Endoplasmic reticulum cargo receptor that mediates the export of lipoproteins by recruiting cargos into COPII vesicles to facilitate their secretion (PubMed:30251625, PubMed:29643117, PubMed:33186557).Acts as a cargo receptor for lipoproteins bearing both APOB and APOA1, thereby regulating lipoprotein delivery and the maintenance of lipid homeostasis (PubMed:29643117, PubMed:33186557).Synergizes with the GTPase SAR1B to mediate transport of circulating lipoproteins (PubMed:33186557).Promotes the secretion of PCSK9 (PubMed:30251625).Also mediates the efficient secretion of erythropoietin (EPO) (PubMed:32989016).May also play a role in the maintenance of the architecture of the endoplasmic reticulum-Golgi intermediate compartment and of the Golgi (PubMed:18287528).