Transmembrane Protein Translocation: Signal Recognition Particle and Its Receptor in the Endoplasmic Reticulum

The membrane-bound ribosomes of the rough endoplasmic reticulum (RER) are engaged in the synthesis of secretory proteins, resident lumenal proteins of the exocytic and endocytic membrane systems, and the majority of cellular integral membrane proteins. The information responsible for the selective delivery of ribosomes to the RER is contained in an amino terminal signal sequence. Ribosomes synthesizing proteins with RER-specific signal sequences are subsequently targeted to a membrane-bound translo­ cation site or translocon. The translocon is a multicomponent protein assembly that mediates the unidirectional transport of proteins or protein domains across the RER membrane. Transport of nascent polypeptide chains across the endoplasmic reticulum (ER) membrane has been proposed