INTERPLAY BETWEEN NUCLEOSOMES AND TELOMERIC PROTEINS. DO THEY INTERACT OR COMPETE FOR TELOMERIC SEQUENCES?

In higher eukaryotes, both specific proteins and the histone octamer bind to telomeric repeats. Whether histones and specific telomeric proteins compete for telomeric DNA binding (and hence occupy different telomere domains) or whether they cooperate in the formation of the telomeric complex is a relevant issue in order to understand telomeric structure and its dynamics. In humans, TRF1 and TRF2 bind as preformed homodimers recognizing two telomeric TTAGGG repeats. Since most of telomeric DNA is packed in nucleosomes, a possible hypothesis is that TRF proteins, after saturating the short DNA linkers between nucleosomes, could interact with nucleosomal binding sites. We have previously shown that hTRF1 forms stable ternary complexes with telomeric nucleosomes. By means of in vitro model systems we show that also hTRF2 recognize nucleosomal binding sites, but with a lower affinity with respect to hTRF1. This difference does not depend on DNA binding domains (DBDs), since the DBDs of hTRF1 and hTRF2 have the same affinity for nucleosomal binding sites. Binding of hTRF1 to naked DNA, but not of hTRF2, is favored by the presence of an adjacent nucleosome. Importantly, hTRF1 binding induces nucleosome sliding, enhancing the intrinsic mobility of telomeric nucleosomes, whereas hTRF2 does not apparently influence nucleosome mobility. These results suggest that the different interactions of hTRF1 and hTRF2 with nucleosomes may play a major role in the architecture of telomeric chromatin.