<i>Tb</i>RAP1 has an unusual duplex DNA binding activity required for its telomere localization and VSG silencing.

Abstract

Localization of Repressor Activator Protein 1 (RAP1) to the telomere is essential for its telomeric functions. RAP1 homologs either directly bind the duplex telomere DNA or interact with telomere-binding proteins. We find that <i>Trypanosoma brucei</i> RAP1 relies on a unique double-stranded DNA (dsDNA) binding activity to achieve this goal. <i>T. brucei</i> causes human sleeping sickness and regularly switches its major surface antigen, variant surface glycoprotein (VSG), to evade the host immune response. VSGs are monoallelically expressed from subtelomeres, and <i>Tb</i>RAP1 is essential for VSG regulation. We identify dsDNA and single-stranded DNA binding activities in <i>Tb</i>RAP1, which require positively charged ₇₃₇RKRRR₇₄₁ residues that overlap with <i>Tb</i>RAP1's nuclear localization signal in the MybLike domain. Both DNA binding activities are electrostatics-based and sequence nonspecific. The dsDNA binding activity can be substantially diminished by phosphorylation of two ₇₃₇RKRRR₇₄₁-adjacent S residues and is essential for <i>Tb</i>RAP1's telomere localization, VSG silencing, telomere integrity, and cell proliferation.