Ribosomes that exist freely in the cytosol or those attached to the ER are intrinsically the same in their translational properties. ER-bound ribosomes are responsible for the production of secretory proteins and integral ER, Golgi, lysosomal, and plasma membrane spanning proteins. Such proteins contain signal sequences that direct their synthesis to the ER membrane. As the nascent polypeptide emerges from the ribosome, a signal recognition particle (SRP) binds to the signal sequence and serves to couple the ribosome to the protein-translocating machinery in the ER membrane. Although the SRP is a 325 kDa ribonucleoprotein, its 54 kDa subunit (SRP54) mediates interaction with, and targeting of, the nascent protein to the ER. Via its C-terminal M-domain, SRP54 associates with the nascent protein and inhibits its elongation. This complex binds to the SRP receptor on the ER, the ribosome is delivered to the translocation machinery, SRP is released, and elongation resumes. Targeting and insertion are tightly coupled to a GTPase cycle that involves SRP54 and SRP receptor. Although the mechanisms are unclear, release of SRP from the ER-bound complex requires GTP hydrolysis.