Figure 1.

Targeting channels to different membranes from the cytosol. Protein synthesis begins in the cytosol before targeting to specific organelles (red lines). This can occur post-translationally as unfolded proteins (mitochondria and chloroplasts) or fully folded proteins (peroxisomes), each protein protected by chaperones (blue circles). Most ER targeting occurs cotranslationally, mediated by SRP (yellow) binding to a signal sequence. Once inserted into the ER membrane, proteins assume the topology that they will retain through all subsequent trafficking steps. Some proteins may pass to the nuclear envelope (NE) or be exported directly from the ER to peroxisomes, but all other proteins pass via the ER−Golgi intermediate compartment (ERGIC) to the Golgi before being sorted to various destinations from the trans-Golgi network (TGN). Routes to and from the PM are shown.