Removing certain FG domains disrupts mRNA export (top row) or protein import (bottom row), but not both.

There's more than one way to pass through a nuclear pore, as Terry and Wente show. Different cargos follow different routes through the channels, suggesting a new mechanism for regulating import and export.

A nuclear pore is a bit like a high school hallway, with molecules and their carriers crowding through in both directions. As they make the crossing, carriers attach to sections of pore proteins (Nups) that harbor repeated stretches of phenylalanine and glycine. How binding to these FG repeat domains helps usher cargos through the pores remains controversial.

Three years ago, Wente's group tested the importance of particular FG repeats by making yeast mutants with different combinations of the domains excised. They found that transport doesn't require FG repeats in the filaments that protrude into the cytoplasm and into the basket that hangs from the nuclear side.

To pin down which FG segments are essential, Terry and Wente continued the research with new mutants. The scientists tracked how the loss of certain domains affected mRNA export from the nucleus and protein import via Kapβ carriers. If the removal of an FG domain blocked mRNA from exiting, it didn't necessarily prevent Kapβ cargos from entering, and vice versa. This difference indicates that the two types of cargos follow different paths through the pore. Cells might be able to control what goes in and out by altering FG domains in specific Nups.

Reference:

Terry, L.J., and S.R. Wente.
2007
.
J. Cell Biol.
178
:
1121
–1132.