Single-particle track colocalization using CoPixie reveals the impact of cancer-associated POT1 mutations on telomerase-telomere interactions

Single-particle imaging and tracking can be combined with colocalization analysis to study the dynamic interactions between macromolecules in living cells. Indeed, single-particle tracking has been extensively used to study protein-DNA interactions and dynamics. Still, identification and quantification of binding events at specific genomic loci remains challenging. Herein we describe CoPixie, a new software that identifies colocalization events between a theoretically unlimited number of imaging channels, including single-particle movies. We employed CoPixie with live cell single-molecule imaging of telomerase and telomeres to test the model that cancer-associated POT1 mutations facilitate telomere accessibility. We show that OB-fold mutants POT1-{Delta}OB, Y223C, D224N or K90E increase telomere accessibility for telomerase interaction and the cumulative dwell-time of telomerase at telomeres. However, unlike POT1-{Delta}OB or D224N mutants, the POT1 Y223C and K90E mutations also increase the duration of long-lasting telomerase interactions at telomeres. Our data reveal that telomere elongation in cells expressing cancer-associated POT1 mutants arises from the dual impact of these mutations on telomeres accessibility and telomerase retention at telomeres. CoPixie can be used to explore a variety of questions involving macromolecular interactions in living cells, including between proteins and nucleic acids, from multi-color single-particles tracks.