Surfeit locus protein 4
Endoplasmic reticulum cargo receptor that mediates the export of lipoproteins by recruiting cargos into COPII vesicles to facilitate their secretion. Acts as a cargo receptor for lipoproteins bearing both APOB and APOA1, thereby regulating lipoprotein delivery and the maintenance of lipid homeostasis. Synergizes with the GTPase SAR1B to mediate transport of circulating lipoproteins. Promotes the secretion of PCSK9. Also mediates the efficient secretion of erythropoietin (EPO). May also play a role in the maintenance of the architecture of the endoplasmic reticulum-Golgi intermediate compartment and of the Golgi.