signal recognition particle

SRP is composed of six polypeptides bound to a small cytoplasmic RNA (7SL RNA). As a ribosome begins translating an mRNA encoding a protein destined for secretion, insertion into the plasma membrane, or delivery to lysosomes, a hydrophobic N-terminal signal sequence emerges from the ribosomal exit tunnel. SRP binds this signal sequence and simultaneously binds the ribosome, causing a temporary pause in translation (elongation arrest). This SRP-ribosome-nascent chain complex is then targeted to the SRP receptor (docking protein) on the cytosolic face of the RER membrane. Upon docking, SRP is released (a GTP-dependent process), translation resumes, and the nascent polypeptide is threaded through the Sec61 translocon channel into the RER lumen (or membrane, for integral membrane proteins). Inside the RER, the signal sequence is typically cleaved by signal peptidase, and the protein undergoes folding, glycosylation, and quality control before trafficking through the Golgi. Clinically, defects in this pathway can be tested conceptually to distinguish proteins synthesized on free ribosomes (cytosolic, nuclear, mitochondrial, peroxisomal proteins) versus those requiring SRP-mediated targeting to the RER (secreted proteins, plasma membrane proteins, lysosomal enzymes). This concept is frequently tested on USMLE Step 1 to assess understanding of protein trafficking and organelle-specific processing pathways. Mutations affecting related exocytic pathway components can cause diseases like I-cell disease (inability to tag lysosomal enzymes with mannose-6-phosphate after RER processing).