diseases associated with transport defects at the NPC, such as cancer.
“We always thought of the NPC as a standalone machine that controls nuclear transport,” noted Prof. Lim. “Now, we have a much greater appreciation for how the systematic interplay of importin alpha and beta are able to regulate the NPC to sustain continuous transport.
“Hence, if importin alpha malfunctions, the revolving door mechanism might get stuck such that essential proteins cannot get to their nuclear destinations. Or if importin beta is defective, the pore might become leaky against unwanted substances that can enter and poison the nucleus.”
“Kaps,” the authors of the Journal of Cell Biology article concluded, “constitute integral constituents of the NPC whose barrier, transport, and cargo release functionalities establish a continuum under a mechanism of Kap-centric control.”