ABSTRACT
The decisive initial step in the biogenesis of most extracellular and many organellar proteins of the eukaryotic cell, i.e. constituent proteins of the endoplasmic reticu-lum, plasma membrane proteins, and proteins of the various other organelles involved in endocytosis and exocytosis, is their transport into the endoplasmic reticulum (ER). Typically, transport requires a signal sequence at the amino terminus of the respective precursor proteins. Transport includes various steps such as association of the precursor protein with the ER membrane, membrane insertion and completion of translocation. The signal sequence not only facilitates membrane specificity, but also helps to preserve the transport competent (non-native) state of the precursor proteins.
