SMGGDS STABILIZES NUCLEOTIDE-BOUND AND NUCLEOTIDE-FREE FORMS OF THE RAC1 GTP-BINDING PROTEIN AND STIMULATES GTP/GDP EXCHANGE THROUGH A SUBSTITUTED ENZYME MECHANISM

The Rac proteins, Rac1 and Rac2, are essential components of the NADPH oxidase system of phagocytes and regulate the actin assembly associated with membrane ruffling. These functions are controlled by the GTP-bound form of Rac. The biochemical interaction between Rac and its only known GDP-dissociation stimulator (termed smgGDS) was characterized. SmgGDS was able to stimulate the incorporation of guanosine 5'-[gamma-thio]-triphosphate GTP[gamma S] into the RhoA, Rac2, Rac1, Rap1A and CDC42Hs GTP-binding proteins, but the activity was greatest toward RhoA and Rac2. Isoprenoid modification of these proteins was not absolutely required for the interaction with smgGDS. Interestingly, the activity of smgGDS toward Rad could not be observed in a [H-3]GDP/GTP exchange assay under conditions where it stimulated incorporation of GTP[gamma S] into Rad. We determined that smgGDS prevented the loss of Rac1 activity during the [H-3]GDP/GTP exchange assay by demonstrating the ability of smgGDS to inhibit the loss of Rac1 GTP[gamma S]-binding during incubations at 30 degrees C. This stabilizing effect was exactly counterbalanced by the ability of smgGDS to stimulate the release of [H-3]GDP from Rac1, thereby producing no net observable effect in the exchange assay. SmgGDS was able to effectively stimulate the release of GDP but not GTP[gamma S] from Rac1. SmgGDS maintains Rad in a nucleotide-free form after release of GDP, indicating that the reaction between Rac1 and smgGDS involves a substituted enzyme mechanism.