Sanford M. Simon
Professor, Head of the Laboratory of Cellular Biophysics
April 3, 2012
The search for the Holey Grail: Transport Mechanism(s) of the nuclear pore
Passage of macromolecules into and out of the cell nucleus, a highly selective process, occurs through the nuclear pore complex (NPC). Transport of cargo with a nuclear localization signal (NLS) is mediated by the interaction of two soluble factors (karyopherins and Ran) and the FG-Nups (flexible filamentous proteins in the NPC center with repeats of the motif phenylalanine-glycine (FG)). The karyopherins are transport chaperones that bind NLS cargo. Ran is a small GTPase whose GTP:GDP gradient from the nucleus to cytoplasm is involved in cargo release. How these interactions give rise to selective transport is debated, though a number of qualitative hypotheses have been proposed.We have used fluorescence anisotropy to study a number of the nuclear pore components. These studies have allowed us to characterize both the orientation of the nuclear pore components as well as their dynamics. Based on our observations we have generated a computational model system based on known structural elements that yields a variety of data in agreement with macroscopic and single molecule experiments. A mechanism for transport emerges that is a hybrid of a number of the pre-existing qualitative hypotheses as well as a Brownian ratchet model, in which a cargo-karyopherin complex remains bound to the same FG-Nups for its entire trajectory through the NPC until RanGTP severs these to effect release into the nucleus.