Structure of full-length ERGIC-53 in complex with MCFD2 for cargo transport

ERGIC-53 is a cargo receptor that promotes the transport of certain subsets of newly synthesized secretory proteins and membrane proteins from the endoplasmic reticulum (ER) to the Golgi apparatus (GA) 1,2 . Despite numerous structural and functional studies since its identification, the overall architecture and mechanism of action of this cargo receptor in its full-length form remain unclear. Here we present cryo-electron microscopy (cryo-EM) structures of full-length ERGIC-53 in complex with its functional partner MCFD2. These structures, in combination with SEC-MALS/SAXS analysis, reveal that ERGIC-53 exists as a homotetramer, not a homohexamer as previously suggested, and comprises a four-leaf clover-like head structure and a long stalk composed of three sets of four-helix coiled-coil followed by a transmembrane (TM) domain. The tetrameric head of ERGIC-53 consists of the vertically assembled carbohydrate recognition domains and the central four-helix coiled-coil. 3D variability analysis visualizes the globally flexible motion of the long stalk and local plasticity of the head region. Notably, MCFD2 has been found to possess a Zn 2+ binding site in its N-terminal lid, which appears to modulate cargo binding. Altogether, unique mechanisms of regulated cargo capture and release by ERGIC-53 via the stalk bending and metal binding are proposed.