Redox response feature and mechanism of Arf1 and their implications for those of Ras and Rho GTPases

The small GTPases ADP-ribosylation factor (Arf), Ras, and Rho cycle between their active GTP-bound and inactive GDP-bound forms. Their regulatory proteins or inorganic redox agents regulate this cycle, which in turn regulates various important cell signals. Unlike regulatory protein-based regulation, redox-mediated regulation that occurs through the redox response of small GTPases to a redox agent is feasible only when the small GTPases are redox sensitive. The known redox-sensitive small GTPases including Ras and Rho have the reactive Cys in their unique redox motif. This study is the first to show the redox-response feature of Arf1 linked to a novel redox-sensitive Cys in the Arf-specific redox motif and its importance for cell functions. The Arf1 redox motif is currently the simplest form as it lacks the additional redox components found in Ras and Rho. The study also identifies critical radical intermediates implicated in the Arf1 redox response, along with the production of chemically modified nucleotides such as a GDP adduct. These results suggest the most elementary radical action-based mechanism for the Arf1 redox response. Although the presence of the radical intermediates was not reported, they were also suggested for the Ras and Rho redox response. Thus, the previously unknown mechanistic aspects of the Ras and Rho redox response are clarified by comparing them with those of Arf1.