Current nuclear pore models do not consider the possibility that transport time might vary under different conditions. But that is just what Yang and Musser found when they increased the concentration of importin β in an in vitro system. Transport speed increased as much as sevenfold. Transport was also more efficient—more of the molecules that entered the pore passed through it successfully.
Structure-based studies suggest that long strands of phenylalanine–glycine (FG) repeats extend from the edge of the nuclear pore into the channel, creating a spaghetti-like network that molecules must wiggle through as they traverse the pore. A single importin β protein can bind to several of these FG repeats at the same time. Yang and Musser hypothesize that, as importin β moves through the channel, it may temporarily rearrange and open up the FG meshwork.
Higher importin β concentrations also increased the rate and efficiency of dextran movement through the pore, even though dextran is small enough to move through the pore without a transport receptor. The team is currently studying the effect of hyperactive import on the rate and efficiency of nuclear export. If excess importin β structurally disrupts the meshwork, it might facilitate movement in both directions.