Characterization of the effector-specifying domain of Rac involved in NADPH oxidase activation

Production of microbicidal oxidants by phagocytic leukocytes requires activation of a latent NADPH oxidase by the coordinated assembly of a membrane-associated flavocytochrome b558, with three cytosolic components, p47phox, p67phox, and the low molecular weight GTP-binding protein Rac. Rac1 and Rac2 have 92% sequence identity and are both active in supporting the oxidase, while CDC42Hs, the closest relative to Rac with 70% sequence identity, only weakly supports oxidase activation in vitro. We have used CDC42Hs as a foil to identify residues in Rac that are critical for oxidase activation. Most of the divergent sequences of CDC42Hs could be incorporated into Rac-CDC42Hs chimeric proteins without affecting cell-free NADPH oxidase activity. However, incorporation of the amino-terminal segment of CDC42Hs (residues 1-40), which differs from Rac1 by only four residues (positions 3, 27, 30, and 33), resulted in a marked loss of oxidase activation capacity. Point mutagenesis studies showed that this was due to changes at residues 27 and 30, but not residues 3 and 33. Conversely, incorporation of the amino terminus of Rac1 (residues 1-40) into CDC42Hs increased its activity to that of Rac1, indicating that this terminus contains the effector-specifying domain of Rac. Taken together, these studies show that the difference in the activity between CDC42Hs and Rac1 is due entirely to differences in amino acids at position 27 and 30.