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.” ▪