The nuclear pore as a chromatin boundary

Exportins are nuclear transport proteins associated with the nuclear pore complex (NPC). Evidence from Ulrich Laemmli (University of Geneva, Switzerland) and colleagues suggests the proteins act as chromatin boundaries, blocking the spread of heterochromatin by tethering the DNA to the NPC.Laemmli's group constructed a boundary-trap yeast strain to screen for proteins that can insulate a reporter gene from the silent mating-type HML locus flanking it on either side. A large number of genes came out of the screen, says Laemmli, but the exportins, including Cse1p, Mex67p, and Los1p, are the first to be characterized.

Their boundary activity (BA) depends on a small COOH-terminal region in each of the proteins, a domain distinct from the Ran/GTP-binding domain required for transport activity. The exportins' BA relies on interaction with Nup2, a protein primarily associated with the nuclear pore....