Predominant Golgi-residency of the plant K/HDEL receptor is essential for its function in mediating ER retention (Plant Cell)

Cells use a sophisticated sorting system to ensure that proteins get to the proper destination. Proteins that are supposed to stay in the ER carry special tags (KDEL or HDEL, recognized by the ERD2 receptor) that prevent them from being swept along with other proteins out of the ER and into the Golgi. Silva-Alvim et al. examined ERD2 function in plants. A key finding is that the position of the fluorescent tag determines ERD2 function, and previous models have been built on studies in which the presence of the tag causes the protein to be inactive. One model for ERD2 function is that it catches K/HDEL proteins that have escaped into the Golgi and carries them in a retrograde direction back to the ER. Interestingly, the authors were unable to detect any ERD2 in the ER, suggesting that it is not directly involved in retrograde transport, or that it leaves the ER very rapidly. As the authors observe, “one of the most conserved steps in the secretory pathway is one of the least understood,” but the tools and assays developed in this work will help to sort things out. (Summary by Mary Williams) Plant Cell 10.1105/tpc.18.00426