Hannett, N. M. | bioRxiv Science

bioRxiv2024

Nucleolar accumulation of APE1 through condensates is mediated by rRNA forming G-quadruplex structures

APE1 (apurinic/apyrimidinic endodeoxyribonuclease 1) is the main endonuclease of the base excision repair (BER) pathway acting on abasic (AP)-sites in damaged DNA. APE1 is an abundant nuclear protein with a higher concentration than other BER pathway enzymes, and therefore, improper expression and localization of this factor could lead to the accumulation of toxic DNA intermediates. Altered APE1 sub-cellular localization, expression levels, or hyper-acetylation are associated with cancer development suggesting the importance of a fine-tuning mechanism for APE1 nuclear-associated processes. Recent work highlighted multi-functional roles of APE1, including rRNA quality control. However, how rRNA influences the sub-cellular localization and activity of APE1 remains poorly understood, but previously underappreciated APE1-RNA interactions may influence the ability of this protein to form biomolecular condensates and tune APE1 partitioning into nucleoli. Since nucleolar accumulation of ectopic proteins could be the result of overexpression strategies, it is imperative to have cellular models to study APE1 trafficking under physiological conditions. Here we created the first cell line to express fluorescently tagged APE1 at its endogenous locus, enabling live-cell imaging. Live-cell imaging demonstrates that APE1 nucleolar accumulation requires active rRNA transcription. When modeled in vitro, APE1 condensate formation depends on RNA G-quadruplex (rG4) structures in rRNA and is modulated by critical lysine residues of APE1. This study sheds light on the mechanisms underlying APE1 trafficking to the nucleolus and formation of RNA-dependent APE1 nucleolar condensates that may modulate a switch between the activity of this factor in rRNA processing and DNA damage repair. Significance StatementWe created and characterized the first endogenous, fluorescently tagged cell line to study APE1 subcellular trafficking under physiological and stress conditions. Using this cell line, we show that APE1 nucleolar enrichment occurs under physiological conditions and, performing in vitro droplet assays, we associate APE1 condensates with active transcription of RNA G-quadruplexes, abundantly present in healthy nucleoli. This work deepens our understanding of APE1s role in healthy cells in the absence of DNA damage and provide a novel mechanism for how this protein responds to stress. Our results suggest that phase separation is an important part of how DNA damage repair proteins switch between their normal physiological functions and their ability to correct DNA lesions.

molecular biology↗

bioRxiv2022

Transcription factors interact with RNA to regulate genes

Transcription factors (TFs) orchestrate the gene expression programs that define each cells identity. The canonical TF accomplishes this with two domains, one that binds specific DNA sequences and the other that binds protein coactivators or corepressors. We find that at least half of TFs also bind RNA, doing so through a previously unrecognized domain with sequence and functional features analogous to the arginine-rich motif of the HIV transcriptional activator Tat. RNA-binding contributes to TF function by promoting the dynamic association between DNA, RNA and TF on chromatin. TF-RNA interactions are a conserved feature essential for vertebrate development and disrupted in disease. We propose that the ability to bind DNA, RNA and protein is a general property of many TFs and is fundamental to their gene regulatory function.

molecular biology↗

bioRxiv2018

Enhancer features that drive formation of transcriptional condensates

Enhancers, DNA elements that regulate gene expression, contain transcription factor (TF) binding sites. TFs bind short sequence motifs that are present throughout the genome at much higher frequency than active enhancers, and so the features that define active enhancers are not well understood. We show that DNA elements with TF binding site valency, density, and binding affinity above sharply defined thresholds can recruit TFs and coactivators in condensates by the cooperative process of phase separation. We demonstrate that weak cooperative interactions between IDRs of TFs and coactivators in combination with specific TF-DNA interactions are required for forming such transcriptional condensates. IDR-IDR interactions are relatively non-specific with the same molecular interactions shared by many TFs and coactivators, and phase separation is a universal cooperative mechanism. Therefore, whether a genomic locus is an enhancer that can assemble a transcriptional condensate is determined predominantly by its cognate TFs binding site valency and density.

molecular biology↗