A conserved motif within RAP1 has diversified roles in telomere protection and regulation in different organisms

Abstract

Repressor activator protein 1 (RAP1) is the most highly conserved telomere protein. It is involved in protecting chromosome ends in fission yeast, promoting gene silencing in Saccharomyces cerevisiae while in Kluyveromyces lactis it is required to repress homology directed recombination (HDR) at telomeres. Since mammalian RAP1 requires TRF2 for stable expression, its role in telomere function has remained obscure. To understand how RAP1 plays such diverse functions at telomeres, we solved the crystal or solution structures of the C-terminal RCT domains of RAP1 from multiple organisms in complex with their respective protein-binding partners. Our comparative structural analysis establishes the RCT domain of RAP1 as an evolutionarily conserved protein-protein interaction module. In mammalian and fission yeast cells, this module interacts with TRF2 and Taz1, respectively, targeting RAP1 to chromosome ends for telomere end protection. While RAP1 repress NHEJ at fission yeast telomeres, at mammalian telomeres it is required to repress HDR. In contrast, S. cerevisiae RAP1 utilizes the RCT domain to recruit Sir3 to telomeres to mediate gene silencing. Together, our results reveal that depending on the organism, the evolutionarily conserved RAP1 RCT motif plays diverse functional roles at telomeres.

The Nuts and Bolts of Transcriptionally Silent Chromatin in Saccharomyces cerevisiae

“…A network of interactions recruits the Sir2/3/4 complex to proteins bound at silencers ( Figure 7A). Sir3 and Sir4 associate with Rap1 and Sir4 associates with ORC-bound Sir1 (Moretti et al 1994;Triolo and Sternglanz 1996;Moretti and Shore 2001;Chen et al 2011). In catalytic mutants of Sir2, the Sir2/3/4 complex is restricted to silencers (Hoppe et al 2002;Rusche et al 2002;Ellahi et al 2015).…”

“…Mutation of Sir3 at the protein-protein interface disrupts Sir4 binding and silent chromatin assembly (Ehrentraut et al 2011). In addition to Sir4 binding, a small region upstream of the AAA + ATPaselike domain (amino acids 456-481) binds Rap1 at silencers to aid nucleation of the Sir2/3/4 complex on chromatin (Moretti and Shore 2001;Chen et al 2011).…”

Human Rap1 Interacts Directly with Telomeric DNA and Regulates TRF2 Localization at the Telomere

“…Rap1 is the most conserved shelterin component among different species (27), and recently, hRap1 was shown to have similar functions to its yeast counterpart. hRap1 regulates the expression level of genes that are in close proximity to its binding sites and regulates telomere length through its BRCT domain (11,26).…”

Extra-Telomeric Roles of Telomeric Proteins

“…The recent availability of several transgenic mouse models of shelterin complex components, as well as the generation of tissue-specific conditional mouse models of these components, has revealed a role for these proteins in cancer susceptibility and age related pathologies even in the presence of normal telomerase activity and under conditions when telomere length is normal (Blasco 2005). Recent work from different laboratories has suggested that Rap1 may not be directly involved in telomere protection, but rather in regulation of DNA repair activities at the telomere (Pardo and Marcand 2005;Bae and Baumann 2007;Sarthy et al 2009;Bombarde et al 2010;Sfeir et al 2010;Chen et al 2011). Sfeir et al have reported that conditional loss of Rap1 alone had little effect on telomere stability (i.e., no overt changes in telomere length, telomere DNA damage foci, or telomeric fusions were noted).…”