During protein translation, the ribosome bind to mRNA to initiate trasncription starting at the N-terminus. The N-terminus peptide end contains a signal recognition particle that binds to the signal recognition particle receptor on the rough ER to allow the protein to be made into the RER.

Uworld QID #2086 Look at explanation for Choice A

Rough endoplasmic reticulum Site of synthesis of secretory (exported) proteins and of N-linked oligosaccharide addition to lysosomal and other proteins. Nissl bodies (RER in neurons)—synthesize peptide neurotransmitters for secretion. Free ribosomes—unattached to any membrane; site of synthesis of cytosolic, peroxisomal, and mitochondrial proteins. Mucus-secreting goblet cells of the small intestine and antibody-secreting plasma cells are rich in RER. Proteins within organelles (eg, ER, Golgi bodies, lysosomes) are formed in RER.

Smooth endoplasmic reticulum Site of steroid synthesis and detoxification of drugs and poisons. Lacks surface ribosomes. Location of glucose-6-phosphatase (last step of glycogenolysis). Liver hepatocytes and steroid hormone– producing cells of the adrenal cortex and gonads are rich in SER.

Signal recognition particle (SRP) receptor, also called docking protein, is a dimer composed of 2 different subunits that are associated exclusively with the rough ER in mammalian cells. Its main function is to identify the SRP units. SRP (signal recognition particle) is a molecule that helps the ribosome-mRNA-polypeptide complexes to settle down on the membrane of the endoplasmic reticulum.

For anyone wondering, the patient has achondroplasia.