Retention mechanisms for ER and Golgi membrane proteins

Unless there are mechanisms to selectively retain membrane proteins in the endoplasmic reticulum (ER) or in the Golgi apparatus, they automatically proceed downstream to the plasma or vacuole membranes. Two types of coat protein complex I (COPI)-interacting motifs in the cytosolic tails of membrane proteins seem to facilitate membrane retention in the early secretory pathway of plants: a dilysine (KKXX) motif (which is typical of p24 proteins) for the ER and a KXE/D motif (which occurs in the Arabidopsis endomembrane protein EMP12) for the Golgi apparatus. The KXE/D motif is highly conserved in all eukaryotic EMPs and is additionally present in hundreds of other proteins of unknown subcellular localization and function. This novel signal may represent a new general mechanism for Golgi targeting and the retention of polytopic integral membrane proteins.

The cytoplasmic tail of heparin-binding EGF-like growth factor regulates bidirectional intracellular trafficking between the plasma membrane and ER

Heparin-binding epidermal growth factor (EGF)- like growth factor (HB-EGF) is synthesized in the ER, transported along the exocytic pathway, and expressed on the plasma membrane as a type I transmembrane protein. Upon extracellular stimulation, HB-EGF, either proHB-EGF or the shed form HB-EGF-CTF, undergoes endocytosis and is then transported retrogradely to the ER. In this study, we showed the essential contribution of the short cytoplasmic tail of HB-EGF (HB-EGF-cyto) to the bidirectional intracellular trafficking between the ER and plasma membrane and revealed several critical amino acids residues that are responsible for internalization from the plasma membrane and ER targeting. We suggest that these anterograde and retrograde sorting signals within HB-EGF-cyto are strictly regulated by protein modification and conformation.