Comparison between the classic GTPase switch and the SRP and SR GTPases. (A) The bimodal GTPase cycles of classical signaling GTPases. GEF, GAP. (B) The nucleotide-driven dimerization cycle of the SRP and SR GTPases. Steps 1a and 1b, nucleotide exchange on SRP and SR, respectively. Step 2, complex formation between the SRP and SR GTPases. Step 3, activated GTP hydrolysis from the SRP•SR complex. Step 4, dissociation of SRP and SR after GTP hydrolysis, returning these GTPases to the basal state.