Moderate discrimination of REP-1 between Rab7·GDP and Rab7·GTP arises from a difference of an order of magnitude in dissociation rates

The kinetics of the interaction of Rab7 with REP-1 have been investigated using the fluorescence of GDP and GTP analogs at the active site of Rab7. The results show that REP-I has higher affinity for the GDP bound form of Rab7 (K-d = 1 nM) than for the GTP bound form (K-d = 20 nM). Both affinities should still be sufficient for the formation of stable completes in the cell. The association reaction proceeds in two steps for the GDP bound form. The initial step is fast (k(+1)=ca, 10(7) M-1 s(-1)) and concentration dependent while the second represents a slow equilibration (k(+2)+k(-2)=3.5 s(-1)) which has little effect on the overall equilibrium. The difference in affinity of the two nucleotide bound forms arises from a difference in dissociation rates (0.012 s(-1) for Rab7 GDP and 0.2 s(-1) for Rab7.GTP). (C) 1998 Federation of European Biochemical Societies