A Data-Analysis Pipeline for High-Throughput Systematic Evolution of Ligands by Exponential Enrichment (HT-SELEX) in the Characterization of Telomeric Proteins

Telomeres are nucleoprotein structures at the ends of eukaryotic chromosomes that safeguard them from triggering inappropriate DNA damage signaling. POT1, a member of the mammalian shelterin complex, binds single-stranded (ss) telomeric DNA and blocks the activation of the ATR kinase-mediated DNA damage response at telomeres. Yet until recently, it was poorly understood how the double-stranded (ds)-ss telomeric junction was protected from DNA damage response factors. An initial study of the DNA-binding activity of human POT1 (hPOT1) using systematic evolution of ligands by exponential enrichment (SELEX) and subsequent investigation revealed that POT1 contains a binding pocket, known as the POT-hole, that binds the 5 phosphorylated dC of the telomeric ds-ss junction. The amino acid residues composing the POT-hole show full sequence identity with telomeric proteins from diverse eukaryotes, including Caenorhabditis elegans POT-1. The current study builds on this SELEX method, developing an extensive analysis pipeline for SELEX datasets sequenced by next-generation sequencing and achieving a deeper analysis of the resulting sequences. We validated our approach by applying it to the DNA-binding domain of hPOT1, yielding results consistent with a previous SELEX study. Furthermore, we employ our pipeline to characterize the DNA-binding activity of C. elegans proteins that are considered homologs of hPOT1: POT-1, POT-2, POT-3, and MRT-1. Our analysis suggests that all four proteins show a binding preference for G-enriched DNA sequences, with POT-1 additionally binding secondary structural elements. Overall, we present a bioinformatics pipeline that is accessible and applicable for determining the nucleic acid-binding properties of a variety of proteins.