Erd2p

Erd2p is a Golgi membrane protein that acts as the receptor for retrieval of endoplasmic reticulum (ER) residents bearing C-terminal KDEL or HDEL motifs in Saccharomyces cerevisiae. It is the yeast homolog of the mammalian KDELR (KDEL receptor) family and plays a central role in the ER retrieval pathway that maintains the composition of the ER lumen.

Localization and structure: Erd2p localizes to the Golgi apparatus and is predicted to be a multi-pass transmembrane protein. Its lumen-facing domain recognizes KDEL/HDEL signals on soluble ER residents, while its cytosolic tail contains motifs for interaction with the COPI coat complex, enabling retrograde transport. The receptor cycles between the Golgi and ER.

Mechanism and function: When ER residents leak into the Golgi, Erd2p binds the KDEL/HDEL-tagged proteins in the Golgi lumen. The receptor–cargo complex is packaged into COPI-coated vesicles and retrieved back to the ER. Cargo release is favored in the higher pH of the ER, allowing soluble chaperones and other residents to remain in the ER, while Erd2p is recycled to the Golgi for another cycle.

Genetic and evolutionary context: The ERD2 gene encodes Erd2p in budding yeast. Orthologs exist in other fungi, and in metazoans the KDELR family serves an analogous function, with KDELR1-3 representing mammalian paralogs. The Erd2p system is a component of broader ER quality control and protein trafficking networks.