Endoplasmic reticulum cargo receptor that mediates the export of lipoproteins by recruiting cargos into COPII vesicles to facilitate their secretion (PubMed:29643117, PubMed:30251625, 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). {ECO:0000269|PubMed:18287528, ECO:0000269|PubMed:29643117, ECO:0000269|PubMed:30251625, ECO:0000269|PubMed:32989016, ECO:0000269|PubMed:33186557}. This is the function of SURF4 (surfeit 4, Ensembl gene identifier ENSG00000148248).