Bidirectional movement of a nascent polypeptide across microsomal membranes reveals requirements for vectorial translocation of proteins.

The translocation of polypeptides across the endoplasmic reticulum is a vectorial process that occurs probably through a protein channel by a mechanism as yet undetermined. Here, we demonstrate bidirectional movement of a 221 residue nascent polypeptide across microsomal membranes and provide evidence suggesting that the retrograde movement is through the translocation channel. Retrograde movement is observed only when the polypeptide is generated from a truncated transcript; addition of a stop codon after codon 221 confers vectorial movement. Retrograde movement can also be prevented by glycosylation of the nascent polypeptide, as well as by inclusion of 32 additional amino acids that may promote folding of the translocated chain. We propose that the protein translocation channel is a passive pore that does not create a directional bias in polypeptide movement and that vectorial translocation is driven by nascent chain elongation and sustained by posttranslocation events that prevent retrograde movement.