Determining the DNA Diffusion Behavior of SA2 on Various DNA Substrates

Abstract

Cohesin is a multi-protein complex involved in sister chromatid cohesion during cell replication and double strand DNA break repair. Cohesin core complex consists of a ring-like trimer and either SA1 or SA2 in somatic vertebrate cells. While SA1 and SA2 share ∼70% homology, only SA1 contains a critical AT hook domain responsible for its binding to telomere sequences. Cohesin-SA1 holds sister chromatids together at telomere regions during cell separation and can be found at specific promoter regions, while Cohesin-SA2 is predominantly located at intergenic and centromere regions. The mechanism by which Cohesin locates specific DNA sequences is currently unknown. To understand the role that SA1 or SA2 has in Cohesin/DNA interactions, we used the single-molecule techniques atomic force microscopy (AFM) and fluorescence imaging of quantum dot labeled proteins on DNA tightropes. Preliminary data indicates that SA1 carries out 1-D diffusion on DNA, binds with high affinity to telomeric sequences, and pauses on telomeric and promoter regions. In contrast, SA2 exhibits static and dynamic populations without pausing for telomere, centromere, and promoter DNA sequences. We propose that 1-D sliding and sequence dependent pausing by SA1 provides binding specificity and stability during the cohesion process at telomeres, while.SA2 alone, lacking the AT hook domain, uses different DNA binding mechanisms.