SIR2 and SIR4 interactions differ in core and extended telomeric heterochromatin in yeast

Yeast core telomeric heterochromatin can silence adjacent genes and requires RAP1, SIR2, SIR3, and SIR4 and histones H3 and H4 for this telomere position effect. SIR3 overproduction can extend the silenced domain. We examine here the nature of these multiprotein complexes. SIR2 and SIR4 were immunoprecipitated from whole-cell extracts. In addition, using formaldehyde cross-linking we have mapped SIR2, SIR4, and RAP1 along telomeric chromatin before and after SIR3 overexpression. Our data demonstrate that SIR2 and SIR4 interact in a protein complex and that SIR2, SIR3, SIR4 and RAP1 map to the same sites along telomeric heterochromatin in wild-type cells. However, when overexpressed, SIR3 spreads along the chromosome and its interactions are dominant to those of SIR4 and especially SIR2, whose detection is decreased in extended heterochromatin. RAP1 binding at the core region is unaffected by SIR3 overproduction and RAP1 shows no evidence of spreading. Thus, we propose that the structure of core telomeric heterochromatin differs from that extended by SIR3.