UC Riverside

Arsenic is an environmentally prevalent metalloid. Arsenic pollution through contaminated drinking water is a leading global public health problem that influences millions of people. Arsenic exposure has been demonstrated to elicit a variety of human diseases including tumorigenesis and neurodegenerative disorders. It is proposed that arsenic interferes with the post-translational modifications (PTMs) that primarily regulate pivotal quality control machineries of the genetic information flow, thereby leading to onset and/or progression of protein-misfolding disorders. Zinc finger (ZnF) proteins have diverse molecular functions, ranging from DNA/RNA binding to regulatory enzymes of protein PTMs, maintaining homeostasis. Arsenite was previously proposed to bind selectively to C3H- or C4-type ZnF by displacing Zn2+ within their zinc coordination spheres, thus altering the functions of the ZnF proteins. The current dissertation aimed to characterize the arsenite-ZnF interaction of ribosome-associated protein quality control (RQC) initiator ZNF598 and epigenetic remodeler TIP60, and examine toxicity of arsenite on small GTPases.

Proteostasis is crucial in maintaining the homeostasis of protein synthesis and turnover, thereby preventing proteotoxic stress. RQC pathway is the frontline against proteotoxic stress in eukaryotes. Its initiation necessitates site-specific regulatory ubiquitinations of the ribosomal protein subunits RPS10 and RPS20 for ribosome stalling of poly-(A) mRNA sequences, which are mediated by ZNF598 E3 ubiquitin ligase. In addition to ZNF598-mediated ribosomal ubiquitination in RQC, histone H4K16 acetylation is important in facilitating DNA repair through modulating chromatin accessibility of DNA damage sites for DNA repair enzymes. Proper regulation of TIP60 histone acetyltransferase is essential for de-compacting chromatin structure to facilitate DNA repair through H4K16Ac, lowering the likelihood of carcinogenesis. Here, arsenite binding to cysteine residues within ZNF598 and TIP60 was proposed to perturb their corresponding PTMs for dysregulating RQC (Chapter 2) and DNA repair (Chapter 3), respectively. Apart from ZnF proteins, a targeted quantitative proteomic method demonstrated that the expression levels of small GTPases were impacted by arenite exposure in Chapter 4. Our results showed that the expression level of RhoB protein in cultured human cells was diminished upon arsenite exposure. Mechanism of As3+-induced down-regulation of RhoB protein was further elucidated by studying ubiquitin-proteasome system, whose function was impaired by arsenite, as reflected by enhanced proteasomal degradation of RhoB protein. These results show ZnF proteins involved in DNA repair and protein quality control are molecular targets for arsenite exposure, which disrupt proteostasis. Overall, these results will be useful for understanding arsenite-induced proteotoxic stress and inform studies on the mechanisms of toxicity of arsenite.