Abstract

ER resident proteins such as chaperones or enzymes involved in post-translational modifications need to be retained in the ER compartment. They also can escape the ER compartment and therefore are recycled from the Golgi apparatus through an ER-to-Golgi retrograde transport. Some short linear motifs allow retention and or/retrieval of ER resident proteins that are carried along with secretory proteins to post-ER compartments.

ER lumen (soluble) proteins bear a KDEL motif at their C-terminus. KDEL is recognised by the KDEL-receptor (an integral membrane protein) which in turns binds to COP I (coatomer) underlying vesicles involved in the retrotransport, thus targeting its ligands to the COP I-mediated retrograde transport.

Other type of retrieving signal exist for ER integral membrane proteins, they interact directly with components of the coatomer through motifs located at the end of the proteins facing the cytoplasmic compartment (depending on their topology): di-Lysine at the C-term, di-Arginine at the N-terminus.



The general criteria defining targeting motifs are that they function in a specific position of a protein, they can be disrupted through mutation, and that they confer residency in a specific organelle when engineered onto a reporter protein.