Investigating the regulation of Arkadia activity in cells

Abstract

Ubiquitylation is a post translational modification the regulates a wide variety of cellular processes. The attachment of ubiquitin onto a substrate requires the activity of three enzymes; E1, E2 and E3. As ubiquitylation is essential to maintaining homeostasis, this modification is tightly regulated. E3 enzymes are regulated by a variety of different mechanisms. Until recently, relatively little was known about the regulation of really interesting new gene (RING) E3 ligase Arkadia. Arkadia has noted roles in amplifying transforming growth factor beta (TGFβ) signalling, facilitating UV-induced DNA damage and degrading proteins in response to arsenic. Some of these functions are facilitated by the SUMO-interacting motifs (SIMs) domains of Arkadia, which classifies Arkadia as a SUMO-targeted ubiquitin ligase.

Recent research has uncovered a novel ubiquitin binding domain on the RING domain of Arkadia that increases ubiquitin transfer activity in vitro. The ubiquitin bound to the novel binding site was termed UbR. This ubiquitin binding site presents a potential mechanism of regulating Arkadia activity. Furthermore, Arkadia is the first RING domain that is known to feature an ubiquitin binding domain. This project aimed to investigate if the novel binding site is important for Arkadia activity in cultured cells.

An Arkadia activity assay was established to measure activation of the TGFβ pathway. This assay was eventually to be used in HAP1 cells to compare activity of WT Arkadia and variant Arkadia that were predicted to lack the RING-UbR interaction. However, expression data of TGFβ-induced genes revealed that the TGFβ pathway was not active in WT HAP1 cells. A pilot experiment into Arkadia-mediated DNA damage repair yielded promising results. Once optimised, this experiment will likely give insight into how the novel binding site on the RING domain affects Arkadia activity in cultured cells.