A conserved cargo recognition mechanism establishes TANGO1 as a selective extracellular matrix cargo receptor

The endoplasmic reticulum (ER) orchestrates the secretion of extracellular matrix (ECM) proteins, many of which exceed the size of conventional transport vesicles and therefore require specialized export machinery. TANGO1, encoded by MIA3, organizes ER exit sites for bulky cargo export, yet the molecular basis of cargo recognition and its relationship to the collagen-specific chaperone HSP47 remain unclear. Through quantitative secretome profiling of TANGO1- and HSP47-deficient fibroblasts, structural modeling, and binding analyses, we show that TANGO1 directly and selectively recognizes a defined subset of ECM proteins, including specific collagen and fibrillin isoforms, through a conserved tyrosine residue in its luminal MOTH domain, independently of HSP47. Novel MIA3 variants identified in individuals with previously undescribed skeletal dysplasia disrupt this cargo-binding interface, leading to selective intracellular retention of ECM proteins. These findings identify direct cargo recognition, rather than ER exit-site assembly, as the primary molecular defect underlying MIA3/TANGO1-associated skeletal dysplasia. More broadly, our work establishes TANGO1 as a selective ECM cargo receptor that functions independently of HSP47, providing a new framework for understanding bulky cargo selection at the ER.