The mechanism behind nuclear pore complexes — the cellular structures that control how materials move between the nucleus and the cytoplasm — has come more sharply into focus. Nuclear pore complexes (NPCs), which span the nuclear membrane in eukaryotic cells, play a fundamental role in many aspects of cellular physiology, including gene expression. Defects in NPC function are implicated in some autoimmune diseases, as well as leukemia and others cancers. Nuclear transport also plays an important role in viral infections. However, it has been unclear how the NPC aids the selective movement of macromolecules across the nuclear membrane. Video imaging captured the moment when a single quantum dot crossed the membrane through the nuclear pore complex. (Image: NetDyaLog) Researchers at the University of California, Berkeley, captured this cellular motion using a custom-built near-total internal reflection microscope. Signals from single, protein-functionalized quantum dot cargos were captured by the system as the particles moved through human NPCs. They found that overall selectivity of the NPC arises from the cumulative action of multiple, reversible substeps and a final, irreversible exit step. “With their extraordinary photostability and brightness, quantum dots have established themselves as very useful tools for cellular analysis,” said Berkeley’s Karsten Weis. “Because of their relatively large size, which is comparable to viral particles, the transport of quantum dots across the NPC is quite slow. This, in combination with their photostability, allowed us real-time tracking over extended periods of time and reconstruction of NPC transport events into high-precision transport trajectories.” Weis’ lab is actively engaged in characterizing and analyzing the molecular machinery responsible for the transport of macromolecules into and out of the nucleus through a combination of genetic, biochemical and biophysical approaches. For more information, visit: www.berkeley.edu