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). Found in a complex composed at least of SURF4, TMED2 and TMED10 (PubMed:18287528). May interact with LMAN1 (PubMed:18287528). Interacts with ZFYVE27 and with KIF5A in a ZFYVE27-dependent manner (By similarity). Interacts with STING1 (PubMed:29251827). Interacts with SAR1B (PubMed:33186557). Interacts with TMEM41B (PubMed:34015269). Cycles between the endoplasmic reticulum and the Golgi. The di-lysine motif confers endoplasmic reticulum localization for type I membrane proteins. Belongs to the SURF4 family.