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

Abstract

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 Zn2+ 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.