*Only 1 GTP molecule is bound per GTPase domain, but the sequences that contribute to the interactions (G1 to G4) are spread over the domain. The key residues are shown in bold in the table below. The G1 motif (on the P-loop) of ras coordinates the phosphates while the T in the G2 motif is involved in catalysis. The G4 motif is involved in guanine and ribose coordination. The G2 motif (a single Threonine residue) is hard to predict by sequence comparisons but has been confirmed experimentally for dynamin (see Marks et al 2001). As the names imply, Switch1 and Switch2 regions in ras are the loops that move (switch) on GTP hydrolysis. Ras effectors can also interact with these sequences. Switch 2 overlaps with G3. In GBP the Threonine 75 in G2 is structurally in the same position as T35 in Ras. T35 in Ras contacts the water molecules involved in catalysis and also makes a direct coordination of the Mg ion. The sidechain of T75 contacts the oxygen of the gamma-phosphate and also contacts a water molecule which in turn contacts the catalytic Mg. We do not know the coordination of T65 in dynamin (and in DymA this region is not resolved in the structure), but the predominant effect of mutations in all these threonine residues are catalytically severely imparied GTPase activities.

The reduction of the third GTP-binding motif N/TKxD to RD in the GBPs leads to an unusual orientation of the guanine base (see pictures above). This sequence is also found in several other large GTP-binding proteins e.g. Atlastin (SP3A) that are likely to interact with the nucleotide in the same way.