Single-Molecule Imaging Reveals Dynamics of SA1-TRF1 Interactions on Telomeric DNA

Abstract

The cohesin complex plays a crucial role in accurate chromosome segregation, organization of interphase chromatin, DNA replication, and post replicative DNA repair in part by promoting DNA-DNA pairing. The core cohesin subunits consist of a tripartite ring and the fourth core subunit Scc3/SA. In somatic vertebrate cells, SA can be either SA1 or SA2. Recent work indicates that while SA2 is important for cohesion at the centromere, SA1 plays a specific role in sister telomere association. In addition, SA1 directly interacts with shelterin subunits TRF1 and TIN2. While these results demonstrate a unique sister telomere cohesion process depending on the SA1-TRF1 complex, the underlying mechanism is still poorly understood. We applied Atomic Force Microscopy (AFM) and Total Internal Reflection Fluorescence Microscopy (TIRFM) to study the interactions between SA1 or SA1/TRF1 complex and various DNA substrates with or without telomeric sequences. DNA tightrope assays were performed and proteins were visualized by conjugating quantum dots. The data demonstrated that 1) SA1 carried out 1-dimensional diffusion on DNA substrate for searching telomeric DNA sequence; 2) SA1 paused at telomeric DNA sequence, while SA2 did not. Interestingly, the AFM data showed that SA1 further stabilized TRF1 mediated DNA-DNA pairing. These data shed more lights on the process of sister telomere association and segregation.