Dynamic scaffolding of the INAD signaling complex via conformation-coupled protein disulfide redox potential cycling

Phototransduction in Drosophila is the fastest known G-protein coupled signalingcascade to date. Studies of Drosophila phototransduction have already unraveled many widely applied mechanisms in signaling pathways. INAD (Inactivation no afterpotential D) protein is a key player in this process, which interacts with essential components of phototransduction including PLCβ, eye-specific PKC and TRP channel. Recently INAD protein has been shown to actively regulate phototransduction activities via its PDZ5 domain. The PDZ5 domain of INAD protein cycles between reduced and oxidized conformers in a light-dependent manner, thereby tuning the “receiver-gain” of photoreceptors. How INAD cycles between the functionally distinct conformers is conceptually challenging, as the redox potential of photoreceptor cells is not known to fluctuate in response to light. In this thesis, we discover that the redox potential of INAD PDZ5 is allosterically regulated by direct conformational coupling with PDZ4. Formation of the PDZ45 supramodule locks PDZ5 in the reduced state by raising the redox potential of the Cys605/Cys645 disulfide by ~330 mV, whereas isolated PDZ5 is stable in the oxidized form. Acidification, potentially mediated via light and PLCβ-mediated hydrolysis of PIP2, uncouples the PDZ45 supramodule, and subsequently disrupts the INAD/TRP channel interaction due to PDZ5 oxidation. The conformation-coupled redox potential cycling of INAD demonstrates that, in addition to being passive redox sensors, proteins can actively modulate their intrinsic redox potentials of disulfide bonds to exert regulatory roles in signaling