TelAP2-null trypanosomes tolerate pervasive telomeric DNA-damage signallin

Chromosome termini comprise G-rich tracts and telosome complexes that both recruit telomerase to complete end-replication and protect chromosome ends from damage-signalling. Specific functions of telosomal components remain incompletely defined, however. The parasitic trypanosomes have unusual telosomal proteins and many telomeric contingency genes with central roles in antigenic variation. We exploited the fact that approximately 75% of telomeres in African trypanosomes are on minichromosomes, reasoning that chromosome end-binding proteins would be specifically released from chromatin in cells lacking these chromosomes. Accordingly, we used Cas9 to eliminate minichromosomes and >70% of all telomeres, and proteomics to assess impacts on soluble and insoluble proteins. Telomere-associated protein 2 (TelAP2) was selectively enriched in the soluble fraction in chromosome-end depleted cells. While telomere extension was unperturbed in telap2 cells, the gH2A damage-signal was dramatically increased. Indeed, chromatin immunoprecipitation indicated specific enrichment of telomere-adjacent gH2A signalling in these cells. Further proteomic and transcriptomic assays indicated that TelAP2 knockout lead to depletion of other telosomal proteins and specific derepression of telome-adjacent genes. We conclude that trypanosomes readily tolerate widespread telomere damage signalling and minichromosome-loss. We also show that telomerase recruitment and end-protection are separable functions of trypanosome telosomal proteins. Specifically, TelAP2 is a telomere-capping factor that specifically suppresses telomeric DNA damage-signalling.