Participates in the reverse transport of cholesterol from tissues to the liver for excretion by promoting cholesterol efflux from tissues and by acting as a cofactor for the lecithin cholesterol acyltransferase (LCAT). Major protein of VLDL, HDL, LDL and in chylomicrons. Expressed in a number of tissues including liver, small intestine, lung, kidney, heart and muscle with highest expression in liver and small intestine. Belongs to the apolipoprotein A1/A4/E family.