TRG_ER_KDEL_1

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 COPI (coatomer) underlying vesicles involved in the retrotransport, thus targeting its ligands to the COPI-mediated retrograde transport.
Other types of retrieving signals 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): the di-Lysine motif at the C-term (TRG_ER_diLys_1) and the di-Arginine motif at the N-terminus (TRG_ER_diArg_1).
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.