Catalyzes the transport of triglyceride, cholesteryl ester, and phospholipid between phospholipid surfaces (PubMed:23475612, PubMed:8939939, PubMed:26224785, PubMed:25108285, PubMed:22236406, PubMed:16478722, PubMed:15897609, PubMed:8876250). Required for the assembly and secretion of plasma lipoproteins that contain apolipoprotein B (PubMed:23475612, PubMed:8939939, PubMed:26224785, PubMed:8876250, PubMed:16478722). May be involved in regulating cholesteryl ester biosynthesis in cells that produce lipoproteins (By similarity). a 1,2-diacyl-sn-glycero-3-phosphocholine(in) = a 1,2-diacyl-sn-glycero-3-phosphocholine(out) a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(in) = a 1,2-diacyl-sn-glycero-3-phosphoethanolamine(out) a cholesterol ester(in) = a cholesterol ester(out) a triacyl-sn-glycerol(in) = a triacyl-sn-glycerol(out) Heterodimer; heterodimerizes with the protein disulfide isomerase (P4HB/PDI) (PubMed:23475612, PubMed:26224785, PubMed:25108285, PubMed:16478722). Interacts with APOB (PubMed:26224785, PubMed:25108285, PubMed:27206948). Interacts with PRAP1 (By similarity). Colocalizes with P4HB/PDI in the endoplasmic reticulum (PubMed:23475612, PubMed:26224785). Liver and small intestine. Also found in ovary, testis and kidney. Positively regulated by cholesterol and negatively regulated by insulin. The disease is caused by variants affecting the gene represented in this entry.