Practice question

Signal recognition particle integrates targeting and translation control to ensure secretory proteins avoid premature cytosolic folding and aggregation. Particle consists of 7SL RNA scaffold folding into Alu and S domains and six proteins. Most critical subunit SRP54 contains N domain four-helix bundle, central GTPase domain and C-terminal M domain rich in methionines that forms deep groove accommodating diverse hydrophobic signal sequences through flexible sulfur-containing side chains providing plastic hydrophobic surface adaptable to many sequences. Binding occurs when signal emerges from ribosome, inducing structural rearrangement transmitted to Alu domain formed by SRP9/14 heterodimer and RNA hairpin that interacts with elongation factor binding site near GTPase center of ribosome, slowing elongation by competing with incoming aminoacyl-tRNA. This pause extends time window for diffusion to ER membrane where heterodimeric SRP receptor SRα-SRβ, both GTPases, captures complex via GTP-dependent dimerization of NG domains. GTP hydrolysis drives hand-off of ribosome-nascent chain to Sec61 and recycles SRP. COPII transport, BiP ATPase activation and vesicle docking are unrelated to primary SRP activity of recognition and pausing, ensuring fidelity and targeting efficiency.