Transport and assembly processes in the endoplasmic reticulum.

Until recently, the endoplasmic reticulum (ER) of eukaryotic cells was regarded as an open corridor for the unregulated movement of newly-synthesized exocytotic proteins from their site of membrane translocation to the vesicles that ferry them from the transitional elements of the ER to the Golgi apparatus. Moreover, it was widely assumed that the folding and assembly of newly translocated polypeptides into their tertiary and quaternary structure is a spontaneous process that does not involve the intervention of other cellular proteins. In this article we review evidence that the ER is a highly discriminatory organelle that grants passage only to proteins that have attained an essentially native conformation, and summarize current knowledge about resident ER proteins that appear to facilitate and/or monitor protein folding and assembly in this organelle.