Nascent-polypeptide-associated Complex: A Bridge between Ribosome and Cytosol

The existence and elucidation of the secretory pathway was closely linked to the observation that a subset of ribosomes in the cell is not free in the cytoplasm but is bound to the endoplasmic reticulum (ER) membrane (for review, see Palade 1975). After it was shown that polypeptides synthesized on membrane-bound polysomes are vectorially discharged into the lumen of the ER (Redman and Sabatini 1966), efforts were made to separate, characterize, and then reconstitute the assembly of membrane-bound ribosomes and the formation of the ribosome-membrane junction in vitro (Adelman et al. 1973a,b; Blobel and Dobberstein 1975a,b; Siekevitz and Zemecnik 1981; Rapoport 1992; Gilmore 1993; Ng and Walter 1994; Sabatini and Adesnik 1995).