Protein translocation at the membrane of the endoplasmic reticulum.

The process of insertion into and translocation across the ER membrane is a significant step in the biosynthesis of a membrane or secretory protein. This commits the protein to a destination within the "secretory pathway" (Palade, 1975) and is part of a complex series of events involving protein targeting, translocation, maturation and sorting, which finally results in a biologically-active protein being delivered to its correct subcellular location. The focus for this review has been the initial events of this process. Proteins which constitute at least a part of the actual translocation site across the ER membrane have been identified and the minimum components required to reconstitute ER translocation in vitro have been defined. A detailed description of the architecture of the ER translocation site and the molecular events occurring during translocation and membrane insertion remain goals for the future. The process occurring in vivo may be more complex since (i) each translocation site may only promote a single round of translocation in vitro whereas in vivo the sites must operate catalytically and go through many cycles of translocation and insertion (see Gilmore, 1993) and (ii) the in vivo requirement for a translocation site which is impermeable to small molecules (in order not to dissipate chemical gradients and the redox potential) is unlikely to be important for in vitro assays. Thus, other components which play a vital role in protein translocation and membrane insertion in vivo may remain to be identified. Our future aim must be to place a detailed understanding of the molecular events of the translocation process into the context of the normal cellular environment.