PROTEIN TRANSLOCATION INTO THE ENDOPLASMIC-RETICULUM (ER) - 2 SIMILARROUTES WITH DIFFERENT MODES

Protein translocation into the endoplasmic reticulum (ER) occurs cotra nslationally with the ribosome tightly bound at the membrane, or post- translationally. Transport of polypeptides is performed by an elaborat e structure in the ER membrane consisting of numerous proteins. Both p athways have been reconstituted in vitro using proteoliposomes that co ntain purified components. Primary structures of these key components have been identified and recent electron microscopic data provide a fi rst impression of the tertiary structure of the translocation apparatu s. However, the precise function of most subunits of the translocation apparatus is still a mystery and little is known about structural cha nges in the translocation site during a transport cycle.