Comparison of proposed models of NADPH oxidase regulation by Rac GTPase.

In each of the proposed molecular models for NADPH oxidase regulation by Rac GTPase, the switch 1 region of Rac (not indicated) interacts with p67^phox, the prenylated tail and polybasic domain of Rac (shown as a zigzag line) interact with the membrane, and the activation domain of p67^phox (a crosshatched section) interacts with cytochrome b₅₅₈. A role for p40^phox remains unclear. In the model of Lambeth and colleagues (model A), Rac is thought to be recruited to the plasma membrane phospholipid bilayer via its prenylated C-terminus and polybasic domain. Indirect evidence supports the idea that the Rac insert domain (hatched section) may interact directly with cytochrome b₅₅₈. Lambeth2 proposes the activation domain of p67^phox is the sole regulator of the electron transfer step from NADPH to FAD. Rac and p47^phox serve as adapters aiding in the interaction of p67^phox with cytochrome b₅₅₈. In model B proposed by Pick and colleagues,37,38 Rac is thought to interact only with plasma membrane phospholipids via its C-terminal prenyl group and polybasic domain and does not interact physically with cytochrome b₅₅₈. In this model, the insert domain is not involved in protein interactions or regulation of the NADPH oxidase. As in the model of Lambeth and colleagues, p67^phox is the sole regulator of electron transfer by cytochrome b₅₅₈, while Rac and p47^phox serve only as adapters to position p67^phox for interaction with cytochrome b₅₅₈. Diebold and Bokoch35 propose a 2-step model (model C) for the regulation of NADPH oxidase by Rac. In step 1, Rac translocates to the membrane and interacts with the phospholipid bilayer via its prenylated/polybasic C-terminus. In addition, Rac, via its insert domain, interacts with cytochrome b₅₅₈ and contributes to the regulation of electron flow from NADPH to FAD without interacting with p67^phox. p67^phox is still required for electron flow to occur in step1 and regulates electron flow via its activation domain. The interaction of the insert domain of Rac with cytochrome b₅₅₈ may induce a conformational change in cytochrome b₅₅₈ that modulates the interaction of p67^phox and cytochrome b₅₅₈. In step 2, the interaction between the switch I domain of Rac and the Rac-binding domain of p67^phox, probably inducing a conformational change in p67^phox, is required for electrons to continue to flow from FAD to the heme groups of cytochrome b₅₅₈. (The step 1 reaction only is depicted in models A and B because this is thought to be the rate-limiting step in the overall electron transfer pathway to molecular oxygen. The step 2 reaction does take place in all 3 models depicted.) Portions of this figure have been used in Diebold and Bokoch.47