Mediates the transport of lipoprotein lipase LPL from the basolateral to the apical surface of endothelial cells in capillaries (By similarity). Anchors LPL on the surface of endothelial cells in the lumen of blood capillaries (By similarity). Protects LPL against loss of activity, and against ANGPTL4-mediated unfolding (PubMed:27929370, PubMed:29899144). Thereby, plays an important role in lipolytic processing of chylomicrons by LPL, triglyceride metabolism and lipid homeostasis (PubMed:19304573, PubMed:21314738). Binds chylomicrons and phospholipid particles that contain APOA5 (PubMed:17997385, PubMed:19304573). Binds high-density lipoprotein (HDL) and plays a role in the uptake of lipids from HDL (By similarity). Mostly monomer, but also homodimer and homooligomer (PubMed:25387803). Interacts with lipoprotein lipase (LPL) with 1:1 stoichiometry (PubMed:17997385, PubMed:26725083, PubMed:27929370, PubMed:29899144, PubMed:30559189, PubMed:19304573, PubMed:25387803). Interacts with high affinity with high-density lipoprotein (HDL) (By similarity). Interacts with chylomicrons. Interacts with APOA5 (PubMed:17997385). The N-terminal acidic region is intrinsically disordered (PubMed:26725083). This region contributes to LPL binding, stabilizes LPL and protects LPL against loss of activity (PubMed:26725083, PubMed:27929370). Glycosylation of Asn-78 is critical for cell surface localization. Sulfation of a Tyr in the N-terminal acidic region increases the affinity for LPL. The missense variant Arg-56 may be associated with severe hypertriglyceridemia and chylomicronemia. The disease is caused by variants affecting the gene represented in this entry.