 However, there are some similarities in how different GEFs alter the conformation of the G protein nucleotide-binding site.GTPases contain two loops called switch 1 and switch 2 that are situated on either side of the bound nucleotide.These regions and the phosphate-binding loop of the GTPase interact with the phosphates of the nucleotide and a coordinating magnesium ion to maintain high affinity binding of the nucleotide.