The RNA tetraloop in mammalian SRP does not play a role in SR recruitment. (A) Schematics to show the electrostatic tethering interaction between the GGAA tetraloop of bacterial SRP RNA and basic residues on FtsY, and the corresponding residues in the mammalian SRP and SR. The negative charges of the RNA tetraloop are highlighted as red –. The basic surfaces on SR and FtsY are highlighted as blue +. The other subunits in the mammalian SRP are shown in light gray and are not labeled, for simplicity. (B) Summary of the k_cat/K_m values for the stimulated GTPase reactions of mammalian SRP and SR with WT 7SL RNA and the indicated tetraloop mutants. The values are compared with the published k_cat/K_m values for the reactions of E. coli SRP and FtsY with WT 4.5S RNA and the same tetraloop mutants (Zhang et al., 2008). (C) Summary of the k_cat/K_m values for the stimulated GTPase reaction of mammalian SRP with WT SR and mutant SR(K537A). The values are compared with the published k_cat/K_m values for the reactions of E. coli SRP with WT FtsY and the homologous FtsY(K399A) mutant (Shen et al., 2011). All values in B and C are reported as mean ± SD, with n ≥ 3.