Chromatin boundaries in budding yeast: the nuclear pore connection

Chromatin boundary activities (BAs) were identified in Saccharomyces cerevisiae by genetic screening. Such BAs bound to sites flanking a reporter gene establish a nonsilenced domain within the silent mating-type locus HML. Interestingly, various proteins involved in nuclear-cytoplasmic traffic, such as exportins Cse1p, Mex67p, and Los1p, exhibit a robust BA. Genetic studies, immunolocalization, live imaging, and chromatin immunoprecipitation experiments show that these transport proteins block spreading of heterochromatin by physical tethering of the HML locus to the Nup2p receptor of the nuclear pore complex. Genetic deletion of NUP2 abolishes the BA of all transport proteins, while direct targeting of Nup2p to the bracketing DNA elements restores activity. The data demonstrate that physical tethering of genomic loci to the NPC can dramatically alter their epigenetic activity.