Catalyzes the transport of triglyceride, cholesteryl ester, and phospholipid between phospholipid surfaces (PubMed:15897609, PubMed:16478722, PubMed:18502767, PubMed:33168624). Required for the assembly and secretion of plasma lipoproteins that contain apolipoprotein B (PubMed:17635917, PubMed:10713055, PubMed:9502759, PubMed:18502767, PubMed:33168624). May be involved in regulating cholesteryl ester biosynthesis in cells that produce lipoproteins (PubMed:18502767). Critical for the development of natural killer T (NKT) cells (PubMed:17312007). Required for the assembly and secretion of plasma lipoproteins that contain apolipoprotein B (PubMed:17635917). 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) Interacts with PRAP1. Heterodimer; heterodimerizes with the protein disulfide isomerase (P4HB/PDI) (PubMed:17635917). Interacts with APOB (By similarity). Heterodimer; heterodimerizes with the protein disulfide isomerase (P4HB/PDI) (PubMed:17635917). Colocalizes with P4HB/PDI in the endoplasmic reticulum (By similarity). Mainly expressed in the intestine and the liver, and at lower levels in white and brown fat cells (PubMed:17635917, PubMed:8660984). Expressed in heart (PubMed:9502759). Ubiquitous, and is the major isoform in hematopoietic cells and adipocytes. Expression in the yolk sac tissues followed by expression in the primordial liver cell nests as early as day 9 post-coitum (9.5 dpc). Intestinal expression is detected around 12.5 dpc and attains full adult expression patterns by 14.5 dpc. Up-regulated by FOXO1. Expressed in a circadian manner in the liver. Cleaved by signal peptidase between residues Gln-33 and Asn-34. Lowers plasma and tissue triglyceride levels, and increases cellular free cholesterol.