Separation of silencing from perinuclear anchoring functions in yeast Ku80, Sir4 and Esc1 proteins.

In budding yeast, the nuclear periphery forms a subcompartment in which telomeres cluster and SIR proteins concentrate. To identify the proteins that mediate chromatin anchorage to the nuclear envelope, candidates were fused to LexA and targeted to an internal GFP-tagged chromosomal locus. Their ability to shift the locus from a random to a peripheral subnuclear position was monitored in living cells. Using fusions that cannot silence, we identify YKu80 and a 312-aa domain of Sir4 (Sir4(PAD)) as minimal anchoring elements, each able to relocalize an internal chromosomal locus to the nuclear periphery. Sir4(PAD)-mediated tethering requires either the Ku complex or Esc1, an acidic protein that is localized to the inner face of the nuclear envelope even in the absence of Ku, Sir4 or Nup133. Finally, we demonstrate that Ku- and Esc1-dependent pathways mediate natural telomere anchoring in vivo. These data provide the first unambiguous identification of protein interactions that are both necessary and sufficient to localize chromatin to the nuclear envelope.