The Ran GTPase system regulates the direction and timing of several cellular events, such as nuclear-cytosolic transport, centrosome formation, and nuclear envelope assembly in telophase. To gain insight into the Ran system's involvement in chromatin formation, we investigated gene silencing at the telomere in several mutants of the budding yeast Saccharomyces cerevisiae, which had defects in genes involved in the Ran system. A mutation of the RanGAP gene, rnal-1, caused reduced silencing at the telomere, and partial disruption of the nuclear Ran binding factor, yrb2-,Delta 2, increased this silencing. The reduced telomere silencing in rnal-I cells was suppressed by a high dosage of the SIR3 gene or the SIT4 gene. Furthermore, hyperphosphorylated Sir3 protein accumulated in the rnal-I mutant. These results suggest that RanGAP is required for the heterochromatin structure at the telomere in budding yeast. (C) 2007 Elsevier Inc. All rights reserved.