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. In yeast strains lacking Nup2 or the NPC-interacting Nup2 domain, the exportins lack BA. Thus, the exportin BA seems to rely on a physical bridge between the DNA and the NPC.

This sort of physical tethering, especially to the NPC, is not the only way chromatin boundaries are formed, and the physiological role of the interaction is not yet established, stresses Laemmli. But, he says, this unexpected turn “gives us a new view of something we don't know that much about.” ▪

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