Local GTP availability is an essential step in regulation of RAC1 activity and cell invasion

Physiological changes in GTP levels in live cells have never been considered a regulatory step of RAC1 activation because GTP concentrations determined by chromatography or mass spectrometry were shown to be substantially higher than the RAC1 GTP dissociation constant (RAC1-GTP Kd) determined in vitro. Here, using a combination of genetically engineered intracellular GTP sensors, recently generated by us, and RAC1 activity reporter, we demonstrate that fluctuations in GTP levels within the range of RAC1-GTP Kd, correlate with changes in RAC1 activity in live cells. We further demonstrate that RAC1 co-localize in protrusions of invading cells with the de novo guanylate biosynthesis enzymes including rate-limiting inosine monophosphate dehydrogenase 2 (IMPDH2) and guanosine monophosphate synthase (GMPS). This colocalization is partly driven by direct interaction of RAC1 with the Bateman domain of IMPDH2. Substitution of endogenous IMPDH2 with the IMPDH2 mutants incapable of binding RAC1 did not affect total intracellular GTP levels but suppressed RAC1 activity. Accordingly, targeting of IMPDH2 away from the plasma membrane did not alter total intracellular GTP pools but decreased local GTP levels in cell protrusions, downregulated RAC1 activity and decreased cell invasion. These data provide previously unrecognized mechanistic insight into the regulation of RAC1 activity by local GTP pools in live cells.